(18c) Separation of Cesium and Strontium from Spent Lwr Fuel and Immobilization of the Product Using Steam Reforming Technology

As part of the Advanced Fuel Cycle Initiative, two solvent extraction technologies are being developed at the Idaho National Laboratory to simultaneously separate cesium and strontium from dissolved spent nuclear fuel. Separation of the Cs and Sr will reduce the short-term heat load in a geological repository and, when combined with the separation of Am and Cm, could increase the capacity of the geological repository by a factor of approximately 100. The first of the two developed Cs/Sr separation processes utilizes a solvent consisting of chlorinated cobalt dicarbollide and polyethylene glycol extractants in a phenyltrifluoromethyl sulfone diluent. Recent improvements to the process include development of a new, non-nitroaromatic diluent and development of new stripping reagents, including a regenerable strip reagent that can be recovered and recycled. Countercurrent flowsheets have been designed and tested on simulated and actual spent nuclear fuel feed streams with both cesium and strontium removal efficiencies of greater than 99%. The second process developed, the Fission Product Extraction (FPEX) process, is based on two highly-specific extractants: 4,4?,(5?)-Di-(t-butyldicyclo-hexano)-18-crown-6 (DtBuCH18C6) and Calix[4]arene-bis-(tert-octylbenzo-crown-6) (BOBCalixC6). The DtBuCH18C6 extractant is selective for strontium and the BOBCalixC6 extractant is selective for cesium. Laboratory test results of the FPEX process, using simulated feed solution spiked with radiotracers, indicate good Cs and Sr extraction and stripping performance.

It is anticipated that the separated Cs/Sr product stream will be immobilized and enter decay-storage, for approximately 300 years, at which time the activity of the Cs and Sr will be below the limits for low-level waste. Steam reforming is one option currently being considered for stabilization of this stream because it can potentially destroy the nitrates and organics present in these aqueous, nitrate-bearing solutions, while converting the Cs and Sr into leach resistant aluminosilicate minerals. A bench-scale steam reforming pilot plant has been operated with several potential feed compositions and steam reformed product has been generated and analyzed.