Behavior and distribution of nuclides in the fluoride volatility process of uranium containing molten salt fuel

Abstract

The recovery of uranium from FLiBe-ZrF4 (LiF-BeF2-ZrF4: 65–30–5 mol%) molten-salt containing irradiated UF4 and ThF4 by the fluoride volatility process (FVP) was carried out. Gamma radiography was applied for in-situ monitoring of the fluorination and desorption steps successfully. The results confirmed that it was feasible to monitor the FVP by detecting the changes of the γ activities of key nuclides in the molten salt and the NaF adsorber. The behavior and distribution of actinides and fission products in each unit of the FVP were investigated by the determination of their activities with the γ-ray spectroscopy. The results indicated the conversion and volatilization rate of UF4 was greater than 99.8%. And the recovery yield of uranium was approximately 80% in two cold traps. A small amount of uranium remained in the molten salt, others was found in the NaF trap and the NaF adsorber. In addition, small amounts of uranium were also found in the off-gas, in the desorption step. As for 95Zr, 140La, 140Ba, 141Ce, 143Ce and 233Pa, existing as nonvolatile fluorides, remained in the fluorinated salt with the overall process gamma (γ) decontamination factors (DFs) in the UF6 product greater than 105∼106. Meanwhile, 99Mo, 131I and 239Np existing as volatile fluorides, were found in the UF6 product, NaF absorber and off-gas with the overall process γ DFs in the UF6 product less than 10. In the case of 103Ru and 132Te also existing as volatile fluorides, the overall process γ DFs in the UF6 product still reached approximately 105∼106 due to their separation from the UF6 product by the NaF adsorber.