Integrated AMP–PAN, TRUEX, and SREX testing. II. Flowsheet testing for separation of radionuclides from actual acidic radioactive waste

Abstract

Three separation processes for the removal of selected fission products, actinides, and Resource Conservation and Recovery Act metals (mercury and lead) have been integrated successfully and tested using actual acidic radioactive waste at the Idaho National Engineering and Environmental Laboratory (INEEL). The separation processes integrated were ion exchange for 137Cs removal, followed by TRUEX solvent extraction for actinide, Hg, and 99Tc removal, and subsequent SREX solvent extraction for 90Sr and Pb removal. A flowsheet comprising these three processes is being developed at the INEEL to reduce the activity of acidic tank waste to allow disposal, after immobilization, as an NRC Class A LLW. Approximately 1350 mL of actual INEEL tank waste was first processed through an ion exchange column for selective Cs removal. The column was packed with a composite ammonium molybdophosphate–polyacrylonitrile (AMP–PAN) sorbent. The ion-exchange system was operated at 26 bed volumes per hour and was sized at a bed volume of 2 cm3. A 137Cs removal of 99.95% was obtained in the ion exchange system without notable interference from other species. The effluent from the ion-exchange (IX) system was stored and subsequently processed several weeks later through a TRUEX solvent extraction flowsheet to separate actinides, Hg, and 99Tc from the tank waste. The TRUEX flowsheet test was performed utilizing 23 stages of 2.0-cm diameter centrifugal contactors. Removal efficiencies of 99.2%, 94.7%, and 63% were obtained for total alpha, Hg, and 99Tc, respectively. Operational problems such as flooding and/or precipitate formation were not observed during the TRUEX flowsheet test. The raffinate from the TRUEX test was stored and subsequently processed several weeks later through a SREX solvent extraction flowsheet to separate 90Sr and Pb, from the tank waste. The SREX flowsheet test was performed using the same centrifugal contactors used in the TRUEX test after reconfiguration. Approximately 99.997% of the 90Sr and 98% of the Pb were extracted with the SREX flowsheet and recovered in the strip and wash sections. In addition, approximately 93% of the remaining alpha activity was extracted and recovered in the strip section. Operational problems such as flooding and/or precipitation formation were not observed during the SREX test.