Synthesis of UF 4 and ThF 4 by HF gas fluorination and re-determination of the UF 4 melting point

Abstract

Basic thermodynamic and electrochemical data of pure actinide fluorides and their mixtures are required for the design and safety assessment of any presently studied molten salt reactor concept based on molten fluoride salt fuel. Since the actinide fluorides are usually not produced commercially, they have to be prepared from the available input materials, typically oxides. In this work, a specially designed facility for synthesis of pure actinide fluorides using pure HF gas is described, as well as a complete procedure of synthesis and characterisation of pure UF4 and ThF4. The fluorination installation consists of a glove box kept under a purified argon atmosphere, a high temperature horizontal fluorination reactor and a HF supply gas line connected to the glove box. The fluorides were synthesised from high specific surface oxides prepared from the respective oxalates by low temperature calcination. The fluorination was partly stationary and partly in a HF gas flow, based on a heterogeneous powder-gas reaction at high temperatures. The products were characterised by X-ray diffraction and differential scanning calorimetry, which confirmed high purity products obtained by this method. Moreover, the melting point of UF4 was revised using a pure sample and a new value is suggested.