Revisiting the Chemistry of Uranium and Rhenium Fluorides: Implication to Fuel Cycle and Nuclear Forensics Science

Fluorine plays a major role in the nuclear industry where F2(g) and HF(g) are critical to the preparation of UF6 and UF4. These materials are prepared for uranium enrichment to increase the isotopic concentration of 235U. During separation efforts, the isotope 99Tc (a high yield fission product, 6.1% from 235U) is extracted along with uranium and can be converted to binary technetium fluorides (i.e., TcF4, TcF6) during re-enrichment of used uranium material. This provides a route for 99Tc to reenter the reactor environment and affect the nuclear properties of fuel pellets made with reprocessed uranium. The objective of this work is to prepare and revisit the chemistry of uranium and rhenium fluorides related to fundamental, nuclear fuel cycle and nuclear forensic applications. Rhenium is used as surrogate material for technetium provided that they show similar chemical behavior. Challenges in the preparation of a number of rhenium fluoride materials are discussed. This was conducted with the aim to isolate binary rhenium fluorides as surrogate materials for the later work on technetium fluorides, and to compare their structures to other metal fluorides. Several routes for the isolation of UF4 were investigated using wet chemistry and dry fluorination. Using tube furnace and thermal gravimetric analysis, the decomposition of (NH4)4UF8 was studied at various temperatures under argon and for the first time, the decomposition products were characterized by powder XRD and scanning electron microscope. The results showed the successive formation of (NH4)3UF7, (NH4)2UF6 and UF4. These materials showed unique morphology. Uranium isotopic ratios were verified using thermal ionization mass spectrometry and the characterization reveal many problems in the measurements of uranium fluoride materials. This work presents an updated procedure for the preparation of (NH4)2ReF6. The structure, morphology and spectroscopic properties of (NH4)2ReF6 was investigated. The decomposition of (NH4)2ReF6 was also reinvestigated. In an alumina crucible, (NH4)2ReF6 reacted with the crucible and resulted in the formation of mixed ReO2 phases. In a platinum crucible, the decompostion of (NH4)2ReF6 produced only metallic rhenium. An updated procedure for the preparation of the alkali (K+, Rb+, Cs+) [ReF6]2- is presented. These materials are isostructural to their technetium analogue and their structure consists of distorted [ReF6]2-octahedra, a result also supported by Raman spectroscopy. Altogether, this work reports detail description on the synthesis and characterization of rhenium and uranium fluorides, and provides promising chemical, isotopic, and morphological signatures for considerations in nuclear forensics and nonproliferation studies. From this, useful information related to processing and intended use of nuclear material can be extracted for nuclear forensics investigations