Radiochemistry Of the Actinides Analysis

15 Radiochemistry of the Actinides

The actinides are the group of 14 elements beginning with thorium (Z 90) and ending with lawrencium (Z 103). Actinium (Z 89), the element preceding this series, is usually included in the group, and the actinides, the actinium-like elements, are named after it. Actinides are characterized by the filling of the seven 5f orbitals, similarly to the lanthanides, in which the 4f orbitals are filled. The similarity between these two groups is indeed extensive, although considerable differences also exist. For the most part, the chemistry of the lanthanides is fairly straightforward, while the chemistry of the actinides is much more complex. Actinides are commonly denoted by the abbreviation An. They can be divided according to their origin into two groups. The light actinides actinium, thorium, protactinium, and uranium, which are members of the natural decay series, and the transuranium elements, artificial actinides neptunium, plutonium, americium, curium, and so on. Since this books focus is on the chemistry of radionuclides appearing in nuclear waste and the environment, elements heavier than curium, rarely occurring in the environment and nuclear waste, are not discussed in detail.

15.1 Important Actinide Isotopes

Table 15.1 presents the most important actinide isotopes. The isotopes fromactinium to curium appear most abundantly in the environment and nuclear waste and are most abundantly generated in nuclear reactors and explosions. For actinides from berkelium onwards, only the longest-lived isotope is listed.

15.2 Generation and Origin of the Actinides

Uranium and the lighter actinides protactinium, thorium, and actinium isotopes are naturally occurring radionuclides belonging to the uranium, actinium, and

j239

Lehto, Jukka, and Xiaolin Hou. Chemistry and Analysis of Radionuclides : Laboratory Techniques and Methodology, John Wiley & Sons, Incorporated, 2010. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/osu/detail.action?docID=645020. Created from osu on 2019-01-29 07:17:21.

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thorium decay series. The uranium isotopes, 238U and 235U, and the thorium isotope, 232Th, are initial elements in the uranium, actinium, and thorium decay series. The Figures 15.115.3 show those parts of the series with actinides. An exception in this category is 236U, which is formed by neutron activation from 235U in nuclear fuel.

Uranium is the initial member of two decay series and also appears as the important, long-lived isotope 234U. Thorium appears in the series as six isotopes, of which 232Th and 230Th are extremely long-lived. Protactinium appears as two isotopes, of which 231Pa is long-lived. Likewise, actinium appears as two isotopes, and, of these, 227Ac is long-lived. Both uranium and thorium are widely dispersed. The concentration of thorium in the Earths crust is 8.1 ppm and that of uranium 2.3 ppm. As early as in the nineteenth century both elements were separated in macro amounts for clarification of their chemical properties. Although all isotopes of these elements emit alpha radiation and are potentially radiotoxic, they do not pose a particular safety problem during handling since the half-lives of the dominant

Table 15.1 Most important long-lived actinide isotopes. All isotopes up to 237Np are natural radionuclides except 236U. All isotopes from 237Np on and 236U are artificial.

Element Major radionuclides Half-life Decay mode

Actinium 227Ac 22 y b/a Thorium 232Th 1.4 1010 y a

230Th 7.6 104 y a 228Th 5.8 y a

Protactinium 231Pa 3.3 104 y a Uranium 238U 4.5 109 y a/SF

235U 7.0 108 y a 236U 2.3 107 a 234U 2.5 105 y a

Neptunium 237Np 2.1 106 y a Plutonium 238Pu 88 y a

239Pu 2.4 104 y a 240Pu 6500 y a 241Pu 14 y b

Americium 241Am 433 y a Curium 244Cm 18 y a/SF

242Cm 0.45 y a/SF Berkelium 247Bk 1380 y a a Einsteinium 252Es 1.3 y a/b/EC Fermium 257Fm 0.27 y a.14 y a/SF/b/EC Nobelium 259No 1 h a/EC/SF Lawrencium 262Lr 3.6 h SF

SF Spontaneous fission.

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isotopes of uranium and thorium are very long, and their specific activities are generally low.

Actinium and protactinium, in turn, are rare. One tonne of uranium ore contains only about 100 grams of protactinium, and the amount of actinium is about 1000 times less. Because the concentrations of Pa and Ac are so low, the separation of macro amounts from natural materials is laborious. Both elements, however, can be produced in a weighable amount in nuclear reactors by bombarding 226Ra and 230Th with neutrons. The activation products 227Ra and 231Th, produced in the irradiation, decay through beta emission to 227Ac and 231Pa, which can be separated from the

Figure 15.1 Onset of the uranium decay series.

Figure 15.2 Onset of the actinium decay series.

15.2 Generation and Origin of the Actinides j241

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mixture chemically after sufficient ingrowth.

226Ran; c227Ra ! b

227Ac

230Thn; c231Th ! b

231Pa

The transuranic elements (those heavier than uranium), that is, neptunium, plutonium, americium, and curium are produced in nuclear reactors and nuclear explosions. In reactors they form through neutron activation of the uranium isotopes 235U and 238U, as shown in Figure 1.1, where neutron capture is followed by beta decay to a heavier element. Corresponding reactions lead to the formation of transuranic elements in nuclear weapons explosions. Larger amounts of transura- nics are formed in nuclear reactors than in nuclear explosions, because the duration of neutron irradiation is considerably longer in reactors. The transuranic elements (Np, Pu, Am, Cm) comprise approximately one mass percent of spent nuclear fuel. Among the transuranic elements, plutonium (239Pu) has been expressly produced as nuclear weapons material by irradiating uranium in a reactor and afterwards separating the plutonium from the uranium. Since the early 1940s, about 300 tonnes of plutonium has been produced for weapons use, and a small percentage has been released into the environment through nuclear weapons testing. Plutonium was also released into the environment in the Chernobyl accident, but the amount was much less than during nuclear weapons tests. In addition, several thousand tonnes of plutonium has collected in spent nuclear fuel from nuclear reactors, mainly power reactors. Hence, plutonium is in no sense a rare element. Like neptunium, it is also naturally present in very small amounts in the environment, formed through the activation of uranium by neutrons from cosmic rays or neutrons released in spontaneous fission of 238U.

Figure 15.3 Onset of the thorium decay series.

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Neptunium, americium, and curium form in nuclear reactors in a combined amount about one-tenth that of plutonium in mass. The activity of americium in the environment is less than half that of plutonium, but the mass is only about one hundredth. Curium is present in the environment in exceedingly small amounts. A rough picture of the activities of the transuranic elements can be obtained from Table 15.2, which shows the activities in the environment and in spent nuclear fuel normalized to the activity of the long-lived plutonium isotopes 239;240Pu. The highest activity in fresh nuclear fallout and spent fuel comes from the beta decaying 241Pu isotope. Its half-life is, however, fairly short (14 y) and it more or less decays in about a hundred years. The decay of 241Pu to 241Am, in turn, considerably increases the activity of the latter during the seventy years following a fallout or the removal of nuclear fuel from a reactor. Regarding the alpha-emitting transuranium isotopes the highest activity is represented by 242Cm, but it decays in a rather short time because of its short half-life. In the long-term, allowing for thousands of years, the activity of 237Np also increases since it is the daughter of 241Am.

Over several decades, these four elements have been prepared in weighable macro amounts for study oftheir chemical properties and for the preparation of adiversity of solid compounds. In particular, these elements have been obtained through chemical separation from the irradiated uranium dissolved in acids. Studies on the chemical properties of these actinides in macro amounts, however, are not easy. To begin with, they are alpha emitters and highly radiotoxic. Compared with uranium and thorium, their half-lives are notably shorter, and their specific activities are correspondingly

Table 15.2 Activities of transuranic elements in Finland in the environment and in spent nuclear fuel relative to 239;240Pu activity. Environmental activities from nuclear weapons testing are estimated total activities at the time of deposition (Salminen, S. (2009) Development of analytical methods for the separation of plutonium, americium, curium, and neptunium from environmental samples. Doctoral

Dissertation, University of Helsinki). The activities in spent fuel are mean values of four Finnish nuclear reactors calculated after 30 years of cooling (Smith, P., Nordman, Pastina, B., Snellman, M., Hjerpe, T., and Johnson, L. (2007) Safety Assessment for a KBS-3H Spent Nuclear Fuel Repository at Olkiluoto Radionuclide Transport Report, Report Posiva 2007-07).

Nuclide Half-life (y) Nuclear weapons tests fallout

Chernobyl fallout

Spent nuclear fuel

237Np 2.1 106 0.004 0.001 0.0005 238Pu 88 0.03 0.54 3 239;240Pu 24 000 and 6500 1 1 1 241Pu 14 16 95 36 241Am 433 0.1 0.37 4 242Cm 0.44 15 244Cm 18 0.08 245Cm 8500 0.0003 246Cm 4700 0.00005

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greatly higher. The handling of macro amounts requires a safe environment, which usually means remote handling in hot cells. Another factor affecting the investigation of chemical properties is the effect of intense alpha radiation on the chemistry of the studied system, especially regarding 241Am. One milligram of 241Am releases about one hundred million alpha particles per second. An amount such as this causes significant changes in the crystal lattices of solid compounds of americium and causes strong radiolysis in the solution, in turn affecting the chemistry of americium in solution.

Of the elements heavier than curium, berkelium and californium were observed for the first time in the fallout from fusion bombs and afterwards have been synthesized by neutron activation in reactors and particle accelerators. Berkelium was first synthesized by the bombardment of americium with alpha particles, in the reaction 241Ama; 2n243Bk. Both berkelium and californium have been prepared and their chemical properties studied in microgram amounts. The heavier actinides, whose half-lives decrease with increasing atomic number, however, have only been syn- thesized in nanogram or picogram amounts (Es, Fm), and in rare cases in amounts of just a few atoms (Md, No, Lr). Neither the chemistry of these elements nor their separation is discussed in this book.

15.3 Electronic Structures of the Actinides

Besides the full shells 1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p6, the acti- nides also have electrons in the 6d and 7s orbitals. The actinides are characterized by the filling of the seven 5f orbitals, whose energy level is a somewhat lower than that of the outer 6d and 7s orbitals. In ground state, the 7s orbital is full, containing two electrons, while the 6d orbital has one or two electrons or none at all. The electronic structures of the actinides are presented in Table 15.3. The outer 6d and 7s orbitals of the actinides shield the 5f orbitals, which are closer to the nucleus. This behavior is analogous to that of the lanthanides, by which the 4f orbitals are filled. The energy differences between the 6d, 7s, and 5forbitals ofthe actinides are very small, however, being about the same as the energy of a chemical bond, and the 5f orbitals of the actinides are less shielded than the 4f orbitals ofthe lanthanides. Thus, whereasin the lanthanides only the 4f electrons of Ce participate in chemical bonding, in the lighter actinides, up to americium, the 5f electrons also take part. This makes the chemistry of the actinides considerably more complex than that of the lanthanides, and, unlike the lanthanides, the lighter actinides readily form complexes and form covalent bonds. With increase in the atomic number of the actinides, the charge on the nucleus grows and increasingly attracts the 5f electrons. Beginning with americium, the heavier actinides largely behave like the lanthanides, and the 5f electrons seldom participate in bond formation. This also applies to the lightest actinide actinium. The bonding of compounds of these heavier actinides, like that of the lanthanides, is mostly ionic. In contrast, the chemistry of the lighter actinides Th, Pa, U, Np, and Pu resembles that of the d-transition elements.

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15.4 Oxidation States of the Actinides

The oxidation states of the actinides vary widely, between II and VII (Table 15.4). For actinium the most stable state is III. The most stable oxidation state then

Table 15.3 Electronic structures of the actinides as gaseous atoms, gaseous M , M2 , M3 , and M4 ions, and M3 ions in aqueous solution. The electronic structures of gaseous M4 ions, except for U, have been predicted but are not known experimentally.

Atom (g) M (g) M2 (g) M3 (g) M3 (aq) M4 (g)

Actinium 6d7s2 7s2 7s Thorium 6d27s2 6d7s2 5f6d 5f Protactinium 5f26d7s2 5f27s2 5f26d 5f2 5f1

Uranium 5f36d7s2 5f37s2 5f36d 5f3 5f3 5f2

Neptunium 5f46d7s2 5f57s 5f5 5f4 5f4 5f3

Plutonium 5f67s2 5f67s 5f6 5f5 5f5 5f4

Americium 5f77s2 5f77s 5f7 5f6 5f6 5f5

Curium 5f76d7s2 5f77s2 5f8 5f7 5f7 5f6

Berkelium 5f97s2 5f97s 5f9 5f8 5f8 5f7

Californium 5f107s2 5f107s 5f10 5f9 5f9 5f8

Einsteinium 5f117s2 5f117s 5f11 5f10 5f10 5f9

Fermium 5f127s2 5f127s 5f12 5f11 5f11 5f10

Mendelevium 5f137s2 5f137s 5f13 5f12 5f12 5f11

Nobelium 5f147s2 5f147s 5f14 5f13 5f13 5f12

Lawrencium 5f146d7s2 5f147s2 5f147s 5f14 5f14 5f13

Table 15.4 Oxidation states of the actinides. The most stable oxidation states are indicated by X, others by x. Oxidation states indicated in parentheses occur only in solid-state compounds.

II III IV V VI VII

Actinium X Thorium x X Protactinium (x) x X Uranium x x x X Neptunium x x X x x Plutonium x X x x x Americium (x) X x x x Curium X x Berkelium X x Californium (x) X (x) Einsteinium (x) X Fermium x X Mendelevium x X Nobelium X x Lawrencium X

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increases systematically, from IV for thorium to V for protactinium and VI for uranium. Afterwards it decreases systematically to V for neptunium, IV for plutonium, and III for americium. The most stable and typical oxidation state for the actinides heavier than plutonium is III, these actinides otherwise closely resemble the lanthanides. The one exception is nobelium, whose most stable oxidation state is II, where it obtains a full 5f shell (see Table 15.3).

15.5 Ionic Radii of the Actinides

The ionic radii of actinides in the same oxidation state systematically decrease with increase of atomic number. Figure 15.4 shows the ionic radii of actinides in the oxidation states III, IV, V, and VI. The reason the ionic radius decreases with the atomic number is the increasing nuclear charge attracting more effectively the 5f electron shell. This decrease, known as the actinide contraction, is analogous to the lanthanide contraction. Decrease in the ionic radius affects the hydrolysis and complex formation of the actinides. The smaller the radius, the more readily the ion hydrolyzes and forms complexes. Otherwise, actinide ions in the same oxidation state behave in a closely similar way. Those actinides that typically appear only in the trivalent state are separated from each other mainly on the basis of difference in ionic radius. As shown in Figure 10.2, in the chromatographic separation of lanthanides and actinides by cation exchange, the greater the atomic number of a lanthanide or

100

120

100989694929088

Atomic number

Io n

ic r

a d

iu s (

p m

)

III

Figure 15.4 Ionic radii of actinides (AcCf) in oxidation states three, four, five, and six at coordination six.

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actinide, the smaller is its ionic radius, the stronger is the complex formed with ammonium a-hydroxyisobutyrate, and the more rapidly it elutes from the column.

15.6 Major Chemical Forms of the Actinides

Actinides appear in oxidation states III to VI as the following forms: III : An3 IV : An4 V : AnO2 VI : AnO2 2

The last two forms, known as actinyl ions, are formed by hydrolysis, in this case by forming covalent bonds with oxygen atoms in the hydration shell. This is a typical reaction for metal cations of high oxidation state ( V to VII). Ionic radii of the actinides are large, however, and charge densities are not, therefore, as high as for smaller metal cations of corresponding oxidation state. As a result, they never bind more than two oxygen atoms, and in oxidation states V and VI do not form oxoanions such as AnO3

or AnO4 2, respectively. Hexavalent actinyl ions, AnO2

2 , do not appear as distinct species in solution only, but also in solid compounds of U, Np, Pu, and Am, in AnO2F2, for example. In complexes, AnO2

2 also appears as a distinct species. Actinyl ions, (OAnO) and (OAnO)2 , are symmetrical and linear. Because of the high charge of actinides in oxidation state six, the bonds with oxygen are stronger than the corresponding bonds in actinides in oxidation state five. Actinyl ions are only found for actinides Pa, U, Np, Pu, and Am. But, whereas actinyl ions are common for the first four, the actinyl ions of Am only appear in highly oxidizing conditions.

The gross charges of the above-mentioned actinide species are 3 (An3 ), 4 (An4 ), 1 (AnO2 ), and 2 (AnO22 ). The positive charge is not, however, equally distributed in the actinyl ions but is localized on the metal, whose effective charge decreases in the order An4 > AnO2

2 > An3 > AnO2 and in the case of

plutonium their charges are 4, 3.2, 3 and 2.2, respectively. Like the ionic radius, the charge on the metal affects the hydrolysis and complex formation occurring in solution. This will be discussed later.

Figure 15.5 shows the distribution of these species for U, Np, Pu, and Am as a function of redox potential (Eh) in 1 M HClO4. More detailed information for each element is given later in their specific Eh pH diagrams.

15.7 Disproportionation

Someof the aforementioned actinide species are not entirely stable insolution. Of the pentavalent actinyl ions, PaO2

and NpO2 are stable but PuO2

and UO2 are

not, and change to other species by disproportionation. Pentavalent Pu easily

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disproportionates in acidic solutions and forms plutonium species in oxidation state four and five according to the following reactions:

2PuO2 4H KPu4 PuO2 2 2H2O

PuO2 Pu4 KPu3 PuO2 2 UO2

disproportionates to U4 and UO2 2 according to the reaction:

2UO2 4H KU4 UO2 2 2H2O

Also, Pu4 , despite being the most stable oxidation state of plutonium, is somewhat unstable in weakly acidic solutions and disproportionates to PuO2

and Pu3 . Figure 15.6 shows the disproportionation of the An4 and AnO2

ions of U, Np, Pu, and Am in a 1 M acid solution. In the oxidation state IV only uranium and neptunium are stable while plutonium disproportionates partly and americium

Figure 15.6 Disproportionation of An4 and AnO2

ions of U, Np, Pu, and Am in 1 M acid. The top part of the diagram shows the original concentration of species and the bottom part

the concentrations at equilibrium. (Cotton, F.A. and Wilkinson, G. (1988) Advanced Inorganic Chemistry, John Wiley and Sons, New York).

Figure 15.5 Distribution of U, Np, Pu, and Am oxidation states as a function of Eh in 1 M HClO4 (Choppin, G., Liljenzin, J.-O., and Rydberg, J. (2002) Radiochemistry and Nuclear Chemistry, 3rd edn, Butterworth-Heineman).

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completely. In the oxidation state Vonly neptunium is stable, uranium more or less completely disproportionates, and plutonium and americium disproportionate to a great extent. In all respect plutonium is the most challenging, since it exists in several oxidation states at the same time.

15.8 Hydrolysis and Polymerization of the Actinides

Actinides in the same oxidation state hydrolyze according to their ionic radius: more strongly when the ionic radius is small and the metal more acidic. The strength of the coulomb interaction between the metal and oxygen atoms in water follows the charge density of the metal. Thus, hydrolysis and complex formation increase in the following order (only those actinides typically appearing in solution are included):

III : Ac3 < Pu3 < Am3 < Cm3 < Bk3 etc:

IV : Th4 < U4 < Np4 Pu4

V : PaO2 > NpO2 < PuO2 VI : UO2 2 < PuO2 2

Some exceptions to this order exist. Tetravalent Np hydrolyses just as strongly as Pu, which is heavier. In addition, pentavalent Pa is clearly more acidic than heavier actinides in the same oxidation state.

For the same actinide, hydrolysis and complex formation increase in the order

AnO2 < An 3 < AnO2 2 < An

because of the already noted growth of the effective charge of the metal. Again, it is a case of the coulombic interaction increasing with the charge density of the metal.

Trivalent actinides form the hydrolysis products AnOH2 , An(OH)2 , and An

(OH)3, the last of which precipitates if the concentration of the actinide in solution is high enough. Tetravalent actinides form the following products: AnOH3 , An (OH)2

2 , An(OH)3 , and An(OH)4. Once again, the last species will precipitate

if the concentration of the actinide is sufficiently high. Since actinides are least soluble at oxidation state IV the precipitation of hydroxides takes place at very low concentrations. Hydrolysis of the tetravalent actinides commences even at pH 23. Before precipitation occurs, the actinides form simple, monomeric An(OH)4 com- plexes and, as the concentration increases, also dimeric and polymeric complexes, such as Th2(OH)2

6 and Th6(OH)15 9 . Pentavalent actinyl ions, which only weakly

hydrolyze and only at pH 89, form soluble AnO2OH complexes. Hexavalent actinyl ions form the mono, di, and trimeric hydrolysis products AnO2OH

, (AnO2)2(OH)2

2 , and (AnO2)3(OH)5 . In the polymeric hydrolysis products, bonds

are formed between metal atoms, hydrogen bonds through the OH group (MOHM) or covalent bonds through oxygen (MOM). The hydrolysis of

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polyvalent actinides is complex. Monomeric hydrolysis products are formed in dilute solutions. As the concentration increases, so does the proportion of polymers, and when the concentration is high enough, precipitation as hydroxide occurs. Besides monomers and polymers, hydrolysis products appear in solution as colloids, small 1100 nm particles of nonstoichiometric composition, which are kept in suspension by Brownian motion.

15.9 Complex Formation of the Actinides

The actinides form a large group of complexes. With respect to the oxidation states of actinides, the stabilities ofcomplexes followthe same general rules as those thatapply in hydrolysis. The strength of complexes formed with inorganic monovalent anionic ligands decreases in the following order:

F > NO3 > Cl > ClO4

With divalent anionic ligands the strength of complexes decreases in the order

CO23 > C2O 2 4 > SO

2 4

In natural waters, carbonate complexes forming alongside hydrolysis products of the actinides are major solution species in the neutral area. In weakly acidic solutions, ligands such as sulfate, phosphate, and fluoride also form complexes with the actinides. Important ligands forming complexes with the actinides in natural waters are humic and fulvic acids, which vary widely in composition and concentration. Humic and fulvic acids are large, complex, and heterogeneous organic molecules with complexing carboxyl groups and oxygen and nitrogen atoms with free electron pairs on the surface. The carboxyl groups and free electron pairs strongly bind metals in solution, especially polyvalent metals, such as the actinides, which readily form complexes.

The actinides form strong complexes with inorganic and organic phosphates. Several phosphate compounds, which strongly bind An4 and AnO2

2 ions, are exploited in analytical separations by solvent extraction. The most important of the organicphosphatesistributyl phosphate(TBP),which isusedinreprocessingfacilities for spent nuclear fuel to separate uranium and plutonium from fission products and other actinides. Other solvent extraction reagents widely used in the separation of actinides have oxygen, nitrogen, or sulfur as complex-forming electron donors.

15.10 Oxides of the Actinides

The actinides form oxides with many different oxidation states (Table 15.5). In many cases, the oxidation state of the actinide in the most stable oxide is the same as it is in

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solution. This is true for Th, Pa, Pu, and Cm. Exceptionally, Np and Am mostly form dioxides, although their most stable oxidation states in solution are V and III, respectively. Thus the most typical stable oxide is AnO2. The most stable oxides of uranium are UO2 and U3O8, the mean oxidation state of U in the latter being 5.33. This is calculated to be the product of two hexavalent uranium atoms and one tetravalent atom per molecule. Many nonstoichiometric compounds are found among the actinide oxides, particularly for uranium. Several crystal phases are formed as the composition of the uranium oxides changes from UO2 to UO3 or the oxidation state varies from IV to VI. At first, the extra oxygen atoms joined to UO2 are distributed randomlyin the UO2 crystal lattice, and the structureremains the same. When the concentration of oxygen increases to 2.4, however, a new crystal phase appears, and, proceeding further to UO3, six new crystal phases are formed one by one. Protactinium also has several nonstoichiometric oxides between Pa2O5 and PaO2, while plutonium has only one, PuO1.61, between PuO2 and Pu2O3. Only stoichiometric oxides have been reported for neptunium (NpO2 and Np2O5) and thorium (ThO2).

15.11 Actinium

15.11.1 Isotopes of Actinium

Actinium has just two isotopes in the natural decay series: in the thorium series 228Ac (Figure 15.3) and in the actinium series 227Ac (Figure 15.2). The first is a beta emitter and has a half-life of just 6.5 h, decaying through many excited states to 228Th, with a maximum beta energy of 2.127 MeV. The energy of the most intense gamma ray of the tens of gamma rays emitted in relaxation of the excited states is 911 keV, with an intensity of 25.8%. Gamma emissions can be used in the determination of this nuclide. 228Ac can also be used to measure the activity of its parent nuclide, the beta- emitting isotope of radium 228Ra (see Chapter 7): the radium isotopes (226Ra and 228Ra) are separated from other nuclides, and after ingrowth into equilibrium with 228Ra, 228Ac is separated, for example, by Ln Resin, and its activity is measured by gamma spectrometry or by liquid scintillation counting.

Table 15.5 Oxides of the light actinides. The most stable oxides are indicated by X, others by x.

Th Pa U Np Pu Am Cm

6 (AnO3) x 4 6 U3O8 5 (An2O5) X x x x 4 (AnO2) x X X X X x 3 (An2O3) x X

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The other natural isotope of actinium, 227Ac, is considerably longer lived, with a half-life of 21.7 years. It decays primarily (98.6%) through beta emission to 227Th with a maximum energy of 45 keV, the rest (1.4%) of the decay occurring through alpha emission to 223Fr (Figure 15.8). Figure 15.7 shows the simulated alpha spectrum. The alpha peaks of 227Ac can be distinguished from those of the daughter nuclides.

After 227Ac, the longest-lived isotope of actinium is 225Ac, which has a half-life of 10 days. It is the daughter of beta-emitting 225Ra and is used as a tracer in 227Ac analyses. Neither 225Ac nor its parent 225Ra occur naturally, but the latter is milked from a generator of artificially produced 229Th. For the utilization of 225Ac as a tracer, its parent nuclide, 225Ra, must be first separated from the solution. 225Ac is an alpha emitter with several alpha and gamma transitions. The intensities of the gamma transitions are very low, the highest intensity being only 0.5%.

15.11.2 Chemistry of Actinium

Actinium is the first member of the actinide series. Its electron configuration is [Rn] 6d7s2 and its only known oxidation state is III; in solution it appears as the Ac3 ion. Although the general characteristic of the actinides is the filling of the 5f shell, in no form does actinium have electrons in its 5f shell. It closely resembles lanthanum in its chemistry, differing from it slightly in size: the ionic radius of the trivalent ion of actinium is 0.112 nm, whereas the radius of the trivalent lanthanum ion is 0.103 nm. Lanthanum is an effective carrier for actinium since its compounds quantitatively precipitate compounds ofactinium.Of all trivalent ions (ofthe lanthanides, actinides, and groups 3 and 13), the Ac3 ion is the largest. It is also, therefore, the least acidic of these, which means that its tendency to hydrolyze and form complexes is the lowest. Hydrolysis of Ac3 begins at pH 67, and at pH 8, 74% of actinium is in the form of

54,94,84,74,6

Energy (MeV)

In te

n s

it y

( %

)

Figure 15.7 Simulated alpha spectrum of 227Ac.

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Ac(OH)2 and 26% in the form of Ac(OH)2 . Actinium forms strong complexes

with oxalate, citrate, and EDTA, and moderately strong complexes with fluoride and sulfate. The chemistry of actinium has been difficult to study because only micro- gram amounts are available. In weighable amounts, actinium has been studied only as its 227Ac isotope. Although the half life of 227Ac (21.7 y) is longer than that of other actinium isotopes, for example that of 228Ac (6.5 h), it is still too short to make it possible to obtain it in larger amounts. Its specific activity is very high, and all operations must be carried out with special handling techniques and great care. The high specific activity also means that autoradiolysis is strong, which influences the chemistry of the investigated system.

15.11.3 Separation of Actinium

Separations of actinium are seldom required. 227Ac is determined for the calculation of the 231Pa=235U ratio in geological dating studies. As 231Pa is difficult to separate in

Figure 15.8 Decay schemes of 227Ac. On the left is the beta decay (98.6%) and on the right the alpha decay (1.4%) (Firestone, R.B., Shirley, V.S., Chu, S.Y.F., Baglin, C.M., and Zipkin, J. (1996) Table of Isotopes, Wiley-Interscience).

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pure form, its daughter, 227Ac, which is in equilibrium with it, is separated and counted instead. Another application is the investigation of the mixing of deep sea waters through measuring the ratio 227Ac=231Pa. A method has been developed to determine Ac and other actinides in marine sediments and particulate matter of seawater (Anderson, R.F. and Fleer, A.P. (1982) Determination of natural actinides and plutonium in marine particulate matter. Anal. Chem., 54, 1142). The sediment is digested in acid, and the pH of the obtained solution is raised to 7 so that the iron and aluminum in the water precipitate as hydroxides, and the hydrolyzable metals, including Ac, are coprecipitated, while alkali and alkaline earth metals remain in solution. The precipitate is dissolved in 9 M HCl and the solution passed through an anion exchange column. Iron, polonium, plutonium, uranium, and most of the protactinium are adsorbed on the resin, while the actinium, thorium, americium, and part of the protactinium pass through the column. The effluent is evaporated to dryness and dissolved in 8 M HNO3, and anion exchange is carried out once again. Th and Pa are taken up by the resin, while Ac and Am pass through. Following this, a few months are allowed for the ingrowth of the daughter of 227Ac, 227Th (t1/2 19 d), and a further anion exchange is carried out in 8 M HNO3. The Ac and Am pass through the column, while 227Th is retained. Thorium is then eluted from the column with 9 M HCl and is finally electrodeposited onto a metal plate for alpha counting. Because 227Th was in equilibrium with 227Ac, its activity also represents the activity of 227Ac.

15.11.4 Essentials of Actinium Radiochemistry

. Actinium has only two natural isotopes: 228Ac in the thorium series and 227Ac in the actinium series. The former is a pure beta emitter with a very short half-life of 6.5 h. The latter has a fairly long half-life of 21.7 years and decays primarily by beta emission (98.6%); the rest of the decays (1.4%) occur by alpha emission. Artificially produced 225Ac, an alpha emitter with a half-life of 10 days, is used as a tracer in 227Ac analyses.

. Actinium is the first member in the actinide series and very much resembles lanthanum in its chemistry. The only oxidation state of actinium is III, the Ac3 ion being formed. Actinium primarily forms ionic compounds and does not hydrolyze strongly.

. Actinium separations are rarely needed. While the determination of the shorter- lived isotope 228Ac as such is of practically of no importance, it is commonly separated and measured to determine the activity of its parent, the beta-emitting radium isotope 228Ra, presuming there is an equilibrium between these two nuclides. 228Ac activity can be determined either by LSC, measuring the 2.127 MeV (maximum energy) beta particles or by gamma spectrometry, mea- suring the 911 keV gamma emission with 25.8 % intensity.

. The determination of the longer-lived isotope 227Ac has some geological and environmental applications. The separation of actinium can be accomplished by ion exchange chromatography, for example. Nonhydrolyzable metals, including

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radium, are first removed by hydroxide coprecipitation. Anion exchange in 9 M HCl removes iron, polonium, plutonium, uranium, and most of protactinium. Another ion exchange step in 8 M HNO3 is carried out to remove thorium and the rest of the protactinium. After these steps, only americium follows actinium. To remove americium, 227Th (t1/2 19 d), the daughter of 227Ac, is allowed to grow in during a few months and one more anion exchange step is carried in 8 M HNO3. Ac and Am pass through the column while 227Th is retained. It is eluted from the column with 9 M HCl and electrodeposited onto a metal plate for alpha counting.

15.12 Thorium

15.12.1 Occurrence of Thorium

Thorium is a relatively common element more common than tin and almost as common as lead. Its abundance in the Earths crust is 8 ppm, about 3.5 times that of uranium. Thorium is homogeneously distributed in small concentrations in the overburden and bedrock. There are two pure minerals of thorium: thorianite (ThO2, thorium dioxide), and thorium silicate (ThSiO4). Both are rare and often occur as mixed minerals with uranium, (Th,U)O2 and (Th,U)SiO4. The main source of thorium is monazite, a lanthanide phosphate mineral, in which thorium may comprise as much as 12%. The solubility of thorium is very low and its concentration in natural water bodies is extremely low: in seawater, its concentration is 1.5 ng L1

and in oxic groundwater less than 1 mg L1.

15.12.2 Thorium Isotopes and their Measurement

Thorium occurs naturally as six isotopes (Figures 15.115.3 and Table 15.6). The longest-livedisotopeisthefirstmemberofthethoriumseries,232Th,whichcomprises 99.9995% of the total mass of thorium. Figure 15.9 shows the decay scheme of this isotope. Two of the isotopes of thorium, 231Th and 234Th, are beta emitters, and the

Table 15.6 Activities and masses of the six natural thorium isotopes relative to 232Th.

Nuclide Half-life (y) Activity Mass Decay mode

227Th 0.051 0.04 1.4 1013 a 228Th 1.91 1 1.4 1010 a 230Th 75 400 0.90 4.8 106 a 231Th 0.0029 0.04 8.2 1015 b 232Th 1.4 1010 1 1 a 234Th 0.066 0.90 4.2 1012 b

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remaining four are alpha emitters. Yield determinations for thorium analyses make use of the artificial isotope 229Th, an alpha emitter with a half-life of 7340 years. It is obtained as an alpha decay product of 233U which, in turn, can be obtained by the neutron bombardment of 232Th:

232Thn; c233Th ! b 233Pa ! b 233U a ! 229Th Alpha spectrometry is the most utilized method to measure the alpha-emitting

thorium isotopes. The alpha energies of the four natural thorium isotopes that decay by alpha emission vary between 3.95 and 6.04 MeV. The alpha energies of these isotopes, as well as the energy of the artificial isotope 229Th used as marker, differ sufficiently for all five isotopes to be identified in the same spectrum from a semiconductor detector (Figure 15.10a). The activity of 227Th, however, compared to other thorium isotopes, is very low, and only 228Th, 230Th, and 232Th are detected in the alpha spectrum, as can be seen in Figure 15.10b.

The intensities of the gamma transitions of 232Th are too low to allow the direct determination of 232Th by gamma spectrometry. The 63.8keV c-ray has the highest intensityof0.26%.Further,theenergiesofthegammaraysaresolowthatself-absorption occurs, causing a marked reduction in the counting efficiency and thus increased uncertainty in the results. 232Th can, however, be measured through its granddaughter nuclide 228Ac,whichhasahighintensity(48%)gammarayat74.7keVenergy.Again,this gamma ray is of such low energy that self-absorption is a problem, and the measurement requires a thin-window semiconductor detector. While the use of the granddaughter nuclide in the measurement of 232Th obviously requires equilibrium with the parent nuclide,thiscanbeassumedinrockssincethehalf-lifeof 228Ac isjust6handthehalf-life of 228Ra, which lies between 228Ac and 232Th, is 5.8 years (see Figure 15.3).

15.12.3 Chemistry of Thorium

The electron configuration of the thorium atom is [Rn]6d27s2. Practically its sole oxidation state is IV, the Th4 ion being formed. The chemistry of thorium

Figure 15.9 Decay scheme of 232Th (Firestone, R.B., Shirley, V.S., Chu, S.Y.F., Baglin, C.M., and Zipkin, J. (1996) Table of Isotopes, Wiley-Interscience).

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strongly resembles that of the group 4 metals Ti, Zr, and Hf. Since thorium has only one oxidation state, it is used as a chemical analog for other tetravalent actinide ions (U4 , Np4 , Pu4 ) in research carried out in conditions where these other actinides may appear in several oxidation states. Although the chemistry of thorium is relatively straightforward compared to that of many other actinides, its research is still demanding. Many factors, such as low solubility and strong absorption on surfaces, create great uncertainty in solubility products, complex formation con- stants, and other values relevant to its chemistry.

As the largest of the tetravalent actinide ions (Figure 15.4), Th4 forms the weakest hydrolysis products and complexes compared to the other actinides. Th (IV) begins to hydrolyze at pH 1, forming mononuclear mono-, di-, and trihydroxy species in weak solutions until, at pH 4, Th(OH)4 predominates in the solution (Figure 15.11). At high concentrations of thorium, polynuclear species such as [Th2(OH)2]

6 , [Th4(OH)8] 8 , and [Th6(OH)15]

9 appear, the latter being domi- nant in 0.1 M thorium solution at pH 46. Colloidal thorium also appears at high concentrations.

Figure 15.10 (a) Simulated alpha spectra of the natural isotopes of thorium and the artificial isotope 229Th, and (b) a real alpha spectrum. The peaks on the left of 228Th are due to the progeny of thorium isotopes.

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The solubility of thorium is dependent on the kinds of oxides or hydroxides in which it occurs: crystalline ThO2 is the least soluble (solubility product 10

54), amorphous Th(OH)4 being somewhat more soluble (10

47). Because of its low solubility, thorium concentrations are very low in natural water, 108 M to 109 M.

Thorium forms moderately strong complexes with fluoride, sulfate, phosphate, and carbonate. The solubility of thorium is strongly increased when carbonate is present.

15.12.4 Separation of Thorium

15.12.4.1 Separation of Thorium by Precipitation Thorium forms sparingly soluble precipitates with hydroxide, fluoride, iodate, oxalate, and phosphate. Hydroxide precipitation separates thorium, along with other hydrolyzing metals, from alkali and alkaline earth metals and anions. Fluoride precipitation separates thorium from penta- and hexavalent actinides, which do not form fluorides; tri- and tetravalent actinides and lanthanides follow thorium in the fluoride precipitation. In fluoride precipitation, iron is also removed because it does not precipitate with fluoride in acidic solution. In addition, iron can be removed in an oxalate precipitation; iron does not precipitate as oxalate when in a trivalent state.

15.12.4.2 Separation of Thorium by Anion Exchange Thorium does not form negatively charged complexes in hydrochloric acid systems and thus does not adsorb to anion exchange resins. Several natural alpha-active nuclides, uranium, protactinium, and polonium, are retained, however. In addition, iron, which interferes in the preparation of the counting source, and the transuranics plutonium and neptunium are retained in the column. When anion exchange is

Figure 15.11 Hydrolysis of thorium in 0.1 M and 105 M solutions. The dashed curves denote regions supersaturated with respect to ThO2 (Baes, C.F. and Mesmer, R.E. (1976) The Hydrolysis of Cations, John Wiley and Sons Inc., p. 166).

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carried out instrong hydrochloric acid,only radium among the natural alpha emitters and americium and curium among the transuranics pass through the column along with thorium. These can be removed by carrying out a second ion exchange, this time in strong nitric acid. In 8 M nitric acid, thorium forms the strong nitrate complex Th(NO3)6

2, which is effectively retained in the column, while radium, americium, and curium pass through. Thorium, separated from other alpha emitters, is eluted from the column with weak nitric acid, and a source for alpha measurement is prepared by coprecipitation with lanthanide fluoride or by electrodeposition onto a metal disk. Alternatively, the amount of thorium can be determined by ICP-MS.

15.12.4.3 Separation of Thorium by Solvent Extraction Thorium can be extracted as a nitrate complex with a number of organic extraction reagents, including ethers, ketones, and phosphates. In nitric acid systems, thorium is effectively extracted into tributylphosphote (TBP), for example, together with uranium and other tetra- and hexavalent actinides. Thorium cannot be extracted from hydrochloric acid medium, however, because thorium does not form a chloride complex. Extraction in hydrochloric acid can therefore be used to remove uranium, since this forms a complex with Cl. The removal of iron is also accomplished in HCl extraction: in 6 M HCl solution, iron is extracted with diethylether as the FeCl3 complex, while thorium remains in the aqueous phase.

15.12.4.4 Separation of Thorium by Extraction Chromatography There are several extraction chromatography resins for the separation of thorium: Actinide, DGA, Diphonix, RE, TRU, UTEVA, and TEVA Resins. Various methods have been reported for the separation of thorium from water using TEVA Resin (Figure 4.13). The functional group in TEVA Resin is trialkyl methylammonium nitrate (or chloride). The actinides are first enriched from water by coprecipitation with calcium phosphate and the precipitate is then dissolved in 3M HNO3. Pluto- nium is converted to Pu(IV) with iron sulfamate and sodium nitrite, and the solution is then poured into a TEVA column pretreated with 3 M nitric acid. Th4 , along with Pu4 and Np4 , are taken up by the resin, while UO2

2 , NpO2 , and Am3 pass

through. Thorium is eluted from the column with 9 M HCl, while plutonium and neptunium remain in the column. Finally, a counting source of the thorium is prepared for the alpha measurement.

15.12.5 Essentials of Thorium Radiochemistry

. Thorium is the most abundant natural radioactive element, constituting 8 ppm of the Earths crust. There are two major thorium minerals, thorianite (ThO2) and thorium silicate (ThSiO4); however, the major source of thorium is monazite, a lanthanide phosphate mineral.

. The most long-lived isotope of thorium 232Th (t1/2 1.4 1010 y) keeps up one of the three natural decay chain, called thorium series. Thorium has another long- lived isotope 230Th (t1/2 75 400 y) in the uranium series. Three other natural thorium isotopes, 227Th, 231Th, and 234Th, are short lived, the half-lives ranging

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from 1.1 to 24 days, while 228Th has a somewhat longer half-life of 1.91 years. 227Th, 228Th, 230Th, and 232Th isotopes are alpha emitters while 231Th and 234Th are beta emitters. As a yield determinant tracer in the analyses of the longer-lived alpha-emitting thorium isotopes (228Th, 230Th, 232Th), 229Th (t1/2 7340 y) is used. Alpha spectrometry is typically used to measure thorium isotopes, but mass spectrometry is also used to measure the two most long-lived isotopes.

. Thorium has only one oxidation state, IV, and its chemistry resembles that of the group 4 metals Ti, Zr, and Hf. Thorium is readily hydrolyzable and highly insoluble. Therefore, its concentrations in natural waters are very low at 108 M to 109 M. High concentrations of carbonate in water considerably increase the solubility of thorium.

. Coprecipitation, ion exchange, solvent extraction, and extraction chromatography are used in thorium separations. Coprecipitations as hydroxide, carbonate, and phosphateareusedinenrichments,oxalateprecipitationintheremovalofiron,and lanthanide fluoride precipitation to prepare a counting source for alpha counting.

. Separations of thorium by anion exchange, solvent extraction, and extraction chromatography are based on the fact that thorium forms a strong negative complex with nitrate anions in strong nitric acid but no chloride complex.

. In anion exchange chromatography, uranium, protactinium, polonium, plutoni- um, and neptunium are removed from thorium by carrying out the ion exchange in strong hydrochloric acid, where the above-mentioned metals are efficiently retained in the column while thorium, radium, and americium pass through. Radium and americium are removed in another anion exchange step in strong nitric acid where thorium is retained in the columns while the other pass through.

. Several extraction chromatography resins have been utilized in thorium separations, the TEVA Resin most commonly. This resin takes up tetravalent actinides (Th4 , Pu4 , Np4 ), while UO2

2 and Am3 pass through. Thorium is then eluted from the resin with strong hydrochloric acid, the other actinides remaining in the resin.

. Several extraction agents have been used in thorium separations, for example TBP in nitric acid to remove thorium and other tetra- and hexavalent actinides. Since thorium does not form a chloride complex it cannot be extracted in hydrochloric medium. However, an HCl medium can be used to remove interfering elements which form chloride complexes, such as uranium and iron.

15.13 Protactinium

15.13.1 Isotopes of Protactinium

Protactinium is represented in the natural decay series by just two isotopes: 234Pa in the 238U series and 231Pa in the 235U series (Figures 15.1 and 15.2). Of these, the former is a short-lived (t1/2 6.75 h) beta emitter, which decays to 234U. 231Pa, in turn,

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is a long-lived isotope with a half-life of 32 760 years. It decays by alpha emission to 227Ac. Figure 15.12 presents the alpha spectrum of 231Pa. In nature, the activity of the longer-lived 231Pa is about 5% of that of 234Pa, but the mass of 231Pa is about two million times as great. In addition to these two natural isotopes, the artificial isotope 233Pa is the third most important, and this can be used as a tracer in determining the chemical yield in 231Pa analyses. 233Pa is a relatively short-lived (t1/2 27 d) beta emitter with a maximum energy of 0.571 MeVand the most intense (38.6%) gamma emission is at 312 keV. To obtain 233Pa, the best method is to separate it from the long- lived 237Np (t1/2 2.1 106 y), which decays by alpha emission to 233Pa. The ingrowth of 233Pa occurs during a few months storage of 237Np, after which 233Pa can then be separated for use as a tracer. Milking 233Pa from 237Np can be repeated at intervals of a few months.

15.13.2 Chemistry of Protactinium

Protactinium is the third actinide and the first in the group to have electrons in the 5f shell. The electron configuration of its ground state is [Rn]5f26d17s2, and its major oxidation state is V. Although the oxidation state IV is also possible, it is unstable and rapidly oxidizes to V. Reduction to the oxidation state IV requires strongly reducing conditions and a highly acidic solution.

Although protactinium is a rare element, it can be separated in amounts even greater than 100 grams in conjunction with the production of uranium; its chemistry, therefore, can be investigated in macro amounts. Nonetheless, studying the chem- istry of protactinium is difficult, since it readily hydrolyzes and adsorbs on various surfaces. The chemistry of protactinium resembles that of the group 5 metals V, Nb, and Ta. The most common oxide of protactinium is Pa2O5; however, the oxide PaO2 is also known.

The high charge on proctactinium causes it to hydrolyze readily. Thus, free Pa5

ions do not occur even in highly acidic solutions. In acidic solutions in which hydrogen ion concentration is greater than 3 M, Pa probably appears as a partly

Figure 15.12 Simulated (a) and measured (b) alpha spectra of 231Pa (The Chemistry of the Actinide and Transactinide Elements, vol. 1, 3rd edn, Springer, p. 167).

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hydrolyzed PaO3 form. From there, the hydrolysis proceeds with decreasing acidity as follows:

PaO3 H2OKPaOOH2 H acid concentration < 3 M

PaOOH2 H2OKPaOOH2 H acid concentration < 1 M

PaOOH2 2H2OKPaOH5 H pH > 3

PaOH5 H2OKPaOH6 H alkaline solution

When protactinium is present in solution in sufficient concentration, it precipi- tates as hydrous oxide at pH 56. Before the solubility product is exceeded, polymeric hydroxide species and colloids form in the solution. The formation of polymers is irreversible, and dilution of the solution does not break them back down into monomers.

In addition to hydroxyl complexes, Pa forms several inorganic complexes, their strengths decreasing in the following order:

F > OH > SO24 > Cl > Br > I > NO3 > ClO

15.13.3 Separation of Protactinium

Protactinium is separated by precipitation, ion exchange, solvent extraction, and extraction chromatography. Precipitation is used for enrichment purposes and the removal of interfering radionuclides. Many protactinium compounds are sparingly soluble, and hydroxide as well as carbonate and phosphate are common coprecipi- tants. Pa adsorbs on anion exchangers from both nitric acid and hydrochloric acids, though only weakly from nitric acid (Figures 4.5 and 4.6). Thus, separations by anion exchange are mainly carried out in hydrochloric acid, where the best separation efficiency is obtained with an acid concentration of 910 M. Of the natural actinides, uranium follows Pa into the resin, while thorium and actinium do not. Further, iron, zirconium, tantalum, and niobium are adsorbed and, like uranium, are removed by chromatography by eluting the column with HClHF mixtures of different strengths. By way of example, Figure 15.13 describes a procedure for the separation of protactinium from rock. Coprecipitation with hydroxide is used to separate radium, which remains in solution while the protactinium is precipitated. To separate Pa from thorium and uranium, anion exchange in hydrochloric acid is used: in a strong HCl, thorium passes through the anion exchange column, while U and Pa are adsorbed. Pa is removed by eluting the column with a mixture of hydrochloric and hydrofluoric acids; fluoride forms a complex with Pa and leads it out of the column, while U remains in the resin. After an additional purification by anion exchange in the same conditions, the activities of 231Pa and 233Pa are counted. The alpha activity of 231Pa is measured with a semiconductor detector and the beta activity of 233Pa with either a liquid scintillation counter or a proportional counter. A

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less laborious method is to determine both radioisotopes in the same sample with a liquid scintillation counter capable of alpha/beta discrimination.

15.13.4 Essentials of Protactinium Radiochemistry

. Protactinium has only two natural isotopes: 234Pa in the uranium series and 231Pa in the actinium series. The former is a short-lived (t1/2 6.75 h) beta emitter while the latter is an alpha emitter with a half-life of 32760 years. As a tracer in 231Pa analyses, a beta-emitting 233Pa (t1/2 27 d) is used.

. Most typically protactinium appears in oxidation state V and is readily hydro- lyzing even in strong acids, forming PaO3 in acid concentrations lower than 3M and further PaO(OH)2

, Pa(OH)5, and Pa(OH)6 as the acid concentrations

decreases (pHincreases). The chemistry of protactinium closely resembles that of group 5 metals V, Nb, and Ta.

. Solvent extraction, as well as ion exchange and extraction chromatographies are used to separate 231Pa. Coprecipitation as hydroxide is used for the enrichment and to remove nonhydrolyzable metals, such as radium. Pa absorbs in an anion

Figure 15.13 Determination of 231Pa in rock by liquid scintillation counting after separation by anion exchange (Saarinen, L. and Suksi, J. (1992) Determination of Uranium Series

Radionuclides Pa-231 and Ra-226 by Liquid Scintillation Counting, Report YJT-92-20, Nuclear Waste Commission of the Finnish Power Companies, Helsinki).

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exchange resin efficiently from 9 M HCl, in which uranium follows Pa into the resin phase while thorium, radium (if any is left after hydroxide precipitation), and actinium are not retained. Pa can be eluted from the column as a fluoride complex with 8 M HCl 0.3 M HF mixture while the uranium remains in the column.

15.14 Uranium

Uranium is the most important radioactive element and the most important subject in radioactivity studies, as it was the first element found to emit radiation and led to the discovery of the phenomenon of radioactivity. Uranium has three isotopes in nature, 234U, 235U, and 238U. The latter two each give rise to radioactive decay series in which there are twenty-five radioactive isotopes of thirteen elements in addition to the isotopes of uranium. Uranium is also highly significant both economically and socially. During the first decades of the twentieth century, its most important use was as a pigment for glass. After the fission of the uranium was discovered at the end of the 1930s, its huge energy was harnessed into weapons use as early as during the following decade. In the 1950s, the first energy-producing nuclear power plants were brought into use. These plants, as did nuclear weapons, made use of neutron-induced fission of the 235U isotope, which had been enriched from the natural uranium in the gas diffusion processes. In the use of both nuclear weapons and nuclear energy production, a large number of radioactive fission products and transuranics have been created some released into the environment as fallout and other emissions. Uranium is also important because plutonium is made fromit inreactors with the aid of neutron irradiation and the subsequent beta decays:

238Un; c239U !b

239Np !b

239Pu:

15.14.1 The Most Important Uranium Isotopes

Uranium has three isotopes in nature: 234U, 235U, and 238U, of which 235U starts the actinium decay chain and 238U the uranium decay chain (Tables 1.1 and 1.2 and Figures 15.1 and 15.2), while 234U belongs to the uranium chain. Of natural uranium, 99.28% is 238U and 0.72% 235U, while the mass fraction of 234U is very low. All these uranium isotopes decay through alpha emissions (Figure 15.14). 236U and 232U, which are also alpha emitters, are used as tracers for chemical yield determinations in uranium analyses. Of these two isotopes, 236U is also generated in nuclear fuel from 235U by neutron activation. The activity of 236U in the spent fuel is at the same level as that of 238U, but because of its much shorter half-life its mass is only less than a percent of thatof 238U. 233U is afurther important anthropogenic isotope of uranium, because it, as a fissile isotope, could be used as nuclear fuel in nuclear power reactors

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in addition to 235U and 239Pu. 233U can be produced by neutron irradiation from 232Th in a reactor through reaction:

232Thn; c233Tht1=2 22:3 min ! b

233Pat1=2 27:0 d ! b

233Ut1=2 1:59 105y 233U is also an alpha emitter with alpha energies of 5.789 MeV (13.2%) and 4.82 (84.4%) (Table 15.7).

Figure 15.14 Decay schemes of 234U, 235U, and 238U (Firestone, R.B., Shirley, V.S., Chu, S.Y.F., Baglin, C.M., and Zipkin, J. (1996) Table of Isotopes, Wiley-Interscience).

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Figure 15.15 shows the simulated alpha spectra of the five uranium isotopes. As can be seen, since the alpha peaks of 232U differ more than those of 236U from the peaks of the three natural uranium isotopes, it is more suitable for use as a tracer. The alpha peaks of 236U overlap, especially with those of 235U; however, since the 235U=238U ratio is constant in natural uranium, the contribution of 235U to the sum peak of 235U and 236U can be calculated from the 238U peak area and subtracted when natural uranium is being studied. However, when uranium isotopes are determined in irradiated uranium fuel, the presence of 236U also prevents it from being used as a tracer. Since the 235U=238U ratio no longer has a constant value. While there are many gamma emissions on all the isotopes, their intensities are low. An exception is 235U which has one gamma emission of 186 keV with high intensity (54%).

15.14.2 Occurrence of Uranium

The average amount of uranium in the lithosphere is 2.3 ppm, and it is distributed moderately evenly in the bedrock, there being some 200 known uranium minerals.

Figure 15.15 Simulated (a) and real (b) alpha spectra of uranium isotopes. In spectrum b no 236U is present.

Table 15.7 Half-lives of the uranium isotopes in nature and in spent nuclear fuel. Figures for average activities and masses are relative to 238U. Two tracer isotopes used in uranium analyses are also presented.

Nuclide Half life (y) Activity Mass

Natural isotopes 234U 2.46 105 1 5.5 105 235U 7.04 108 0.046 0.0073 238U 4.47 109 1 1 Isotopes found in irradiated uranium 233U 1.59 105 0.0002 in spent fuel 108 236U 2.34 107 1 in spent fuel 0.005 Tracer isotopes 232U 67 236U 2.34 107

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However, only a few uranium minerals are found as ores, of which uraninite, the ideal formula being UO2, is the most important. In uraninite, uranium is ideally in the oxidation state IV. In practice, part of the uranium in uraninite is partly oxidized; this mineral is called pitchblende (UO2 x), formally a mixture of UO2 and UO3. Another important ore mineral is carnotite, K2(UO2)2(VO4)2 3H2O, in which uranium has an oxidation state of VI. To be economically usable, the uranium concentration should be at least 0.1%, that is, 100 times the average content of uranium, although poorer concentrations than this have been exploited. Uranium ores were formed when oxic ground waters oxidized the uranium to the hexavalent uranyl ion, which is highly soluble as carbonate complexes UO2(CO3)2

2 and UO2(CO3)3

4. When uranyl-containing waters meet a redox front, where oxic conditions have changed to reducing anoxic conditions, uranium is reduced back to its tetravalent state and reprecipitated as UO2. The reducing conditions are mainly due to the presence of pyrite mineral (FeS) or organic material.

The concentrations of uranium in natural waters vary over a wide range. In seawater, the concentrations are moderately even at 24 mg L1. In other waters, however, concentrations vary greatly, from 0.1 mg L1 to 1 mg L1. The highest concentrations are found in groundwaters, where they can be even as high as milligrams per liter.

15.14.3 Chemistry of Uranium

The electron configuration of uranium is [Rn]5f36d7s2. Uranium can occur in solution in four oxidation states, III to VI (Figure 15.16), of which the oxidation states IV and VI are the most stable. Uranium in the higher oxidation states of V and VI does not occur as free U5 and U6 ions in solution; these hydrolyze because of their high charge, even in highly acidic solutions, and form the uranyl species UO2

and UO2 2 , which appear as linear OUO units. U3

only occurs in very reducing circumstances and oxidizes rapidly in aqueous solutions, releasing hydrogen from water. Pentavalent uranium (UO2

), in turn, only appears in a very narrow redox potential range. It can be produced even in millimolar concentrations in solution by reducing UO2

2 with hydrogen, for example. Pentavalent uranium is not thermodynamically stable but slowly oxidizes

Figure 15.16 Distribution of the oxidation states of uranium in 1 M perchloric acid.

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back to a hexavalent form. It also disproportionates into the tetravalent and hexavalent uranium as follows:

2UO2 4H KU4 UO2 2 2H2O Because trivalent uranium is not found in conventional chemical conditions and

because the stability range of pentavalent uranium is narrow and its concentrations are very low, these two uranium species are not discussed further in this book. The following will focus on the chemistry and analysis of the two prevailing forms U4

and UO2 2 . Of these, the latter is more stable, and, in solution, U4 oxidizes slowly

to UO2 2 . Although tetravalent uranium is more stable in acidic solutions, it oxidizes

in them to the hexavalent form, especially when complex-forming agents are present. Figure 15.17 shows the Eh pH diagram for uranium, with uranium species presented as a function of pH and redox potential. It reveals the high stability of hexavalent uranium, UO2

2 (and its hydrolysis species), which occurs overthe whole pH range in oxidizing conditions. In reducing conditions, however, the prevailing species is UO2, which is highly insoluble. The standard electrode potential of the uranium reduction reactions are as follows:

Figure 15.17 Eh pH diagram of uranium in 1010 M solution (Atlas of Eh-pH diagrams, Geological Survey of Japan Open File Report No. 419, 2005).

UO2 2 e ! UO2 E0 0:163 V UO2 4H e ! U4 2H2O E0 0:273 V U4 e ! U3 E0 0:520 V

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The first and the last reactions are fast since there is only an electron transfer involved, while the middle reaction is slow since it also requires breaking the covalent bond between uranium and oxygen.

15.14.4 Hydrolysis of Uranium

Tetravalent uranium U(IV) hydrolyzes strongly. In a strongly acidic solution, it occurs as U4 ion but already starts to hydrolyze when the acid concentration decreases below 0.5 M. At even lower acid concentrations, the mono-, di- and trihydroxide species U(OH)3 , U(OH)2

2 , U(OH)3 are formed; and at pH above 4, the

prevailing species is the neutral soluble U(OH)4(aq) complex. If the uranium concentration is high enough, precipitation of U(OH)4/UO2 takes place, the solu- bility minimum being at pH about 5. In the alkaline region, however, uranium does not form anionic hydroxide species such as U(OH)5

. Hexavalent uranium also hydrolyzes readily, but not as strongly as tetravalent

uranium. In10 mM solutionhydrolysis starts atpH 3,when cationic UO2OH andits

dimeric form (UO2)2(OH)2 2 are formed (Figure 15.18). When the pH increases to

56, the prevailing species will be (UO2)3O(OH)3 . It was previously assumed to be

(UO2)3(OH)5 and is presented as such in Figure 15.18.

15.14.5 Formation of Uranium Complexes

Uranium is a strong Lewis acid and thus readily accepts electrons from various ligands. It therefore forms strong complexes with fluoride and ligands that contain free electron pairs of oxygen and nitrogen. The stabilities of halide complexes

Figure 15.18 Hydrolysis of hexavalent uranium in 105 M solution. a: UO2 , b: UO2OH

, c: (UO2)2(OH)2

2 , d: (UO2)3O(OH)3 . (Baes, C.F. and Messmer, R.E. (1976) The Hydrolysis of

Cations, John Wiley & Sons).

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decrease in the order of their increasing ionic size: F >> Cl > Br > I. While the fluoride complex is strong, complexes with other halides are rather weak. The complexes with oxoanions weaken in the order PO4

3 > CO3

2 > SO4

2 > NO3

, which shows that the higher the ligands valence and the stronger its basicity the stronger are the complexes.

The most important uranium complexes in natural waters are formed with carbonate. In acidic waters (pH < 6), uranium forms complexes, especially with sulfate, phosphate, and fluoride ions; however, in neutral and alkaline solutions, uranium occurs practically solely as carbonate complexes. Figure 15.19 shows the uranium species in solution as a function of pH when carbon dioxide is present at a partial pressure of 102 bar, a typical value for deep groundwaters. As can be seen, at pH 6 the prevailing species is UO2CO3, and, at higher pH, di- and tricarbonate complexes appear the former at pH 6 and the latter at pH 7. The presence of uranium as anionic carbonate complexes makes uranium very soluble and mobile in groundwaters. Carbonate complexation increases the solubility of uranium minerals considerably by two to three orders of magnitude. In surface waters, where the carbon dioxide partial pressure is lower at 103.5, hydrolysis competes with carbonate complexation to some extent; however, even in these circumstances, carbonate complexes are the predominant species at pH above 6.5.

With organic molecules, uranium forms numerous complexes which are impor- tant both in the analytical and industrial separation processes of uranium. The most notable example is tributylphosphate (TBP (CH3CH2CH2CH2O)3PO), which forms complexes with tetra and hexavalent actinides but not with penta and trivalent actinides. TBP is widely utilized in the reprocessing of spent uranium fuel. The fuel is first dissolved in strong nitric acid, after which the uranium is extracted with TBP as the neutral UO2(NO3)2(TBP)2 complex. Only tetravalent plutonium follows uranium into the organic phase, while most of the other actinides and fission products remain in the aqueous phase.

Figure 15.19 Carbonate species of UO2 2 as a function of pH at the carbon dioxide partial

pressure of 102 bar. Uranium concentration 106 M. (Langmuir, D. (1997) Aqueous Environmental Geochemistry, Prentice-Hall, Inc.).

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15.14.6 Uranium Oxides

The most common uranium oxide is UO2, in which the oxidation state of uranium is IV. As a mineral it is known as uraninite, but this is seldom found as pure UO2, as described earlier. Although UO2 is very stable over a wide pH range in reducing conditions, as the conditions become more oxidizing, it transforms into uranium oxides with higher oxygen content. The following stoichiometric uranium oxides, in addition to UO2, are U4O9, U3O7, U3O8, and UO3, which are listed in the order of increasing oxidation state. In the first three, uranium occurs formally in the two oxidation states IV and VI, which can also be presented as mean values of 4.5, 4.67, and 5.33. UO3, in which the oxidation state of uranium is VI, is the most oxidized uranium oxide. As a mineral it is known as schoepite. It is very soluble, forming uranyl ions UO2

2 in aqueous solution, and thus it is a rare mineral, found only in oxic places with no water. The intermediate oxides between uraninite and schoepite (U4O9, U3O7, U3O8) are stable in the pH range typical for natural waters 48; however, the Eh range where they are stable is rather narrow, at about 200 mV.

15.14.7 From Ore to Uranium Fuel

Uranium ore is first ground to a fine powder, and from it the lighter rock, called gangue, is removed by flotation, for example. The ground ore is then roasted at a high temperature to oxidize the uranium and thus enhance its solubility. The uranium is then leached from the ore by either sodium carbonate or sulfuric acid. To further oxidize the uranium, either manganese oxide (MnO2) or sodium chlorate (NaClO3) is added to the mixture. In sulfuric acid leaching, the UO2(SO4)3

4 species is mainly formed, while in carbonate leaching UO2(CO3)3

4 is formed. Carbonate extracts uranium much more selectively than sulfuric acid. Sulfuric acid leaching is also used as an in-situ method: sulfuric acid is pumped into holes bored in the bedrock and the uranium dissolved by the acid is extracted from adjacent holes.

The sulfate and the carbonate leachates are dried, and a crude uranium product, yellow cake, is formed, which contains mainly U3O8 as well as some UO2 and UO2SO4. Yellow cake also contains many impurities. The crude product is dissolved in strong nitric acid, after which the uranium is purified by solvent extraction using TBP, with which uranium forms the complex UO2(NO3)2(TBP)2. The uranium is stripped back to the aqueous phase with water. The extraction is very selective, producing pure uranium except for very small amounts of thorium, which follow it to the end product. The aqueous uranium solution is dried and calcined to UO3 before being reduced to UO2 with the help of hydrogen.

UO2 obtained in the above process can be used as a fuel in reactors that use unenriched (natural) uranium. Most reactors, however, use enriched UO2 as a fuel, in which the fraction of 235U has been increased from its naturally occurring value of 0.72% to about 3%. The enrichment is performed by transforming uranium with the help of hydrogen fluoride into the gaseous UF6, which is then conducted through

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several consecutive gas centrifuge units. The separation of 235U from 238U takes place because of their small mass difference. Finally, the uranium is converted to UO2, and the end product of uranium dioxide enriched with respect to its 235U content is obtained as fuel for nuclear reactors. In addition, as a side product, the process produces depleted uranium, in which the fraction of 235U is lower than its natural value of 0.72%.

15.14.8 Measurement of Uranium

If the concentration of uranium is high enough, its concentration can be measured by conventional methods such as titration, gravimetry, X-ray fluorescence, and spectrophotometry. These methods are used when larger amounts of uranium are being treated, for example, in uranium fuel-producing industries. These methods, however, are not discussed further since the focus of this book is on radionuclides in the environment and nuclear waste, where the levels of uranium are usually so low that more sensitive methods are needed. For environmental and waste samples, radiometric and mass spectrometric methods are normally used.

Since all uranium isotopes are alpha emitters, alpha spectrometry is an obvious choice for measuring them. The alpha energies of the natural uranium isotopes differ from each other so much that they can all be detected from the same sample (Figure 15.15). The detection of 236U, however, is impossible, since its peaks overlap with those of 235U. For the alpha measurement, the uranium is either electrode- posited on a stainless steel plate or microcoprecipitated with lanthanide fluoride on a membrane. While the latter is somewhat more straightforward, the former results in a better peak resolution.

Gamma spectrometry can be used to a limited extent to measure 238U and 235U. The intensities of 238U gamma rays are very low one percent and below; however, the gamma rays of its short-lived progeny nuclides 234Th (63.3 keVand 92.5 keV) and 234mPa (1001 keV) can be used in some cases to measure 238U, provided there is an equilibrium between them. 235U emits several gamma rays with fairly high inten- sities, the 186 keV gamma ray being the most intense at 54%. 226Ra has a gamma emission at the same energy and therefore needs either to be separated or its contribution to the gamma intensity at this energy corrected by calculation. The latter method requires that there should be equilibrium between the uranium and radium. While liquid scintillation counting can also be used to measure uranium, because of its poor energy resolution it is more suitable for measuring the total uranium content from radiochemically separated samples.

Mass spectrometry is the most sensitive method for the measurement of uranium, especially when determining its isotopic ratios. The most accurate method to measure the isotopic ratio is thermal ionization mass spectrometry (TIMS); however, preparing the sample is most laborious and only one element can measured at a time. ICP-MS with a quadrupole mass analyzer is suitable for the measurement of the major isotopes 235U and 238U, while high-resolution ICP-MS with double-focusing

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mass analyzers is needed for the accurate determination of the minor uranium isotopes 234U and 236U. High-resolution ICP-MS is also the most sensitive method for uranium detection: a detection limit as low as 1 pg in a 1 mL sample can be obtained.

15.14.9 Reasons for Determining Uranium Isotopes

There are several reasons to determine uranium and its isotopic ratios. The choice of the analytical method very much depends on the type and amount of uranium in the sample. Typically, uranium analyses are carried out for the purposes described below.

Ratios of 234U to 238U, as well as of uranium to its progeny isotopes, are used to evaluate geological processes. In general, equilibrium prevails between 238U and its progeny isotopes; however, during certain geological processes, such as the mineralization of uranium by the reduction of uranyl ions, this equilibrium may become disturbed. Determining the isotope ratios provides data relevant to the time when this disturbance occurred.

Studies of uranium behavior in uranium deposits are used as analogs for the final disposal of spent nuclear fuel from nuclear power plants. Other important research topics in this field include the dissolution, reprocessing, and final disposal of spent fuel.

Uranium, thorium, and their progeny are responsible for some 5% of the total radiation dose to humans because of the consumption of food and drinking water. The levels of these radionuclides are usually quite stable; in some cases, however, they are elevated and cause higher doses, for example, in wells drilled in the bedrock where the concentrations of uranium can be very high, even as high as the milligrams per liter range.

As an environmental pollutant, uranium is not typically a problem because of its low specific activity compared to that of most transuranium elements. Studies of environmental uranium from nuclear weapons tests fallout and from accidental or intentional releases are thus not as extensive as those of plutonium, for example. Studies on uranium particles, however, have been carried out from nuclear weapons tests and from Chernobyl and other accidents. The main environmental impacts of uranium, and especially its progeny, have been created by mining it by conventional methods and by in-situ leaching, which have mobilized uranium both in surface waters and groundwaters. In these studies, the main emphasis has been put on the levels of uranium, its solubility, and its chemical forms.

The latest concerns in uranium studies relate to nuclear forensics, nuclear trafficking, and undeclared nuclear activities violating the nonproliferation of nuclear weapons. The origin of illegal nuclear material caught by customs, for example, has been studied by mass spectrometry. A major tool for the detection of undeclared nuclear activities has been air sampling to collect uranium- and plutonium-bearing particles from the air. The origin and intended purpose of these materials can be determined by the uranium and plutonium isotope ratios in the particles.

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15.14.10 Separation of Uranium

Radiochemical separation methods for the determination of uranium are based on three main methods: ion exchange, solvent extraction, and extraction chromatogra- phy, which are usually optional but also supplement each other. A major challenge in the separation of uranium for the radiometric measurement, mostly done by alpha spectrometry, is to remove other alpha-emitting radionuclides, 210Po, 226Ra, thorium isotopes, plutonium isotopes, and 241Am. If studies are carried out with samples that contain no artificial radionuclides, such as rocks, there is no need to remove the transuranium isotopes. Various methods are used to separate uranium, the most important of which are presented below.

15.14.10.1 Separation of Uranium from Other Naturally Occurring Alpha-Emitting Radionuclides As shown in Figure 4.7, uranium, in a solution obtained by leaching a rock sample with acid, can be separated from other natural alpha-emitting radionuclides by ion exchange using a strongly basic anion exchange resin. In this type of sample, there are no transuranium elements requiring separation. In this ion exchange method, uranium, as an anionic complex (UO2Cl4

2) in 9 M HCl is retained in an anion exchange resin bed. Since thorium and radium do not form chloride complexes, they pass the column in the effluent. In addition to uranium, 210Po is also retained in the column as the PoCl6

2 complex. When the uranium is eluted out of the column by 0.1 M HCl, which decomposes the anionic uranyl complex, polonium still remains in the column. The uranium can thus be removed from interfering radionuclides in a single step.

15.14.10.2 Determination of Chemical forms of Uranium in Groundwater The same anion exchange method, used above for the rock sample, can be applied to determine uranium and its chemical forms in groundwaters that do not contain transuranium elements. The possible physical and chemical forms of uranium in groundwaters are hexavalent uranyl ion, usually present as anionic carbonate complexes UO2(CO3)x

22x, tetravalent uranium as UO2(aq), and uranium attached to particle matter. In most cases, the first is the prevailing form. These three forms can be distinguished by using filtration, anion exchange in 9 M HCl, and precipi- tation with lanthanide fluoride. Filtration is used to remove particles the pore size of the filter determining the minimum size of the particles. The filter is digested in concentrated acids and the amount of uranium in it is measured after the anion exchange separation (see above) to get the fraction of uranium associated with particles. To separate tetravalent and hexavalent uranium from each other in the filtrate, lanthanide fluoride precipitation is carried out LnF3 coprecipitates tetra- valent but not hexavalent uranium. The precipitate is collected on a filter membrane and dissolved along with the membrane in aqua regia (a mixture of concentrated HCl and HNO3), evaporated to dryness, and dissolved in 9 M HCl to separate the uranium by anion exchange to obtain the fraction of uranium in the tetravalent state. The

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supernatant from lanthanide fluoride precipitation is evaporated to dryness and dissolved in 9 M HCl, and the uranium is separated again by anion exchange to get the uranium fraction in the hexavalent state.

15.14.10.3 Separation of Uranium from Transuranium Elements by Anion Exchange or by Extraction Chromatography Several environmental samples also contain transuranium elements, especially plutoniumandamericium,whichneedtobeseparatedpriortothealphameasurement of uranium. These samples include soil, sediment, surface water, vegetation, animals, and aerosol particles collected by air filtration. In addition, transuranium elements coexist with uranium in many nuclear waste samples. Transuranium elements can be removedfromuraniumbybothanionexchangeandextractionchromatographyusing the same methodology, plutonium being reduced to the trivalent state. This (like americium) is not retained in the anion exchanger or in the used extraction chroma- tography column. Solid samples (soil, sediment, precipitate from enrichment of actinides from water, etc.) are first digested in concentrated acids and evaporated to dryness. The residue is dissolved in 9 M HCl for the anion exchange separation and in 3 M HNO3 for the extraction by chromatographic separation using a UTEVA column; yield determinant tracers are then added for uranium and other radionuclides of interest. Plutonium is then reduced to the trivalent state by adding ferrous sulfamate and ascorbic acid. The 9 M HCl solution is poured into an anion exchange column preconditioned with 9 M HCl, and the 3 M HNO3 is poured into a UTEVA column preconditionedwith 3 M HNO3.Inboth cases,theuranium remainsinthecolumn,as an anionic UO2Cl4

2 complex in the anion exchange column and as a neutral UO2(NO3)2 complex in the UTEVA column. The trivalent plutonium and americium, in turn, pass the columns without retention (see Figures 4.6 and 4.12). The whole procedure for the separation of U, Pu, and Am from air filters is represented in Figure 4.13.

15.14.10.4 Separation of Uranium by Solvent Extraction with Tributylphosphate (TBP) As mentioned, tibutylphosphate (TBP) has been used in analytical and industrial processes for the separation of uranium. In the analytical separation, the sample is prepared in 8 M HNO3 and plutonium is reduced to the trivalent state to prevent its extraction along with uranium. Only the thorium follows uranium into the organic phase when extracted with TBP. The thorium can be removed from the uranium by back extraction with 1.5 M HCl, while the uranium remains in the organic phase. Finally, the uranium is stripped into the aqueous phase by water.

15.14.11 Essentials of Uranium Radiochemistry

. Uranium is the most important radioactive element and has three isotopes in nature: 234U, 235U, and 238U. The first isotope belongs to the decay chain beginning with 238U, while the last two are parents in two natural decay chains.

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In irradiated uranium, 236U is also present, which is formed from 235U by neutron activation. All these uranium isotopes decay by alpha emission.

. Uranium has been widely mined for both nuclear fuel and weapons material. The natural abundance of uranium is 2.3 ppm, and the most important uranium ore mineral is uraninite with an ideal formula of UO2. In natural uranium, the abundance of the fissile isotope 235U is 0.72%, while that of 238U is 99.28%. To be used as nuclear fuel, 235U is enriched to about 3% in gas centrifuge facilities; for weapons material, its enrichment percentage is above 90%. Uranium is also a source of 239Pu, another fuel and weapons material formed by neutron irradiation from 238U in nuclear reactors.

. Uranium has four oxidation states between III and VI, of which IV and VI are the most predominant. Trivalent uranium can only be found in very reducing conditions in acidic solutions. The pentavalent state only occurs in small proportions over a limited redox potential range and readily disproportionates to the tetra- and hexavalent states. Tetravalent uranium occurs as the U4 ion in acidic solutions, while hexavalent uranium occurs as the uranyl ion, UO2

2 . . Tetravalent uranium readily hydrolyzes, forming U(OH)4, even in slightly acidic

solutions. Hexavalent uranium hydrolyzes less readily and forms UO2OH and

(UO2)2(OH)2 2 at pH above 4 and (UO2)3O(OH)3

at higher pH values. . Uranium forms the following stoichiometric oxides: UO2, U4O9, U3O7, U3O8,

and UO3, in which the oxidation state gradually increases from IV with UO2 to VI with UO3.

. The stabilities of complexes with halides decrease in the order: F >> Cl > Br

> I; and with common oxoanions in the order: PO4 3 > CO3

2 > SO4 2 >

NO3 . In natural waters of pH 6 and above, uranium typically occurs as carbonate

complexes UO2(CO3)2 2 and UO2(CO3)3

4, which makes uranium very soluble and mobile in natural waters.

. Uranium isotopes can be measured by both alpha spectrometry and mass spectrometry (TIMS and ICP-MS), while LSC is only used to measure total uranium concentrations. High-resolution ICP-MS and TIMS are the most sensitive methods with which to measure uranium isotopes.

. If alpha spectrometry is used to measure uranium, the key factor is to separate it from the other alpha-emitting radionuclides, 210Po, 226Ra, and 241Am, as well as thorium and plutonium isotopes.

. If samples that have no transuranium elements present are studied, a single anion exchange step in 9 M HCl is needed to separate uranium as anionic UO2Cl4

from 226Ra and thorium isotopes, which do not form chloride complexes and are thus not retained in the column. Uranium is eluted from the column with dilute HCl solution but polonium still remains.

. ToseparateuraniumfromthetransuraniumelementsPuandAm,anionexchange and extraction chromatographies as well as solvent extraction can be utilized. For the separation, plutoniumis reduced to Pu3 , in which oxidation state americium is initially inthe samples. Unlikeuranium, trivalent Pu and Am are not retained in ananionexchangecolumnin9 M HCl,inaUTEVAResincolumnin3 MHNO3,or in tributyl phosphate, and are thus removed from the uranium.

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. The separation of oxidation states in solutions can be accomplished by copreci- pitation with lanthanide fluorides (LnF3), which coprecipitate tetravalent but not hexavalent uranium.

15.15 Neptunium

15.15.1 Sources of Neptunium

Neptunium is the first transuranium element. The most important long-lived isotope of neptunium is 237Np, with a half-life of 2.14 106 years. Since the half-life is three orders of magnitude shorter than the Earths age, all primordial neptunium decayed long ago and therefore no decay chain, such as shown in Tables 1.11.3, originating from this nuclide exists. Though 237Np is naturally found in minute amounts in uranium ores it can be considered purely an artificial element. The main source of neptunium is the nuclear power industry, and it is formed by neutron activation and through beta decays from uranium in nuclear fuel in the following reactions:

235Un; c236Un; c237U ! b 237Np

238Un; 2n237U ! b 237Np Since the 237U isotope is short-lived (t1/2 6.8 days), the formation of 237Np is

rapid. In spent fuel, the activity fraction of 237Np is quite small. The activity of 237Np is only one thousandth of that of the plutonium isotopes 239Pu and 240Pu, for example, mainly because of its considerably longer half-life. After a few hundred thousand years it will be the prevailing transuranic element in spent fuel. In spent nuclear fuel reprocessing, where plutonium and uranium are separated for further use, neptu- nium goes to the high-activity waste fraction for final disposal. Concern arises about the behavior of neptunium in final disposal not only because of its long half-life, but also because it is fairly mobile in oxic conditions as the neptunyl ion NpO2

. 237Np is also formed in nuclear explosions, with a total of about 2500 kg having

been released to the environment. This is approximately the same amount as that of plutonium isotopes, but because of the longer half-life of 237Np its activity in the environment is a hundred to a thousand times lower. The total environmental release of 237Np from the Chernobyl accident was about five thousand times lower than that from weapons tests fallout. Another source of environmental 237Np is in the release from nuclear fuel reprocessing plants, especially La Hague in France and Sellafield in the UK, together resulting in increased 237Np levels in the nearby seas by a factor of as much as one thousand compared to the levels caused by the nuclear weapons tests fallout (1015 to 1014 g L1). 237Np is also formed by the alpha decay of 241Am, and therefore its activity in nuclear fuel and the environment is increasing. In conclusion, the levels of 237Np are highonly in spent nuclear fuel. In the environment, inturn, the

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activity levels are very low compared to other actinides (Th, U, Pu, Am), and therefore the determination of 237Np from environmental samples requires a high separation efficiency and a sensitive measurement technique.

15.15.2 Nuclear Characteristics and Measurement of 237Np

237Np decays by alpha emission to 233Pa. The main alpha particles have energies of 4.788 MeV (intensity 51%) and 4.770 MeV (19%). Because of the proximity of the energies of these two emissions, alpha spectrometry cannot distinguish between them and only gives a sum peak. 237Np also emits 29.4 keV gamma rays with an intensity of 15%, but these gamma rays cannot be used for direct measurement from environmental and waste samples because of their low energy and especially the low activity of 237Np compared to other coexisting radionuclides. Therefore, 237Np needs to be separated from samples for measurement by alpha spectrometry, for which detection limits around 0.1 mBq are achieved. Much lower detection limits than those achieved by alpha spectrometry can be obtained by neutron activation analysis, in which 237Np is activated by neutrons to form short-lived (t1/2 2.1 days) gamma- emitting 238Np. Neutron activation analysis requires separation of neptunium both before and after irradiation. Even lower detection limits, clearly below microbe- querels, can be obtained by mass spectrometry, especially with high-resolution ICP- MS, which has become a favored method for measuring 237Np. A critical factor in ICP-MS measurement is the removal of the neighbor isotope 238U from the samples to avoid downmass tailings to the mass peak of 237Np. A description of 237Np measurement with ICP-MSis given in Chapter 17. The most sensitive method for the measurement of 237Np is AMS, but its use is hindered by the limited accessibility and high costs.

15.15.3 Chemistry of Neptunium

The electron configuration of neptunium is [Rn]5f46d17s2. It can occur in all oxidation states between III and VII. Of these the oxidation states IV and V are the most important and only these are discussed. Figure 15.20 gives the Eh-pH diagram for neptunium. In oxidizing conditions the prevailing form is the pentavalent dioxocation NpO2

, called the neptunyl ion. The neptunyl ion is fairly soluble, and it is the form in which neptunium typically occurs in oxic natural waters. As the redox potential decreases, neptunium is reduced at a rather high Eh value to tetravalent neptunium Np4 . Both tetravalent and pentavalent neptunium hydro- lyze, the former much more strongly than the latter. Hydrolysis of Np4 starts at the low pH of about 1, while that of NpO2

only at a pH above 7. At higher neptunium concentrations, both form hydrous oxides, NpO2xH2O and NpO2OH. The former is highly insoluble and is the solubility-limiting solid phase occurring in final disposal conditions for spent nuclear fuel, that is, NpO2xH2O is formed in these conditions if

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the neptunium concentration is high enough. Heating the hydrous tetravalent oxide results in the formation of anhydrous NpO2, which is a very stable insoluble compound. Pentavalent neptunium also forms anhydrous Np2O5, but only under special conditions.

Pentavalent neptunium disproportionates in acidic solutions to tetravalent and hexavalent forms in the following way:

2NpO2 4H KNp4 NpO2 2 H2O

It is obvious from the equation that by increasing neptunium concentration and acidity the disproportionation reaction is promoted. The equilibrium constant of the reaction is fairly small, but the formation of complexes with Np4 and NpO2

promotes the reaction to the right. For example, in acids, such as sulfuric acid, both tetravalent and hexavalent states form complexes with sulfate, driving the reaction to the right.

Because of its much higher ionic potential, tetravalent neptunium forms consid- erably stronger complexes than the pentavalent form, as is also the case in hydrolysis. The stabilities of the Np4 complexes with monovalent ligands, decreases in the following order:

F > H2PO 4 > SCN

> NO3 > Cl > ClO4

Figure 15.20 Eh-pH diagram of neptunium (Firestone, R.B., Shirley, V.S., Chu, S.Y.F., Baglin, C.M., and Zipkin, J. (1996) Table of Isotopes, Wiley-Interscience).

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and with divalent ligands, which form stronger complexes than monovalent ligands, in the following order:

CO23 > HPO 2 4 > SO

2 4 :

15.15.4 Separation of 237Np

In the separation of neptunium from the sample matrix, ion exchange, solvent extraction, and extraction chromatography have been widely used. Precipitations are not typically used as major separation steps in neptunium analyses. As for other actinides, coprecipitations with Fe(OH)2, Fe(OH)3, and MnO2 are used to precon- centrate neptunium from large water volumes. For the coprecipitation, neptunium is reduced in a slightly acidic solution to Np(IV). Another precipitant used in neptunium analyses is fluoride, which precipitates Np(IV) but not Np(V). It is also used to prepare sources for alpha counting after reduction of all the neptunium to Np(IV).

The major challenge in most neptunium analyses is uranium removal, which is in great excess compared to neptunium. 238U has alpha emissions in the same region as 237Np and, therefore, needs to be completely removed. This is even more important in ICP-MS measurement, since excessive 238U causes downmass tailings to the 237Np peak. Thorium, plutonium, and americium do not have major alpha peaks in the energy region of the alpha emissions of 237Np, but they too are in great excess to neptunium and cause interference; hence removal is necessary. Removal of neptu- nium from these most important interferences makes use of the different behavior of neptunium, as well as the interfering actinides, in various oxidation states. Amer- icium exists solely in oxidation III and plutonium can also be converted to oxidation III; consequently they are removed from neptunium in this oxidation state since it does not form oxidation state IIIin the same conditions. Separation of uranium is typically carried out in redox conditions where neptunium is in a IV state, while uranium is in a VI state. This requires reduction of neptunium to Np (IV), leaving UO2

2 unreduced (see Figures 15.7 and 15.20). Thus, a major challenge in 237Np analyses is a careful adjustment of the oxidation state of neptunium. Typically, reduction from a pentavalent state to a tetravalent state is done by addition of ferrous iron or iodide ions, hydrazine hydrochloride, sulfite, or sulfamate. Thorium is typically separated from neptunium based on differences in forming chloride and nitrate complexes in HCl and HNO3 media. Both Np and Th form anionic nitrate complexes and adsorb on anion exchange resins, from which thorium is eluted with hydrochloric acid while neptunium remains.

15.15.4.1 Neptunium Tracers for Yield Determinations In 237Np separations, 239Np is typically used as a tracer. It is a short-lived (t1/2 2.4 days) neptunium isotope that can be obtained by separation from its parent nuclide 243Am in approximately one week intervals. 239Np decays by emitting beta particles of 330 keV (40.5%) and 436 keV (45.3%) energies accompanied by gamma rays of

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106.1 keV (27.2%) and 277.6 keV (14.4%). Measurement of 239Np is easily carried out by gamma spectrometry. Figure 15.21 shows a separation procedure of 239Np from its parent 241Am using anion exchange chromatography. Neodymium is first added as a carrier, and neptunium is reduced to Np(IV) with iodide ions. Americium is then removed by two-step anion exchange chromatography in 9 M HCl; neptunium is retained as an NpCl6

2 complex, while americium passes through. Americium in the effluent from the first ion exchange step is recycled for further milkingof 239Np after sufficient ingrowth time. The neptunium is eluted from the column with a dilute mixture of HNO3 and HFand purified in a second ion exchange step. Addition of HF enhances the elution of Np by forming a complex with neptunium that is not retained in the anion exchange resin.

236Np (t1/2 1.54 105 y) is another potential tracer for 237Np determinations. It decays through beta emission (12.5%), electron capture (87.3%), and alpha emission (0.16%). Because of its long half-life and very small alpha emission intensity, 236Np is notanidealisotopictracerfor 237Np determinationwhenusingalphaspectrometryfor the measurement of 237Np. It is a good tracer for mass spectrometric measurement, however, since both 237Np and 236Np can be measured simultaneously. 236Np can be produced by the neutron capture reaction of 237Np in a nuclear reactor, through 237Npn; 2n236;236mNp, but a problem is that the produced 236Np cannot be separated fromthetargetisotope237Np.Inaddition,theproductionrateof236Np islowerthanthat

Figure 15.21 Preparation of an 239Np tracer by anion exchange chromatography from 243Am (La Rosa, J., Gastaud, J., Lagan, L., Lee, S.-H., Levy-Palomo, I., Povinec, P.P., and Wyse, E.

(2005) Recent developments in the analysis of transuranics (Np, Pu, Am) in seawater. J. Radioanal. Nucl. Chem., 263, 427).

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ofits isomer 236mNp. 236Np can bealsoproducedfromuranium in a cyclotronthrough reactions 238Up; 3n236Np and 235Ud; n236Np, but 237Np is also formed in the irradiationbythereaction 238U(p,2n)237Np.Toobtainthedesiredreaction,selectionof the proper projectile energy is an essential issue. The produced Np can be separated from the uranium target by dissolution and ion exchange or extraction chromatogra- phy. Because of the very long half-life of 236Np, its production rate is very low, making production of a large amount difficult and expensive. Therefore, 236Np is not easily available to most researchers, and no commercial 236Np is presently available.

15.15.4.2 Preconcentration of Neptunium from Large Water Volumes Neptunium can be preconcentrated with Pu and Am from large water volumes using the same method as that for Pu. The water is first acidified to pH 12, then tracers are added and Np and Pu are reduced to Np(IV) and Pu(III) by using NaHSO3 after addition of iron as FeCl3, which reduces to Fe(II) by the action of the sulfite. NaOH or NH4OH solution is then added to adjust the pH to 910 to coprecipitate Np and Pu with Fe(OH)2 and separate it from the water. The precipitate is separated and dissolved with HNO3 for further separation of Np and Pu from other interferences and from each other.

15.15.4.3 Separation of 237Np by Extraction Chromatography Extraction chromatography allows a rather straightforward separation of neptunium from other actinides (Figure 15.22). Eichrom/Triskem TEVA Resin efficiently takes uptetravalentactinides,butnothexavalentortrivalentactinides(Figure4.13).Thus,if

Figure 15.22 Separation of 237Np from other actinides by extraction chromatography using Eichrom/Triskem TEVA Resin (Maxwell, S.L. III (1997) Rapid actinide-separation methods. Radioact. Radiochem. 8, 36).

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neptunium is adjusted to a tetravalent state by ferrous sulfamate, for example, it is very efficiently retained as an Np(NO3)6

2 complex in a TEVA column in 2.5 M HNO3. Sulfamate reduces plutonium to a trivalent state, which passes through the column with americium. Uranium, initially as uranyl ions, is not reduced by sulfamate and therefore not retained in the column. Thus, only neptunium and thorium remain in the column. Thorium can be removed by changing the eluant to 6 M HCl, and, since thorium does not form a chloride complex, it elutes out. Tetravalent neptunium forms a NpCl6

2 complex and remains in the column. Finally, neptunium is eluted from the column with dilute HNO3 for measurement by alpha spectrometry or by ICP-MS.

15.15.4.4 Separation of 237Np by Anion Exchange Chromatography Anion exchange can be used to separate 237Np in both HNO3 and HCl media, where tetravalent neptunium forms anionic complexes Np(NO3)6

2 and NpCl6 2.

For both media, neptunium is first reduced to Np(IV) with ferrous sulfamate, for example. This reduces all plutonium species to Pu(III). In HNO3, the solution is loaded into an anion exchange column at an acid concentration of 78 M (the nitrite ions present in concentrated nitric acid oxidize Pu(III) to Pu(IV)). Np(IV), Th(IV), and Pu(IV) are retained in the column, while Am(III) and U(VI) pass through and are completely removed by rinsing the column with 8 M HNO3. Next, thorium is eluted with 8 M HCl, Th(IV) does not form a strong anionic complex with chloride. The Pu in the column is then reduced with hydrazine hydrochloride in 8 M HCl to Pu(III) and then eluted out. Finally, neptunium is eluted out with dilute nitric acid. To carry out ion exchange in HCl media, neptunium is first reduced to Np(IV) and

plutonium to Pu(III), and the solution is introduced into the column at an acid concentration of 9 M, where Th(IV), Pu(III), and Am(III) are not retained in the column. U(VI) is then rinsed out with 8 M HNO3, and finally Np(IV) is eluted with 0.5 M HCl.

15.15.4.5 Separation of 237Np by Solvent Extraction Many solvent extraction agents have been used in the separations of neptunium. Of these, the most widely used agent is thenoyltriacetone (TTA). For extraction, neptunium is reduced to Np(IV) and the coexisting plutonium to Pu(III). Then neptunium is extracted into TTA in toluene in 1 M HNO3. In these conditions, Pu (III) and U(VI) remain in the aqueous phase. Neptunium is back-extracted in either 8 M HNO3 or 1 M HNO3 after oxidation to Np(V). If solid samples, such as soil and sediment, containing large amounts of iron, are

analyzed for neptunium, the interfering iron can be removed by solvent extraction with isopropyl ether or with 4-methyl-2-pentanone (MIBK).

15.15.5 Essentials of Neptunium Radiochemistry

. Neptunium, the first transuranium element, has only one important isotope, 237Np, an alpha emitter with a half-life of 2.14 106 years. 239Np (t1/2 2.4 d) is

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used as a tracer in neptunium analyses. It can be obtained through separation from its parent, 243Am.

. 237Np can be measured by alpha spectrometry, but because of very low activity in environmental samples the measuring times are long. Activation analysis and especially high-resolution ICP-MS both offer shorter measurement times and lower detection limits.

. The most prevalent oxidation state of neptunium is V, existing as neptunyl ion, NpO2

, in aqueous solutions. It is fairly easily reduced to the tetravalent Np4

ion. The neptunyl ion is rather soluble and does not readily hydrolyze, while the tetravalent form hydrolyzes easily and forms the very insoluble hydrous oxide NpO2xH2O. NpO2 also disproportionates in acidic solutions to form Np4 and NpO2

2 . . For the separation of 237Np, neptunium is typically first reduced to the tetravalent

state. Preconcentration of neptunium is achieved by coprecipitation with Fe (OH)2, Fe(OH)3, or MnO2.

. The major task in 237Np analyses is uranium removal, since 238U has alpha emissions at the same energies as 237Np. Removal of uranium is also important in ICP-MS measurements, since excessive 238U causes downmass tailings to the 237Np peak. UO2

2 can be separated from Np4 by anion exchange and extraction chromatography as well as by solvent extraction. In 8 M HNO3, Np

is retained in an anion exchange column while UO2 2 passes through. The

same separation occurs in 2.5 M HNO3 in a TEVA column. In addition, neptunium can be extracted with TTA in 1 M HNO3, while UO2

2 remains in the aqueous phase.

. Thorium can be removed by absorbing Np4 and Th4 from nitric acid in an anion exchange or a TEVA column and then eluting Th4 with hydrochloric acid, leaving neptunium in the column.

. Plutonium can be removed from Np4 by reducing it to Pu(III), which absorbs in neither TEVA nor anion exchange columns nor is extracted with TTA. Americium, as a trivalent ion initially, follows plutonium in these treatments.

15.16 Plutonium

Plutonium is a fundamental element in the nuclear industry, employed in both nuclear weapons and nuclear power production. As a consequence, it is of essential concern in the investigation of environmental radioactivity and in waste treatment.

15.16.1 Isotopes of Plutonium

Plutonium is an anthropogenic element, though minute amounts occur naturally on the Earth, produced through neutron-induced reactions of uranium. Numerous

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isotopes of Pu have been synthesized, with atomic numbers from 228 to 247, and all of them are radioactive, with half lives ranging from 1 second to 8 107 y. The most prevalent isotopes in the environment and nuclear waste are 238Pu, 239Pu, 240Pu, and 241Pu. Table 15.8 lists the physical properties of these four isotopes, and, in addition, those of 236Pu, 242Pu, and 244Pu, which can be used as yield tracers in plutonium analysis. Isotope 242Pu is also formed in nuclear reactors but in very small amounts compared with the four first-mentioned isotopes.

The most important isotope of plutonium is 239Pu, which undergoes fission with thermal neutrons, providing a highly effective fuel for nuclear reactors. The critical mass of 239Pu, that is, the minimum amount required to maintain a fission chain reaction, is 10 kg, or about one-fifth the amount of 235U that is required. Because of its low critical mass, 239Pu has also been used as weapons material in nuclear bombs. Most 239Pu is produced by bombardment of 238U with neutrons in a nuclear reactor. The 239U that forms decays to 239Np, and ultimately to 239Pu:

238Un;c239Ut1= 2 23:5 min!b

239Npt1=

2 2:36 d!b

239Put1=

2 24110y

Isotope 238Pu, with a high power density of 6.8 Wcm3 (or 0.57 Wg1) because of its alpha decay, has been applied in power systems, where its alpha energy is transformed into electricity and used to power space satellites and remote instrument packages. It is produced in considerable amounts in nuclear reactors and nuclear explosions.

Table 15.8 Nuclear properties of the major plutonium isotopes in the environment and in nuclear waste.

Isotope Half-life (y)

Specific activity (Bq g1)

Principal decay mode

Alpha/beta energy (MeV)

Example of production method

236Pu 2.56 2.17 1013 a 5.768 (69%), 5.721 (31%)

235Ua; 3n

238Pu 87.7 6.338 1011 a 5.499 (70.9%), 5.456 (29.0%)

238Np daughter

239Pu 2.411 104 2.296 109 a 5.157 (70.77%), 5.144 (15.1%), 5.105 (11.5%)

239Np daughter

240Pu 6.561 103 8.401 109 a 5.168 (72.8%), 5.124 (27.1%)

Multiple n capture

241Pu 14.35 3.825 1012 b>99.99% 20.8 keV Multiple n capture

a 2.45 103% 4.896 (83.2%), 4.853 (12.2%)

242Pu 3.75 105 1.458 108 a 4.902 (76.49%), 4.856 (22.4%)

Multiple n capture

244Pu 8.08 107 6.710 105 a 4.589 (81%), 4.546 (19.4%)

Multiple n capture

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238Pu is formed from 238U in the following reactions:

238Un;2n237Ut1= 2 6:75min!b

237Npn;c238Npt1=

2 2:12d!b

238 Pu

and

235Un;c236Un;c237Ut1= 2 6:75 min!b

237Npn;c238Npt1= 2 2:12 d!b

238Pu

The higher-mass Pu isotopes are formed as the result of successive neutron capture reactions of 239Pu: 239Pun; c240Pun; c241Pun; c242Pu, and so on.

The relative amounts, or isotopic ratios, of the isotopes produced in nuclear reactors and nuclear weapons explosions vary with the neutron flux and duration of the irradiation, and therefore the isotopic ratios ofPu is used for source identification.

15.16.2 Sources of Plutonium

World production of Pu (mainly 239Pu) in 2005, existing in the form of spent nuclear fuel, nuclear weapons material, and nuclear waste, was approximately 2000 tonnes. In the reprocessing of spent nuclear fuel, uranium and plutonium are separated from fission products and transuranium elements (Np, Am, Cm) and can be used as mixed UPu fuel in nuclear power reactors. Only a small fraction of spent nuclear fuel has been reprocessed, however, and most of the plutonium remains in the spent fuel. The relative activities of plutonium isotopes in spent uranium fuel after 30 years cooling time (normalized to the amount of the longest- lived isotope, 239Pu), are 238Pu: 8, 239Pu: 1, 240Pu: 1.6, 241Pu: 100, 242Pu: 0.007. Only 239Pu and 240Pu are relevant in the final disposal of spent nuclear fuel, since 238Pu and 241Pu are short-lived as compared with the expected life-time of the technical barriers preventing the release of radionuclides from spent fuel in geological conditions, and the fraction of 242Pu is minor.

Weapons plutonium is produced by irradiating uranium in nuclear reactors and separating the plutonium by more or less the same process as that applied in reprocessing of spent nuclear fuel. Because the irradiation time in weapons pro- duction reactors is much shorter than that in power reactors, the isotopic compo- sition is essentially different. Weapons plutonium contains practically no 238Pu or 241Pu, and the fraction of fissile 239Pu is much higher. Furthermore, the mass ratio 240Pu=239Pu is only one-fifth of that in spent fuel, or about 0.05.

Plutonium is present in the environment as the result of nuclear weapons testing in the 1950s and 1960s, nuclear accidents (e.g., burn-up of a SNAP satellite in 1964, accidents of aircraft carrying nuclear weapons at Palomares in 1966 and at Thule in 1968, and the Chernobyl accident in 1986), and discharges from nuclear fuel reprocessing facilities and nuclear power plants. Table 15.9 lists the sources of Pu isotopes in the environment and shows that by far the largest source is nuclear

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weapons testing. The fallouts of 330 TBq of 238Pu, 7.4 PBq of 239Pu, 5.2 PBq of 240Pu, 170 PBq of 241Pu, and 16 TBq of 242Pu from nuclear weapons tests are estimated to comprise about 85% of the total environmental plutonium activity. Releases from nuclear power plants (not included in Table 15.9) are minor compared with the amounts listed in the table.

15.16.3 Measurement of Plutonium Isotopes

Of the seven Pu isotopes listed in Table 15.8, all those except 241Pu decay by alpha emission. 241Pu decays by beta emission. The most common radiometric methods used for the measurement of plutonium isotopes are alpha spectrometry and liquid scintillation counting. Alpha spectrometry is used to determine 238Pu, 239Pu, and 240Pu, and liquid scintillation counting for 241Pu. Alpha spectrometry cannot separate the close-lying alpha peaks of 239Pu (5.16 MeV) and 240Pu (5.17 MeV), but measures the sum activity of 239Pu and 240Pu. Thus, the pluto- nium activities are presented in the form 239;240Pu when measured with alpha spectrometry. Furthermore, the decay of 241Am, 210Po, 224Ra, 229Th, 231Pa, 232U, and 243Am commonly interferes with the alpha-spectrometric determination of 238Pu and 239;240Pu, and effective chemical separation of Pu from these interfering radionuclides is required before measurement. The detection limit of alpha spectrometry is about 0.020.1 mBq for 238Pu and 239;240Pu, depending on the measurement time.

241Pu is a low-energy beta emitter with maximum energy of 20.8 keV and can be measured by liquid scintillation counting (LSC). This technique requires a thorough chemical separation of Pu from the matrix and from all other radionuclides, especially interfering beta emitters. A detection limit of about 10 mBq is achievable for 241Pu measured by LSC. Abetter detection limit can be achieved by measuring the 241Pu activity indirectly, that is by measuring the alpha-emitting daughter 241Am by alpha spectrometry. This requires fairly long ingrowth times, however. Detection limits down to 0.3 mBq have been reported with use of alpha-spectrometry and 13 years ingrowth time.

Another common technique used to measure plutonium isotopes is mass spec- trometry (see Chapter 17). Methods include inductively coupled plasma mass spectrometry (ICP-MS), accelerator mass spectrometry (AMS), thermal ionization mass spectrometry (TIMS), and resonance ionization mass spectrometry (RIMS). Of these, ICP-MS is currently the most popular method. Attractive features are the relatively short measurement time and the provision of isotopic information on 239Pu and 240Pu separately. The most serious problem in employing ICP-MS to measure low-level samples for Pu isotopes is the interference from uranium, because the mass concentration of uranium is typically 106109 times that of Pu. The tailing (abun- dance sensitivity) of 238U to the mass range of 239Pu and formation of the polyatomic ion 238U1H are major disturbances at mass 239. Thus, uranium must be removed before measurement. The detection limit of ICP-MS, especially of high-resolution

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Table 15.9 Sources of plutonium isotopes in the environment (Bq).

Radionuclide 238Pu 239Pu 240Pu 241Pu 242Pu 239 240Pu Total

Nuclear weapons testing 3.3 1014 6.5 1015 4.4 1015 1.4 1017 1.6 1013 12.6 1015 1.7 1017 Burn-up of SNAP-9A satellite, 1964 6.3 1014 6.3 1014 Aircraft accident in Palomares, Spain 1966 5.5 1010 5.5 1010 Aircraft accident in Thule, Greenland, 1968 1 1013 1 1013 Nuclear power plant accident at Chernobyl, 1986 3.5 1013 3 1013 4.2 1013 6 1015 7.0 1010 7.2 1013 6 1015 Reprocessing plant at Sellafield 1.2 1014 2.2 1016 6.1 1014 2.2 1016 Reprocessing plant at La Hague 2.7 1012 1.2 1014 1.7 109 3.4 1012 1.4 1014

288j 15

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ides

Lehto, Jukka, and X iaolin H

ou. C hem

istry and A nalysis of R

adionuclides : Laboratory T echniques and M

ethodology, John W iley &

S ons,

Incorporated, 2010. P roQ

uest E book C

entral, http://ebookcentral.proquest.com /lib/osu/detail.action?docID

= 645020.

C reated from

osu on 2019-01-29 07:17:21.

ICP-MS, is comparable with that of alpha spectrometry for 239Pu and 240Pu (0.020.05 mBq), and with that of LSC for 241Pu (10 mBq).

Plutonium isotopes can also be determined by AMS (see Chapter 17). AMS offers abundance sensitivity well suited for ultra trace detection of 239Pu, 240Pu, 241Pu, 242Pu, and 244Pu, with a detection limit of 0.452 mBq for 239Pu and 1.5 mBq for 241Pu. The main advantage of AMS over other mass spectrometric techniques is that the interference of 238U1H with 239Pu can be completely removed. The determina- tion of 238Pu remains difficult because of serious interference from 238U.

TIMS measurements are made on a small volume (down to 1 mL) of aqueous solution containing the target nuclide. Careful separation ofPu from matrix elements andinterferingradionuclidesandconcentrationtoafewmicrolitersisrequiredbefore measurement.ProblemsduetouraniumarelesssevereinTIMSthaninICP-MSsince uraniumand plutoniumhavedifferent ionization potentialsandareemittedfromthe filamentatdifferenttemperatures(plutoniumleavesthefilamentfirst).Furthermore, the dry sample introduction considerably reduces the interference due to UH, and theabundancesensitivityisgenerallyordersofmagnitudebetterinTIMSthaninICP- MS.Adetectionlimitofabout1 fg(2 mBq)239Pu hasbeenreportedwithTIMS,whichis a much lower level than for the radiometric method and comparable with that offered by AMS.

Of the various methods for measuring plutonium, alpha spectrometry is more straightforward than the mass spectrometric methods, and the risk of interfering signals is lower than with ICP-MS. There is also no interference from microgram amounts of stable elements in the activity measurement source after chemical separation and electrodeposition. The major disadvantage of alpha spectrometry is the lengthy measurement time, up to one week for low level samples. In ICP-MS, several polyatomic species, as well as tailing of 238U, may appear in the mass range 230245 if microgram amounts of uranium, lead, mercury, thallium, and rare earth elements are remaining in the sample after separation. Mass spectrometric methods such as AMS and TIMS are highly sensitive and allow measurement of 239Pu, 240Pu, and 241Pu simultaneously. In contrast, use of mass spectrometric methods to measure 238Pu in environmental samples is very difficult owing to the low mass concentration and serious isobaric interference from more abundant 238U. For these reasons, the mass spectrometric methods should be seen as a complement to rather than a replacement for conventional alpha spectrometry for the measurement of Pu isotopes. Mass spectrometric methods are also inadequate for accurate measurement of short-lived 241Pu (k1=2 14.4 y) in environmental samples.

15.16.4 The Chemistry of Plutonium

15.16.4.1 Oxidation States and Plutonium The electron configuration of plutonium is [Rn]5f67s2, and plutonium can occur in all oxidation states between IIIand VII. Oxidation state VIIis rare, while III, IV, V, and VI are all common, and can even exist in solution simultaneously.

15.16 Plutonium j289

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Figure 15.23 shows the Eh-pH diagram for plutonium. The chemical properties of Pu greatly depend on its oxidation state. Pu ions in the lower oxidation states (III and IV) are more stable under acidic conditions, whereas Pu(VI) is more stable in alkaline media and Pu(V) in neutral media. Pu(IV) is the most stable and most thoroughly studied oxidation state. Ever since the discovery of plutonium in the early 1940s, studies on its solution chemistry have been motivated by the need to separate it from uranium and fission products for use in weapons production and as a fuel for power production. Under noncomplexing, strongly acidic conditions such as in perchloric or trifluoromethanesulfonic acid (triflic acid) solutions, both Pu(III) and Pu(IV) exist as the simple hydrated (or aquo) ions, Pu3 (aq) and Pu4 (aq), respectively, retaining their overall formal charge. Pu(V) and Pu(VI) cations have such large positive charges that they immediately hydrolyze in aqueous solution to form dioxo cations, PuO2

and PuO2 2 , which are commonly referred to as plutonyl ions. The effective charges

of the plutonium forms decrease in the order

Pu4 4 > PuO2 2 3:3 > Pu3 3 > PuO2 2:2

15.16.4.2 Disproportionation In acidic solutions in the absence of complexing ligands, Pu(IV) reacts by dispro- portionation, as follows:

3Pu4 2 H2OK2Pu3 PuO2 2 4H

Figure 15.23 Eh-pH diagram of plutonium, at total Pu concentration of 1010 M (Atlas of Eh-pH diagrams, Geological Survey of Japan File report No. 49, 2005).

290j 15 Radiochemistry of the Actinides

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