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

This paper will discuss the results of TEM analysis that was performed on two samples taken from the low flux and high flux sides of the fuel plate with U-7Mo fuel particles dispersed in U-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 x 10{sup 27} f/m{sup 3} and 90 C, and 6.31 x 10{sup 27} f/m{sup 3} and 120 C, respectively. The results of this work 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 Si-rich interaction layers that develop around the fuel particles at the U-7Mo/matrix interface during fuel plate fabrication and irradiation become amorphous 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 lowmore » flux and high flux condition, respectively.« less