Development of U-Mo for applications on high density MTR fuels

The application of U-Mo alloys is considered in the RERTR (Reduced Enrichment for Research and Test Reactors) program as an alternative for obtaining MTR nuclear fuels with higher uranium densities than those obtained with U3Si2. The CCHEN Fuel Fabrication Plant has developed studies since 2002 to obtain U-Mo alloys through induction melting followed by physical, chemical and metallurgical characterization and studying the phase transformations that occurs in the binary U-Mo system. The alloys were developed and obtained through induction melting in Ar inert atmosphere and then poured from 1350oC to 1550oC. The chemical characterization was carried out using atomic absorption spectrometry (ICP) and X-ray fluorescence (FRX). Optical and electronic microscopy, together with microanalysis by dispersive energy (EDAX) was used for the metallurgical characterization of the alloys. The microstructure was identified, the macro-segregations were quantified, the microstructure and phase compositions were revealed and concentration profiles and presence and composition of precipitates were determined. The alloys UMo 7wt%, UMo 8wt% and UMo 10wt% were obtained and characterized without important macrosegregations, a cubic gamma phase like matrix was founded, metastable at room temperature and a minority and discontinuous phase located on the grain boundaries identified as alpha phase (orthorhombic). The study of the solid state phase transformations was carried out with thermal treatments of homogenization and isothermal cooling for the alloy U-7%Mo. The conditions were a homogenization temperature of 980oC, with isothermal cooling between 400oC and 590oC and times of 1 to 44 hours. The result was the experimental determination of the TTT-IT diagram for this alloy (author)