TEM characterization of U-7Mo/Al-2Si dispersion fuel irradiated to intermediate and high fission densities

This paper will discuss the results of transmission electron microscopy (TEM) analysis performed on two samples taken from the low-flux and high-flux sides of the same fuel plate with U-7Mo fuel particles dispersed in Al-2Si matrix. The corresponding local fission density of the fuel particles and the peak fuel-plate centerline temperature between the low-flux and high-flux samples are 3.32 × 1027 f/m3 and 90 °C, and 6.31 × 1027 f/m3 and 120 °C, respectively. The results showed the presence of a bubble superlattice within the U-7Mo grains that accommodated fission gases (e.g., Xe). The presence of this structure helps the U-7Mo exhibit a stable swelling behavior during irradiation. The change in bubble distribution at the high fission density suggests that the bubble superlattice is stable as the U-7Mo matrix remains crystalline. It appears that there is a threshold Si content in the fuel particle above which the U-Mo turns to amorphous under irradiation. The threshold Si content is approximately 8 at.% and 4 at.% for low-flux and high-flux conditions, respectively.