Comparison of sphere-pac and pellet (U,Pu)O2 fuel pins in low burnup instrumented irradiation tests

Three irradiation tests were conducted in the Oak Ridge Research Reactor to evaluate and compare the low-burnup (0.5% FIMA) performance of Sphere-Pac and pellet fuels. The stainless-steel-clad (U, Pu)O2 fuel pins were fabricated at a smear density of 82% of theoretical and irradiated in a fully instrumented capsule. The capsule position was varied to control fission heating, and the fuel center temperatures, cladding temperatures, and heat generation rates were measured. The data from these tests showed, at the 99% confidence level, that the thermal conductance of the Sphere-Pac fuel pin was 11.7 ±1.2% better than that of the pellet fuel pin. This result was attributed to better heat transfer across the fuel-cladding interface with the Sphere-Pac fuel form. In addition, the Sphere-Pac fuel was more compatible with cladding than the pellet fuel, even though the maximum cladding temperature was higher on the Sphere-Pac fuel pin. Both fuel types restructured thermally at the same temperatures.