Application of Sulfide Process to Fuel Debris Treatment

In nuclear reactor accident at Fukushima NPP, the reactor core was damaged by the severe conditions such as very high temperature and sea water injection, forming a huge amount of nuclear fuel waste so called debris fuel. Since UO2 fuel pellets are packed in the zircaloy cladding and the fuel assemblies were charged in the reactor core, the fuel debris consists of many types of the mixture of nuclear fuel and structural materials. Some of them are the mixture of metal and oxide, or mixtures after melting. Sulfurization method is one of the promising techniques for the separation and recovery of rare metals. Especially, the dry process has an advantage for the reduction of radioactive waste in nuclear engineering field. In this paper, sulfuization behavior of uranium oxides, zirconium oxide and metal as fuel and structural materials was studied by using carbon disulfide for the sulfurization treatment of fuel debris components. No reaction was observed for Uranium oxides in the presence of CS2 at T<773 K, while metal components, such as zirconium metal shows sulfurization at lower temperature. After the selective sulfurization treatment, separation of sulfurized materials from oxides or metals was examined by acid leaching method. Application of mechanochemical treatment was considered for the size reduction and effective sullfurization. When the mixture of simulated fuel debris was mechanochemically treated, the interaction of zirconium with UO2 seems to occur forming the UO2-ZrO2 solid solution.