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Complexation Reactions in Nuclear Separations

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Complexation Reactions in Nuclear Separations Complexation Reactions In Nuclear Separations Raymond G. Wymer Vanderbilt University Nuclear separations are a mainstay of the nuclear industry, and complexation reactions play a major role in nuclear separations. This paper concentrates not only on those separations that are both of primary importance to spent nuclear fuel reprocessing and waste management and the complexation reactions that make those separations possible, but also on some of the most unusual aspects of the chemistry involved. Nuclear separations find applications in all parts of the nuclear fuel cycle from mining and milling of ores, to purification of nuclear materials, to uranium enrichment, to reactor fuel fabrication, to reactor spent fuel reprocessing, and to radioactive waste management. Separations in these areas rely to a very large extent on reactions with chemical complexation species that form chemical complexes with a relatively small number of radioactive elements. The chemistries of some of the elements of greatest importance in the nuclear fuel cycle are quite remarkable and make possible many of the separations processes that are fundamental to the nuclear industry. Significant Elements in Separations and Complexation Reactions Isotopes of a relatively small number of radioactive elements have special significance in the context of the nuclear fuel cycle. Their complexation reactions are illustrative of the important role played by complexes. The elements selected for attention here are: technetium (Tc), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), and curium (Cm). Some radioisotopes of these elements that are especially important in the nuclear fuel cycle are given in Table 1. Table 1. Some Radioisotopes of Importance in the Nuclear Fuel Cycle Element Isotopes of Interest Half life Technetium 99Tc 2.111E+05 yr Uranium 232U, 233U, 234U, 235U, 238U Various Neptunium 237Np 2.144E+06 yr Plutonium 238Pu, 239Pu, 240Pu, 242Pu Various Americium 241Am 432.2 yr Curium 242Cm, 244Cm 162.8 days, 10.1 yr These elements and their isotopes have importance for a variety of reasons which are discussed below. Some of the reasons for importance in separations of specific isotopes of the six elements listed in Table 1 are given in Table 2. Table 2. Some Reasons for Importance in Separations of Specific Radioisotope Radioisotope Reason for Importance 232U Present with 233U; Hazardous gamma emitter in its daughter chain 233U Potential reactor fuel; weapons usable 234U High specific activity alpha emitter; naturally occuring 235U Reactor fuel; weapons usable 238U Fertile isotope for 239Pu production; good radiation shield 99Tc Dose limiting isotope in geologic repository release path 237Np Dose limiting isotope in repository release path; precursor to 238Pu 238Pu Producer of heat for thermoelectricity production in space 239Pu Potential reactor fuel; long-term heat producing isotope in repository 240Pu Heat producing isotope in repository 241Am Important intermediate-term heat producing isotope in repository 244Cm Heat producer in repository; high specific activity alpha biohazard Complexation reactions of elements are very strongly dependent on the valence states of the elements. Changing the valence of an element dramatically changes its chemistry and consequently changes its complexation reactions and separations chemistry. Table 3 lists common valence states of the elements of interest here as well as some of the important features of them. The most common valence states are in bold face. Table 3. Important features of Some Common Valence States Element Valences Features - Tc +4, +5, +6, +7 Environmentally mobile as TcO4 ; Tc2O7 is volatile at relatively low temperatures 2+ +4 U +3, +4, +5, +6 UO2 forms extractable species; U is used in oxide fuels; UF6 is volatile - Np +3, +4, +5, +6, NpO2 : Environmentally mobile; extractable in organic solvents +7 +4 +4 +4 Pu +3, +4, +5, Mobile as Pu colloid; Pu is extractable in organic solvents; Pu is used in oxide fuels +6, +7 +3 6+ Am +3, +4,+5, +6 Am is very stable in aqueous media; Am is potentially useful in separations from other actinides +3 Cm +3, +4 Cm is the only common valence state in aqueous solution; it behaves much like rare earths Uranium Uranium is at the heart of commercial nuclear power and is vital to the entire nuclear enterprise. Its presence and use worldwide has resulted in a vast literature, not only on uranium complexation and separations reactions, but also on all aspects of the uranium fuel cycle: mining and, milling; isotope enrichment; reactor fuel manufacturing; spent reactor fuel reprocessing; and weapons production. Only a small fraction of that literature is covered in this discussion of complexation reactions used in nuclear separations. The chemistry of uranium is very unusual, and although it cannot be said to be unique, it certainly can be said to be remarkable. The number of valence states of uranium that are easily obtainable under ordinary conditions make possible a wealth of compounds and complexation reactions that present almost unparalleled opportunities for separations processes, both of uranium from contaminants and of uranium isotopes. Trivalent uranium as a chloride complex in water is used in a quite unusual example of uranium isotope separations chemistry. The trivalent chloride is thermodynamically unstable in aqueous media in the presence of metallic ions that catalyze its reaction with water to form hydrogen and U+4, but when catalytic ions are absent it is stable indefinitely. This unusual meta-stability has been used in a practical uranium isotope separation process called the Chemex process. Table 4 lists common aqueous ionic uranium chemical species and some chemical properties of interest in separations processes. Table 4. Common Aqueous Ionic Uranium Chemical Species Aqueous Species Chemical Properties U+3 Thermodynamically unstable in aqueous media but kinetically stable +4 - 2- - - U Forms complexes with Cl , SO4 , F , CNS , et al.; hydrolyzes easily + +4 2+ UO2 Transient existence; disproportionates to U and UO2 +2 UO2 Predominant aqueous species; some salts are stable to 300° C Uranium in the tetravalent state forms colloids and gels that are easily formed into different shapes, e.g., small spheres that find application in preparing certain types of reactor fuels. The colloidal dispersions (sols) of uranium hydroxide are gelled by precipitation with a chemical base or by removal of water as a step in the preparation of UO2 for use in uranium dioxide reactor fuel. Because it is highly charged the U+4 ion easily forms a wide variety of complex ions. It is also readily oxidized or reduced by a variety of redox reagents. It precipitates as the fluoride which is the chemical form used to produce uranium metal by thermochemical reduction with alkali metals. 2+ The uranyl ion (UO2 ) is the most common uranium ion in aqueous media. The oxygen atoms in the uranyl ion are bound extremely tightly and do not detach readily in chemical reactions. 2+ 2+ Thus UO2 behaves much as a divalent monatomic cation. Reduction reactions of (UO2 ) to produce U+4 are slow due to the stability of the uranyl ion. Uranyl salts and complexes are - - -2 - 3+ formed with most common anions such as NO3 , Cl , SO4 , F and PO4 . These anions may react with the uranyl ion to form anionic complexes that are useful in carrying out separations using anion exchange resins. Uranyl phosphate is found in phosphate deposits and may be economically recovered as a byproduct in fertilizer manufacture. Uranyl ion complexes extract readily into organic solvents. This is the basis of the widely used Purex Process for reprocessing spent nuclear reactor fuel. Uranyl nitrate in nitric acid reacts with tributyl phosphate when contacted with a TBP phase to form UO2(NO3)2·2TBP which is highly soluble in a TBP (tributly phosphate) solvent formed by mixing TBP with a hydrocarbon diluent such as dodecane or kerosene. Most fission products and some actinides do not extract in that solvent except under conditions of considerably more concentrated TBP than is use in the Purex Process (see the discussion of curium below). Whether or not other actinides extract depends strongly on the valence state of the actinides. Uranyl salts are often exceptionally stable at temperatures well above the boiling point of water, e.g., 300° C. This property found application in the aqueous homogeneous reactor which employed a solution of UO2SO4 at temperatures well above the boiling point of water at atmospheric pressure. UO2(NO3)2 has been proposed for use in an aqueous homogeneous reactor for the production 99Mo which is a high-yield fission product and is the parent of 99mTc, a widely used isotope in medical diagnostics. Addition of chemical bases such as NH4OH and NaOH to solutions of uranyl salts precipitate uranium as a diuranate, for example as (NH4)2U2O7, a rather ill-defined but useful compound called ammonium diuranate that finds use in uranium milling and in reactor fuel manufacture. The uranyl ion reacts in mildly acidic solutions of hydrogen peroxide to form the insoluble complex peroxide salt UO4·2H2O. This reaction, although not unique to uranium, is unusual and may be used to purify uranium. A complexation reaction of uranyl ion with sodium and zinc acetates forms the unique precipitate NaZn[UO2(CH3O2)3]3 that may be used for the quantitative determination of sodium. The very unusual and highly volatile compound UF6 is at the heart of commercial uranium isotope separations, both by gaseous diffusion and by gas centrifugation. Because it can be distilled UF6 finds use in uranium purification. -4 A very useful and extraordinarily stable uranyl tricarbonate complex anion, UO2(CO3)3 , forms with carbonate anions. This extraordinary anionic uranyl tricarbonate complex finds use in uranium solution mining, in fuel fabrication, and in separations from a host of cations that do not form such anionic complexes. The capacity of uranium to form slightly non-stoichiometric uranium dioxide makes possible adjustments to its composition to optimize its behavior as fuel in nuclear reactors.
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