A laboratory-scale process for producing dilithium beryllium tetrafluoride (FLiBe) with dissolved uranium tetrafluoride

Abstract

Flibe Energy, Incorporated (FEI)'s conceptual Lithium Fluoride Thorium Reactor (LFTR) incorporates a chemical processing facility aimed at recovering uranium and other valuable volatile radionuclides while managing harmful radionuclides from the used fuel. The fuel utilized in this reactor is a combination of dilithium beryllium tetrafluoride (Li2BeF4 or FLiBe) and uranium tetrafluoride (UF4), (FLiBe/U). FEI's plan involves extracting the uranium and other valuable volatile fluoride-forming radionuclides using nitrogen trifluoride (NF3). To facilitate laboratory-scale testing of uranium extraction using NF3 and address the toxicity and physical hazards associated with beryllium and beryllium fluoride (BeF2), we used a two-step process to prepare the simulated fuel salt. The first step entailed thermally decomposing ammonium beryllium tetrafluoride [(NH4)2BeF4] (ABeF) through a nominal 3-step process, combined with appropriate amounts of lithium fluoride (LiF) and UF4, resulting in the formation of beryllium fluoride (BeF2). In the second step, the mixture was repeatedly melted and frozen at the melting point of FLiBe to prepare the eutectic FLiBe with dissolved UF4. Although the concept appears straightforward, the production of FLiBe/U involved various challenges. These challenges included transporting the gaseous decomposition products of ABeF, hydrogen fluoride (HF) and ammonia (NH3), while preventing the formation of ammonium fluoride (NH4F). Additionally, it was necessary to control the reaction between the higher-than-anticipated water content in the commercial ABeF with NH3, HF, and the condensed NH4F, protect UF4 from forming an unknown black compound, select suitable structural materials to mitigate fluoride corrosion, address the risks associated with beryllium toxicity through equipment design and operational protocols, and monitor process conditions. This article provides an account of the thermal decomposition chemistry observed in the commercial ABeF, describes the FLiBe/U production apparatus, describes the experiences and process refinements developed to prepare FLiBe/U, and presents our characterizations of prepared FLiBe/U.