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.