Purex process modifications

As described above, various separation processes and CFCs have been developed and proposed, aiming at the modification of the current PUREX process and reformation of the Improved PUREX and also aiming at the establishment of advanced reprocessing processes. Figure 1.10 shows a classification scheme for these processes and CFCs. 

Major Compounds HDBP and H2MBP. HDBP, the main degradation product of TBP, is partially soluble in aqueous phases the distribution ratio is highly dependent on the composition of the two phases (89,112,113). This molecule can form strong complexes with Pu(IV), Th(IV), Zr(IV), U(VI), Np(VI), and other cations, causing modification of An(IV) and An(VI) distributions (89, 114, 115) and various problems in the PUREX process (27, 89, 116-119). The behavior of HDBP solution complexes is not easily understood because various compositions could be present in solution or as solid compounds. 

In one modification of the Purex process, the plutonium is not separated from the uranium. In this version, the first cycle has only two columns instead of three. In addition to reducing the criticality risk, this modification reduces the risk of unauthorized diversion of the plutonium. 

Solvent extraction can be carried out in pulsated extraction columns, in mixer-settlers or in centrifuge extractors. Organic compounds such as esters of phosphoric acid, ketones, ethers or long-chain amines are applied as extractants for U and Pu. Some extraction procedures are listed in Table 11.11. The Purex process has found wide application because it may be applied for various kinds of fuel, including that from fast breeder reactors. The Thorex process is a modification of the Purex process and has been developed for reprocessing of fuel from thermal breeders. 

A more radical modification of the Purex process, the Aquafluor process, developed by General Electric for its Midwest Fuel Recovery Plant, retained only a single TBP co-decontamina-tion cycle followed by a continuous anion exchange contactor in which plutonium was to be removed from the U-Pu nitrate solution. The performance of this plant was never tested with plutonium, since General Electric decided to forego operation of the plant after technical difficulties developed during the "cold" checkout trials. 

The Purex process will continue to be the main method for the reprocessing of nuclear reactor fuels. The inherent flexibility of this process allows for modifications needed to accomodate a large range of fuel compositions and product specifications. Among the several plutonium partitioning methods developed, those avoiding the introduction of extraneous metal ions... 

A solvent extraction process similar to Purex using TBP was developed by the Commissariat a I Energie Atomique [Gl] for use in the French plutonium separation plant at Marcoule. Since then, the Purex process has replaced the Butex process at Windscale [W3], has been used in the Soviet Union [Sll], India [S7], and Germany [S3], and by now is the universal choice for separation of uranium and plutonium from fission products in irradiated sUghtly enriched uranium. Fuel from the liquid-metal fast-breeder reactor (LMFBR) is also reprocessed by the Purex process, with modifications to accommodate the higher concentrations of plutonium and fission products. 

El-Said, N., Rahman, N.A., Borai, E.H., Modification in Purex process using supported liquid membrane separation of cerium(III) via oxidation to cerium(IV) from fission products from nitrate medium by SLM, J. Membr. Sci. 198, 23, 2002. 

The aim of the present improvement work on the PUREX process is to make the separations more selective and to create effluent streams of high purity. Thus, modifications are performed to make neptunium end up in a fraction for later transmutation in a reactor or accelerator-driven system. This can be achieved by a better control of redox conditions in the process. Today neptunium is partially co-exlracted with plutonium and uranium. There are also suggestions to withdraw product streams with Tc and respectively, i.e., long-lived nuclides that might be of interest for transmutation. 

Over the past 10 years, modifications to the PUREX process have made it possible to more effectively separate neptunium. To effect the efficient separation of Np within the conventional PUREX process, Np is oxidized to VI state by nitrous add and is extracted in the first cycle along with U and Pu into the organic phase. The extracted Np( VI) follows the uranium stream and is later separated during the second purification cycle of uranium. In the RFC, the neptunium is sent to vitrification and disposed of as HLW but in an AFC option, the neptunium can be blended with MOX fuel or fabricated into special targets for later transmutation. The other minor actinides, ameridum and curium cannot be separated by reasonable modifications to the PUREX process. These elements will require the addition of special processing steps to separate them from the PUREX high-level waste stream. 

Modifications to the traditional PUREX process can also lead to the separation of two of the long-lived fission products, technetium and iodine. During dissolution, 10-20% of the... 

A variation of the PUREX process is being proposed by the US Department of Energy as a possible alternative partitioning scheme for the transmutation of wastes. This aqueous process called UREX, only removes uranium from spent commercial LWR fuel and leaves plutonium in the HLW stream with the other minor actinides and fission products. Nonaqueous pyroprocessing, a variation of the ANL electrorefining process, is then proposed to be used to separate both the plutonium and minor actinides so that they can be transmuted in an ADS. For further information related to potential modifications of this process for other accelerator transmutation of waste applications, see ANL-99/15 (1999). 

Purex [Plutonium and uranium recovery by extraction] A process for the solvent extraction of plutonium from solutions of uranium and fission products, obtained by dissolving spent nuclear fuel elements in nitric acid. The solvent is tri- -butyl phosphate (TBP) in kerosene. First operated by the U.S. Atomic Energy Commission at its Savannah River plant, SC, in 1954 and at Hanford, WA, in 1956. Now in operation, with modifications, in several countries. Sites include Savannah River (SC), Cap de la Hague (France), Marcoule (France), Sellafield (England), Karlsruhe (Germany), and Trombay (India). See also Recuplex. 

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