Simultaneous measurement of lithium isotope and lithium/beryllium ratios in FLiBe salts using MC-ICP-MS

Abstract

Fluoride salt-cooled high-temperature nuclear reactors (FHR) use solid nuclear fuel and molten salt as a coolant for energy production. The mixture of 2LiF–BeF₂ (FLiBe) is used as the heat transfer fluid in the reactor primary loop due to favorable thermophysical properties and nuclear cross sections. While the salt composition is defined by the stoichiometric formula (Li₂BeF₄), variations in the Li/Be ratio can have large effects on the properties of the salt. Previous methods used to characterize salt composition are well-suited for trace element contaminant levels, but typical uncertainties in concentration measurements result in high uncertainty (∼5–8%) in the Li/Be ratio. Measurement of elemental ratios in analyte materials can be achieved through either direct concentration measurements or isotope dilution. Certain elements, such as beryllium, preclude the use of isotope dilution for concentration determination for practical reasons. Here, we provide a new method for high precision characterization of the Li/Be ratio in FLiBe using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). To achieve this, we produced gravimetric standards with independently characterized Li/Be ratios as reference standards for use in sample-standard bracketing analytical techniques. Results for FLiBe with previously reported compositions show that the Li/Be ratios measured by MC-ICP-MS are more precise and accurate than traditional concentration measurements. When combined with trace metal analysis by sector field (SF) ICP-MS, the new method provides a path forward for additional characterization of synthesized FLiBe for use in molten salt nuclear reactors.