No abstract

Ternary uranium molybdenum alloys are being examined for use as dispersion and monolithic nuclear fuels in research and test reactors. In this study, three such ternary alloys, with compositions U-10Nb-4Zr, U-8Mo-3Nb, and U-7Mo-3Ti in wt.%, were examined using scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM) with high angle annular dark field (HAADF) imaging via scanning transmission electron microscopy (STEM). These alloys were homogenized at 950°C for 96 h and were expected to be single-phase bcc-c-U. However, upon examination, it was determined that despite homogenization, each of the alloys contained a small volume fraction precipitate phase. Through SEM and XRD, it was confirmed that the matrix retained the bcc-c-U phase. TEM specimens were prepared using site-specific focused ion beam (FIB) in situ lift out (INLO) technique to include at least one precipitate from each alloy. By electron diffraction, the precipitate phases for the U-10Nb-4Zr, U-8Mo-3Nb, and U-7Mo-3Ti alloys were identified as bcc-(Nb,Zr), bcc-(Mo,Nb), and bcc-(Mo,Ti) solid solutions, respectively. The composition and phase information collected in this study was then used to construct ternary isotherms for each of these alloys at 950°C.

This study examined the growth kinetics of intermetallic phases that develop in solid-tosolid diffusion couples assembled with U-7, 10 and 12wt.%Mo vs. Al alloys (Al, Al-2wt.%Si, Al- 5wt.%Si, 4043 and 6061) after a diffusion anneal at 550°C for 24 hours. Based on interdiffusion microstructure and integrated interdiffusion coefficients, the addition of Si into the Al matrix alloy was observed to significantly reduce the growth rate of the intermetallic layer that primarily consisted of (U,Mo)Al4 phase. Growth rate of the (U,Mo)Al4 intermetallic layer also increases slightly with Mo content; however, it was not significant compared to the effect of alloying Si into Al alloys. Growth kinetics of (U,Mo)Al4 intermetallic layer appear highly sensitive to composition of U-Mo fuel alloy and Al cladding alloys, and must be an important criteria in alloy development/selection for optimum fuel performance with due consideration for compositiondependent multicomponent interdiffusion.