Microstructural dependence on fuel matrix composition in irradiated U-Mo dispersion fuels

Abstract

Microstructural characterization of U-Mo dispersion fuel plates irradiated in the RERTR-7 experiment in the Advanced Test Reactor was performed to study the dependence of fuel matrix composition in irradiated U-Mo fuels. The evolution of the fission gas pores and fuel matrix interaction in U-7Mo fuel plates with pure Al and Al-2wt%Si matrix compositions were characterized using optical and scanning electron microscopy in conjunction with image processing techniques. In addition, the combined contribution of Mo composition, fission power, and temperature was studied. Image analysis allowed determination of the fission gas pore (FGP) size, morphology and distribution, as well as the growth of the fuel matrix interaction (FMI) layer. The isolated effect of the Al matrix on the microstructural evolution of U-7Mo fuels revealed that the growth evolution of FGPs as well as the FMI layer is influenced by the matrix. The mean FGP area and FMI growth are 31% and 25% higher in U-7Mo/Al compared to U-7Mo/Al-2Si dispersion fuel, suggesting that U-Mo dispersion fuel with a thicker FMI layer (low thermal conductivity) has the potential to grow larger FGPs in the U-7Mo, possibly because of the higher temperature in the fuel meat. Previous studies have shown that there is an inverse relationship between Mo composition and the FMI, as a result, the microstructural response of U-10Mo and U-7Mo was also investigated and compared. Because the FMI layer thickness is 40% higher in the U-10Mo specimen than the U-7Mo specimen, the combined effect of increased fission power, and temperature for the U-10Mo sample may counteract the propensity of U-Mo fuels with increased Mo content to have a decreased amount of FMI.