Immobilization of Radioactive Salt Waste into Silica-Based Waste Form via Chemical Conversion

Abstract

The molten salt waste from a pryochemical process to recover uranium and transuranic elements is one of the problematic radioactive wastes to be solidified into a durable waste form for its final disposal. This paper suggests a new method as a dechlorination approach to the immobilization of salt waste. The inorganic composite consists of SiO₂, Al₂O₃, and P₂O₅ (SAP), which can generate a series of reactive sites for metal chlorides when in contact with molten salt. Under an oxidative condition, metal chlorides were successfully dechlorinated and converted into metal aluminosilicate, metal aluminophosphate, and metal orthophosphate, which are manageable at a high temperature. The optimum mixing ratio of SAP/salt in weight is about 2, and a borosilicate glass shows good compatibility with the reaction products containing phosphate compounds. By using a glass, a highly monolithic waste form was successfully fabricated at 1100°C, and more than 33 wt% of mixing ratio of glass as a chemical binder increased the chemical durability of the waste form. Use of SAP as an effective stabilizer can offer a chance to avoid Cl-induced problems and control the vaporization of volatile elements. This allows a high degree of freedom in the fabrication of monolithic waste form.