Research and development is underway on three classes of nuclear reactors that use salt: (1) Fluoride Salt-Cooled High-Temperature Reactors (FHRs) 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 1950 to the 1970s there was significant work on salt processing associated with MSR programs—but until recently 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 separations options include distillation, electrochemical and other processes. Waste form requirements depend upon (1) the chemical and radio-isotopic composition, (2) laws and regulations and (3) disposal site waste acceptance criteria. For high-level wastes (HLWs), 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 HLWs in geological repositories. Significant work will be required to sort out preferred options and address major uncertainties.