Development of new reactor fuel materials: hydrogenation properties of U-Th-Zr alloys and neutron irradiation effects on their hydrides

Hydrogen absorption properties of four U-Th-Zr alloys with compositions of 2:1:6, 1:1:4, 1:2:6 and 1:4:10 in U:Th:Zr ratio were examined for developing a new U-Th mixed hydride fuel. It was revealed by hydrogen absorption measurement that for all the specimens hydrogen capacities normalized by the sum of Th and Zr contents were similar to or higher than that of the U-ZrH2-x alloy, TRIGA fuel, at temperatures from 773 to 1073 K and under hydrogen pressures from 102 to 105 Pa. Regarding the microstructure, the alloy hydrides consisted of three phases; α-U, ZrH2-x and ThZr2H7-x, which are finely and homogeneously mixed with each other probably because they were formed from one solid solution phase stable at high temperatures. In the case of Th-rich alloys the α-U was about 1 μm in diameter and dispersed in the bulk of ThZr2H7-x and ZrH2-x. Such microstructure is quite similar to that of U-ZrH2-x fuel. The hydrides irradiated to 7.4 x 1023 n/m2 in the reactor showed a high phase stability. (orig.)