Studies on hydrogen absorption-desorption properties of U-Th-Zr alloys for developing new reactor fuel materials

In order to develop a new U-Th mixed hydride fuel, 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 at temperatures from 773 to 1173 K and under hydrogen pressures from 10(2) to 10(5) Pa. Hydrogen capacities normalized by the sum of Th and Zr contents of all the specimens were similar to or higher than that of U-ZrH2-x alloy which has been utilized as a TRIGA fuel. Regarding the microstructure, the alloy hydrides consisted of three phases; alpha-U, ZrH2x 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 alpha-U was about 1 mu m 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. Hydrogen desorption properties examined on the 1:1:4 alloy showed better holding of hydrogen than ZrH2-x at 1173 K.