Aqueous reprocessing PUREX

Modern Aqueous Reprocessing 14.5.1 Basic Commercial Process PUREX Process... 

As an alternative, closed fuel cycles with MOX or hybrid U-Th fuel are considered for the GT-MHR. An option to apply fuel reprocessing is mentioned for the HTR-PM and the GTHTR-300 for the latter it has already been investigated. In case of TRISO coated particles, aqueous reprocessing methods (e.g., PUREX) cannot be applied directly. Specifically, the problem is with the SiC coating layers, which are not dissolved in mixtures of acids and, therefore, require a mechanical treatment to be removed. 

The Purex process is used for almost all fuel reprocessing today. Irradiated UO2 fuel is dissolved in HNO3 with the uranium being oxidized to U02(N03)2 and the plutonium oxidized to Pu(NC>3)4. A solution of TBP in a high-boiling hydrocarbon, such as n-dodecane, is used to selectively extract the hexavalent U02(N03)2 and the tetravalent Pu(NC>3)4 from the other actinides and fission products in the aqueous phase. The overall reactions are... 

As U is the major component of a SNF see Table 1.2, its initial separation in reprocessing alleviates the mass burden of following steps and is considered preferable. The UREX process developed in the AFCI program of the United States is based on the PUREX process (30 vol % TBP in n-dodecane) and suppression of extractions of Pu and Np by reduction/complexation (175-182). Plutonium and Np are reduced by acetohydroxamic acid (AHA, CH3CONHOH) to Pu(III), Np(V), and Np(IV). U is kept in an extractable U(VI) state. Although Np(IV) is also extractable, AHA forms a complex with Np(IV) that is soluble in the aqueous phase. In the case where reoxidation of Pu(III) occurs, the Pu(IV) also transfers to the aqueous phase by forming a Pu(IV)-AHA complex. Thus, U is exclusively extracted. AHA decomposes to hydroxylamine and acetic acid (176). 

Reprocessing is based on liquid-liquid extraction for the recovery of uranium and plutonium from used nuclear fuel (PUREX process). The spent fuel is first dissolved in nitric acid. After the dissolution step and the removal of fine insoluble solids, an organic solvent composed of 30% TriButyl Phosphate (TBP) in TetraPropylene Hydrogenated (TPH) or Isopar L is used to recover both uranium and plutonium the great majority of fission products remain in the aqueous nitric acid phase. Once separated from the fission products, back-extraction combined with a reduction of Pu(I V) to Pu(III) allows plutonium to be separated from uranium these two compounds can be recycled.2... 

Reprocessing of nuclear fuel by the Purex process leads to the following amounts of waste per ton of U 1 m HLW (fission products and actinides in HNO3 solution), 3 m MLW as organic solution, 17m MLW as aqueous solution, 90m LLW (aqueous solution). By further processing a volume reduction is achieved 0.1m HLW, 0.2m MLW (organic), 8m MLW (aqueous), 3m LLW (aqueous). 

Liquid-liquid extraction (LLE) systems using neutral phosphorus-based organic compounds have been the subject of extensive study since Warf (1) first reported the use of tributyl phosphate, TBP, as a useful extractant for cerium(IV), uranyl and thorium nitrates. After more than twenty years, liquid-liquid extraction systems (such as the Purex and Thorex processes) employing TBP dissolved in a suitable diluent versus an aqueous HNO3 phase remain the most widely accepted systems for reactor fuel reprocessing. 

Aqueous Process. In 1967-68, a hot reprocessing test had been conducted using the spent fuel (ca. 600 MWD/T) from JRR-3 (Japan Research Reactor-3) (6). About 200 g of purified plutonium was recovered by a modified PUREX process from aluminum-claded uranium fuels of natural isotopic composition. 

Natural zeolites have played important roles as in clean-up from nuclear accidents. After the Three Mile Island incident, the SDS (Submerged Dcmineraliser System) made use of a 60/40 mixture by volume of IE-96 and LTA zeolite (A-51) from the then Union Carbide Corporation to immobilise 340,000 Ci of fission products from >1.5 million gallons of water [128], Phillipsite tuff, from Pine Valley Nevada, clinoptilolite, A-51, and IE-96 have all been used at pilot plant scale to deal with high salt, high activity, aqueous wastes at West Valley, New York- site of the PUREX plant used for reprocessing nuclear fuels from 1966 to 1972. 

Figure 10.29 shows the principal steps in applying the Purex process to irradiated LMFBR fuel, step 7 of Fig. 10.28. The flow scheme and the compositions and locations of solvent, scrubbing, and stripping streams have been taken from the process flow sheet of a 1978 Oak Ridge report [Oil] describing a planned experimental reprocessing facility designed for 0.5 MT of uranium-plutonium fuel or 0.2 MT of uranium-plutonium-thoiium fuel per day. As that report gave process flow rates only for the uranium-plutonium-thorium fuel. Fig. 10.29 does not give flow rates for the uranium-plutonium fuel of present interest. This flow sheet shows the codecontamination step, in which flssion products are separated from uranium and plutonium the partitioning step, which produces an aqueous stream of partially decontaminated...

The main part of the HLLW is aqueous raffinate from the Purex cycle. It contains 99.9% of the nonvolatile FPs, <0.5% of the uranium, <0.2% of the plutonium, and some corrosion products. For each ton of uranium reprocessed about 5 m of HLLW is produced. This is usually concentrated to 0.5-1 m for interim tank storage specific activity is in the range 10 GBq m. The amounts of various elements in the waste and their concentration in 0.5 m solution is shown in Table 21.9. The HNO3 concentration may vary within a factor of 2 depending on the concentration procedure. The metal salt concentration is 0.5 M it is not possible to keep the salt in solution except at high acidity. The amounts of corrosion products, phosphate, and gadolinium (or other neutron poison added) also may vary considerably. Wastes from the HTGR and FBR cycles are expected to be rather similar. 

Some part of the spent fuel of atomic reactors is reprocessed separating uranium, plutonium, and the fission products, in order to produce new fissionable fuel or to collect some part of the valuable fission products. While several reprocessing methods have been proposed, the Purex process is the most widely used all over the world. The process uses 30% tributyl phosphate, TBP, as extractant in dodecane or kerosene solvent that is used to decrease the viscosity and the density of the liquid. The mixture is easily separated from water. The spent fuel is dissolved in concentrated nitric acid and the aqueous solution is mixed with the organic extractant. U and Pu present in the aqueous phase in the forms U02 and Pu are extracted to the organic phase, the fission products remain in the aqueous solution. After reduction of Pu by chemical or electrochemical method, Pu goes back to the aqueous phase, while the uranium remains in the organic phase (Benndict et al., 1981 Choppin et al. 1995 Katsumura 2004). 

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