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Part Ii: Technical Analysis and Systems Study

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PART II: TECHNICAL ANALYSIS AND SYSTEMS STUDY 111 1. PARTITIONING 1.1 Aqueous separation techniques This section briefly describes aqueous separation techniques currently used on industrial scale and research activities in the field of new separation methods for more effective separation of minor actinides and fission products. There has been a large number of reports published until now and a selection of the important ones is listed in Annex D. 1.1.1 PUREX process The PUREX process, see Figure II.1, which is universally employed in the irradiated fuel reprocessing industry, is a wet chemical process based on the use of TBP, a solvent containing phosphorus. As shown in Table II.1 this solvent displays the property of extracting actinide cations in even oxidation states IV and VI, in the form of a neutral complex of the type M•An•2TBP (where M is the metallic cation and A an anion, generally nitrate ion), from an acidic aqueous medium. Conversely, the actinide cations with odd oxidation state are not significantly extracted, at least in the high acidity conditions prevailing during reprocessing operations. Uranium and plutonium, whose stable oxidation states in nitric medium are VI and IV, respectively, are co-extracted by TBP and thus separated from the bulk of the fission products which remain in the aqueous phase. This is the basic principle of the PUREX process. Table II.1 Extractability of actinide nitrates in 3 M nitric acid by TBP Oxidation state III IV V VI U (m) (l) m Np (m) l m Pu (l) m (l) (m) Am l (m) Cm l m: extractable by TBP, l: not extractable by TBP, ( ): unstable in the media Uranium and plutonium are recovered with an industrial yield close to 99.9% (including losses in secondary wastes). 113 1.1.1.1 Minor actinides Americium and curium Among the minor actinides, americium and curium, which are stable in valency III, are not extracted by TBP and remain in the aqueous phase. They accordingly follow the path of the fission products and are currently managed like the latter by conditioning in a glass matrix. Neptunium Another minor actinide, neptunium, whose stable oxidation state is V, is hence very slightly extractable in this species by TBP. However, in the chemical conditions of the first cycle extraction operation of the PUREX process (presence of nitrous acid), part of the Np(V) is oxidised to VI, and accordingly extracted in the organic phase. The operating results of the UP3 plant reveal that the majority of the Np is extracted by TBP, follows the uranium stream, and is separated from the latter in the second uranium purification cycle. The effluent containing neptunium is currently added to the high-level waste stream. Hence all the neptunium is sent to vitrification apart from the proportion following the Pu product. The behaviour of neptunium is independent of the type of fuel reprocessed. 1.1.1.2 Long-lived fission products Among the fission products with long-lived isotopes, three elements (technetium, zirconium and iodine) display specific behaviour in the PUREX process, which could be exploited for their separation (see Figure II.1). Technetium During fuel dissolution, part of the technetium, probably in metallic or oxide form, does not go into solution. This fraction, estimated at 10 to 20% of the total Tc for a UO2 fuel, accompanies the “insoluble residues” essentially consisting of noble metals (Ru, Rh, Pd). These insoluble residues are currently incorporated in the vitrified or cemented wastes. In nitric medium and in the absence of a reducing agent, the dissolved technetium is in its highest valency (VII) which is the most stable. It occurs in anionic form in the state of pertechnetate ion, - TcO 4 . This species can be extracted by TBP at the same time as a metallic cation by substitution of a pertechnetate group for a nitrate group in the neutral complex extracted. The occurrence of this co-extraction was observed with the main metallic cations extracted by TBP, and particularly with the cation ZrO2+ (see below), which is present in large amounts in the feed solution to the extraction cycle. This extraction of Tc proved to be a serious hindrance because of its interference with the chemical mechanisms of the partitioning operation (U/Pu separation). The PUREX process accordingly had to be adapted to limit the extraction of Tc. 114 Figure II.1 Behaviour of long-lived elements in PUREX process (An example of UP3 La Hague) Hulls and end pieces Activation I (>90%) products Dissolution Off-gas treatment U/Pu Purification 1st Cycle FP Tc U/Pu Extraction U scrubbing scrubbing separation stripping Am,Cm Np (minor fraction) Insolubles Cs, Zr (100%) Tc (up to 90%) U Np (major fraction) U purification Pu Vitrification Pu purification Tc (>10%) The final flow chart, which includes a specific stripping step for the Tc extracted by the TBP, produces an effluent containing a large fraction of the element. This effluent is currently added to the main fission product stream. Zirconium Zirconium, which is present in the form of the ion ZrO2+, can be extracted by TBP, but to a lesser degree than uranium and plutonium. This element is effectively stripped in the “FP washing” operation, which immediately follows extraction. The aqueous phase from the washing must be recycled to extraction due to the large amounts of U and Pu which it contains. In the present status of the process, zirconium is not specifically isolated. Iodine Iodine presents a special case in so far as this element is extremely volatile in the elemental state. This property is exploited for the containment of this fission product in the “process head end”, 115 thus preventing its dispersion in the downstream operations where its controlled management would be quite difficult. The operating conditions of fuel dissolution are selected to ensure that the iodine is brought to and maintained in the elemental state, and to entrain it in the off-gas. The iodine is recovered in an aqueous solution by caustic scrub of the off-gas. This specific effluent of iodine, which is discharged into the sea today, thus contains nearly all the iodine initially present in the irradiated fuel. Some reprocessing plants envisage the use of iodine immobilisation by adsorption on silver impregnated zeolites. Other long-lived fission products As to the other long-lived fission products, it is clear that the PUREX process cannot be used to separate caesium and strontium, since these mono- and divalent elements are unextractable by TBP. The behaviour in the PUREX process of the other fission products which have long-lived isotopes (Pd, Se, Sn) is not precisely known. A combined electrolytic extraction of Pd2+ with the other platinum group 3+ 3+ - 2- elements (RuNO , Rh ) and TcO 4 (and probably SeO 4 ) seems to be promising from even higher acidic PUREX liquors [1]. 1.1.1.3 Long-lived activation products The activation products formed in the fuel element structural metals (stainless steels, inconel and zircaloy) mostly remain in these materials and are found in the corresponding “hulls and end pieces” waste stream. The 14C issue should receive increasing attention because of this isotope’s impact on the biosphere. 1.1.1.4 Conclusions The behaviour of the minor actinides and long-lived fission products in the PUREX process can be divided into three categories: · elements already partially separated by the PUREX process: neptunium, technetium and iodine. For these elements, the R&D objective involves process extensions to achieve the desired separation performance. This first aspect is discussed further in Section 1.1.2. · elements separable by TBP, for which a complementary step to the present PUREX process can be developed. This applies to zirconium. · elements that cannot be separated by the PUREX process: – americium and curium, – caesium, strontium and probably the other fission products (Pd, Se, Sn). To separate these elements, it is necessary to develop new classes of extractants, or to resort to different separation methods. The corresponding developments are discussed in Section 1.1.3 (Am and Cm) and Section 1.1.4 (FPs). 116 1.1.2 Improved separation of long-lived elements in PUREX process 1.1.2.1 Neptunium Neptunium is present in the irradiated fuel dissolution liquor (nitric acid medium) in oxidation states V and VI. The extractability of Np(V) by tributylphosphate (TBP) organic solution (the PUREX solvent) is rather poor whereas that of Np(VI) is good, approaching that of U(VI) and Pu(IV). Hence one alternative to separate neptunium from the wastes is to extract this element in the first cycle of the PUREX process together with U(VI) and Pu(IV). This requires the oxidation of the whole neptunium inventory to the VI oxidation state, allowing Np extraction by TBP. Thus the redox reaction to convert Np(V) to Np(VI) is the key point to be addressed to achieve ~100% Np extraction in the first cycle. Np(V) can be oxidised to Np(VI) by various means including: · chemical oxidation using nitric/nitrous acid mixture, like those present in the fuel dissolution liquors, intentionally adding oxidants such as vanadium (V) compounds; · using force field oxidation like: b, g radiolysis oxidation, photochemical oxidation, sonochemical oxidation. After its co-extraction with U and Pu, neptunium can be selectively separated from these elements, either using the regular PUREX cycles (for example in the second uranium cycle) or under the action of specific reagents like butyraldehydes. Computer codes of the PUREX process are available for calculating the behaviour of Np in the PUREX extraction cycles. The current research trend in this area is to define more refined chemical models for Np behaviour that are more elaborate than those hitherto employed.
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