(389c) Salt-Bismuth Extraction Pyroprocessing for Molten Salt Thorium Fueled Reactors

The Molten Salt Reactor (MSR) is a high temperature, liquid fuel, fluoride salt based nuclear reactor concept. It employs a thorium-uranium fuel cycle and is designed to sustain its energy production via breeding the fissile U-233 from fertile Th-232. The MSR was initially developed and successfully operated at Oak Ridge National Laboratory (ORNL) from the 1960s to the early 1970s. The program was cancelled in 1972 in favor of focusing support onto the liquid sodium cooled Integral Fast Reactor (IFR). While sodium cooled fast reactors such as the IFR are still being considered for future development by the Department of Energy, the MSR concept has also gained renewed interest from a variety of sectors.  Notably, this includes commercial electric utility companies.

The reactor relies on continuous (on-line) reprocessing of its liquid fuel. There are two main goals of on-line pyroprocessing – to clean the fuel of neutron poisons (lanthanides); and to isolate protactinium – the newly bred precursor to fissile uranium. An additional objective of pyroprocessing is to return actinides back into reactor for incineration and to prevent them from reaching waste streams designated for lanthanides (fission products).

The focus is put on the isolation of protactinium (and uranium) from the thorium-rich salt matrix. One particular unit operation (reductive extraction) is being investigated by our group. This step requires efficient separation of relatively small amounts protactinium (and uranium) into molten bismuth from a molten fluoride salt with a high concentration of thorium fluoride. Either lithium or thorium metals can act as reductants. Electrochemical sensors are proposed to monitor the composition of the molten salt in this and other unit operations of the pyroprocessing scheme.

Calibration curves were initially developed for using voltammetry to measure the concentration of thorium and uranium in the molten salt.  This was needed to provide real time analysis of the concentrations during extraction experiments.  Calibration curves were linear over the range of concentrations of interest.  Then reductive extraction experiments were performed in which U/Th loaded fluoride salt was contacted with liquid bismuth containing lithium as the reducing agent.  The results of these experiments are significant for calculating the size and residence time requirements for the extraction unit operations and will be reported in this talk.  Selectivity between U and Th extraction is also important, and initial results of co-extraction experiments will be presented.