High level waste, separation

By-Products. The PUREX process is efficient at separating uranium and plutonium from everything else in the spent fuel. Within the high level waste stream are a number of components which have, from time to time, been sufficiendy interesting to warrant their recovery. The decision to recover a particular isotope is usually based on a combination of market incentives and desired waste reduction. 

After the separation of the actinides from the high-level waste, it is desirable to remove certain other fission products from the nuclear wastes. Some Cs and Sr are low-charged cations that react well with macro-cyclic ligands (e.g., crown ethers, calixarenes). Research to synthesize and investigate the properties of macrocyclic ligands for application in nuclear waste treatment has been an active effort internationally. Some of the results obtained are discussed in section 12.7. 

Long-lived ty = 2.1 x 10 years) Tc, present as TCO4 in Purex process HNO3 feed solutions, is partially coextracted with uranium and plutonium in the first cycle. Unless separated in the Purex process, Tc contaminates the uranium product subsequent processing of the U02(N03)2 solution to UO2 can release some of the technetium to the environment. The presence of technetium in the purification steps as well as in the uranium product causes several other complications. Thus it is desirable to route all Tc into the high-level waste. Efforts in this direction have been described in some recent flow sheets [37]. 

Several tests using countercurrent separation in mixer-settler or centrifugal extractors with simulated and genuine high-level waste showed that the recovery of An(III, IV, VI) is quantitative. The back extraction of An(III) was complete. These early flow sheets were not designed to strip U(VI) and Pu(IV). The distribution ratios of these ions at low acidities are lower than those measured with CMPO and one can guess that the stripping... 

Fig. 12.18 Concepts for partitioning radionuclides from Purex process high-level waste (a) today s technology (b) tomorrow s technology. Note Numbers in parentheses indicate separations that must be performed remotely.

In tlie PUREX process, the spent fuel and blanket materials are dissolved in nitric acid to form nitrates of plutonium and uranium. These are separated chemically from the other fission products, including the highly radioactive actinides, and then the two nitrates are separated into tv/o streams of partially purified plutonium and uranium. Additional processing will yield whatever purity of the two elements is desired. The process yields purified plutonium, purified uranium, and high-level wastes. See also Radioactive Wastes in the entry1 on Nuclear Power Technology. Because of the yield of purified plutonium, the PUREX process is most undesirable from a nuclear weapons proliferation standpoint,... 

The primary separation of plutonium and uranium from the fission products involves a solvent extraction with 30 vol % TBP at room temperature. The activity levels in this separation are quite high ( 1700 Ci/L for the fission products) and the aqueous waste, which contains 99+% of the fission products, is a high-level waste. Am and Cm are not extracted and Np is partially extracted. Because of the high radiation levels, there are radiolysis problems with TPB, leading to solvent degradation. Primary products of the radiolysis of TBP are the dibutyl- and monobutylphosphoric acids along with phosphoric acid. These degradation products are removed in the solvent purification steps. 

High-level waste — Efficient separation and robotic system... 

Forsberg, C.W. 2000. Rethinking high-level waste disposal Separate disposal of high-heat radionuclides (90Sr and 137Cs).Nucl. Technol. 131 252-268. 

Moyer, B.A., Bazelaire, E., Bonnesen, P.V. et al. 2005. Next generation extractants for cesium separation from high-level waste From fundamental concepts to site implementation. FY 2005 Annual Report, Environmental Management Science Program, Project 73803. 

Mathur, J.N. Murali, M.S. Iyer, R.H. Ramanujam, A. Dhami, P.S. Gopalkrishnan, V. Rao, M.K. Badheka, L.P. Baneiji, A. Extraction chromatographic separation of minor actinides from purex high-level wastes using CMPO, Nucl. Technol. 109 (1995) 216-225. 

Naik, P.W. Dhami, P.S. Misra, S.K. Jambunathan, U. Mathur, J.N. Use of organo-phosphorus extractants impregnated on silica gel for the extraction chromatographic separation of minor actinides from high level waste solutions, J. Radioanal. Nucl. Chem. 257 (2003) 327-332. 

Sharma, J.N., Suri, A.K., Manohar, S., Chitnis, R.R., Shah, G.J., Wattal, P.K. 2004. Solvent extraction studies of synthetic high level waste using novel extractant tetra (2-ethylhexyl) dilycolamide (TEHDGA). SESTEC-2004 Emerging Trends in Separation Science and Technology, July, Mumbai, India. 

EPA s current definition of high-level waste was first developed in 1985 (EPA, 1985) and is contained in 40 CFR Part 191 (EPA, 1993a). This definition defers to NWPA, and spent fuel is defined separately from high-level waste as in the Act. Thus, EPA has adopted the traditional, source-based definition of high-level waste. 

During the starting period, a simulated "high level" waste from a separate tank is used as feed. 

Separation of Actinides from High-level Waste (HLW). From the point of view of seeking a possible approach to the ultimate disposal of the HLW from the reprocessing of spent nuclear fuels, processes of solvent extraction and ion-exchange techniques have been studied to recover both americium and lanthanides from the HLW and to separate those subsequently. 

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