Distribution and behavior of fission product ${}^{95}$Nb in FLiBe salt

The ${}^{235,238}$UF${}_4$ was irradiated by photo-neutrons, distribution and behavior of the fission product ${}^{95}$Nb from irradiated ${}^{235,238}$UF${}_4$ in FLiBe salt were investigated by the measurement of its activity in the salt with the γ-ray spectroscopy. The experiments indicated that a part of ${}^{95}$Nb deposited on the surfaces of graphite and Hastelloy, as the moderator and the structural materials of molten salt reactor (MSR), respectively, and the majority of ${}^{95}$Nb maintained in molten salt. Addition of lithium metal made ${}^{95}$Nb in salt to be reduced and settled, leading to the decrease in its activity. Degree of the decrease was found to be correlated with niobium concentration. The experimental results supported the statement proposed early by ORNL, that ${}^{95}$Nb might be used as a redox indicator for MSR. Finally, the problem met with on-site monitoring for redox potential in MSR was pointed, and a possible protocol to resolve the problem was proposed.