SIDEBAR 3.1: REPROCESSING
The RT-1 plant extracts plutonium and uranium from SNF using the PUREX process. PUREX is an aqueous process in which the declad and crushed fuel matrix is dissolved in nitric acid yielding a feed to a multi-stage cascade that extracts and strips uranium and plutonium (U and Pu). The solutions contain high concentrations of particulates (graphite, silicon, and others) in suspension, so the solution is clarified with filters and organic flocculants. U and Pu (and, currently, neptunium) are extracted with an organic solvent, tributyl phosphate (TBP) in a saturated hydrocarbon similar to kerosene, which leaves behind essentially all of the other constituents. Nitric acid with a reducing agent strips the Pu from the TBP and contact with dilute nitric acid strips the U back into the aqueous phase. RT-1 repeats this process twice. The PUREX process is very effective at recovering nearly all the U and Pu, leaving insignificant levels (one part in one hundred million) of residual contamination with fission products and minor actinides, but the process generates large amounts of waste and cannot separately recover (fractionate) other constituents for recycling or specialized disposal. Radiolysis and chemical processes degrade the TBP, which must be continuously purified, and this purification process also generates large volumes of waste. Equipment choices, such as centrifugal contactors, can achieve some reductions in volume by promoting faster reaction resulting in less exposure and fewer radiolytic effects, but the clarification and extraction processes still generate large amounts of waste.
As processing of SNF continues in Russia, and particularly if the program is to expand to accept VVER-1000 and RBMK SNF and SNF of international origin, the Russian Federation must examine and pursue ways to improve the radiochemical processes employed to carry this out. Other schemes might improve the characteristics of the process with respect to the environment, proliferation (theft of special nuclear material), safety, and economics. These aspects all must be examined.
Several enhancements and alternatives to PUREX are close to being ready for production-scale deployment. These include UREX, TRUEX, volatilization using AIROX; dry reprocessing technologies using flourination, or electrochemical separation in molten salt; and several others.
UREX is a modification of the front end of the PUREX process that uses the reagent AHA (acetohyroxamic acid) to complex Pu and reduce its valence so that the Pu will remain in the aqueous phase when the uranium is extracted into TBP, as in PUREX. This allows high-purity recovery of the U from SNF, leaving the Pu with the minor actinides and fission products. UREX is attractive in systems that keep Pu and the minor actinides together for proliferation resistance and actinide burning (see, e.g., the integral fast reactor concept with pyroprocessing, or transmutation in general with other partitioning techniques).