Chapter 7 - Salt cleanup and waste solidification for fission and fusion reactors

Abstract

This chapter has provided by Dr. Charles Forsberg of Massachusetts Institute of Technology, Nuclear Engineering Department, who this author has collaborated with and together they coauthored many publications. Dr. Forsberg has presented this chapter in 2018 International Congress on Advances in Nuclear Power Plans (ICAPP 20180; Charlotte, NC, USA April 8–11, 2018). Research and development is underway on three classes of nuclear reactors that use salt: (1) the title of presentation was “Salt cleanup and waste solidification into iron phosphate or borosilicate glass with halide recycle for fission and fusion salt reactors.” Fluoride-salt-cooled high-temperature reactors with clean fluoride salt coolants and solid fuel; (2) molten salt reactors (MSRs) with fuel dissolved in either a fluoride or chloride salt; and (3) salt-cooled fusion reactors with fluoride salts for cooling, tritium production, and shielding. These reactors require salt coolant cleanup systems for corrosion control and removal of impurities (corrosion products, activation products, and fission products) with solidification of the waste products for disposal. From 1950s to the 1970s, there was significant work on salt processing associated with molten salt reactor programs—but until recently there was little new research on salt purification and conversion of halide wastes into acceptable waste forms. Since the 1970s, major developments in related fields have created the technology base for advanced salt cleanup and waste solidification processes—the backend of salt reactor fuel cycles. We describe pathways from (1) the molten salts in the reactor systems to (2) separations with recycle of salt to the reactor and a waste salt stream to (3) conversion of waste salts into final waste forms. The separation options include distillation, electrochemical, and other processes. Waste form requirements depend upon (1) the chemical and radioisotopic composition, (2) laws and regulations, and (3) disposal site waste acceptance criteria. For high-level wastes, the waste treatment options include converting waste salts into iron phosphate or borosilicate waste glasses with recycle of the chloride (especially if chloride-37 is used) or fluoride anion. Iron phosphate and borosilicate are the standard chemical forms for disposal of high-level wastes in geological repositories. Significant work will be required to sort out preferred options and address major uncertainties.