Study on an innovative fast reactor utilizing hydride neutron absorber - Fabrication and high temperature behavior of hafnium hydride pellets

Fabrication tests of hafnium hydride pellets were performed for several sizes in a diameter range of 5 to 10 mm and a height range of 2 to 10 mm in order to establish the fabrication process for hafnium hydride neutron absorber Hafnium metal pellets were hydrogenated at high temperature while controlling the amount of the supplied hydrogen to get hafnium hydride pellets with several different hydrogen to hafnium ratios. The samples were expanded by hydrogen absorption and slight barrel shape transformations were observed. No cracks, however, were observed in the obtained hafnium hydrides. The dependence of density on the hydrogen to hafnium ratio showed good reproducibility without any effects from the experience of hydrogen desorption and/or re-hydrogenation process. This indicated that the hydrogen content can be adjusted easily in the fabrication process of hydride pellets. Out-of-pile experiments were performed at high temperature using the obtained hafnium hydrides to evaluate the integrity of the control rod with hafnium hydride absorbers during heat cycle tests between 500 to 700 °C and temperature transient tests at 25 to 30 °C/min. After the temperature transient tests and the heat cycle tests, the integrity and the hydrogen to hafnium ratio of hafnium hydrides were evaluated. No cracks were observed in any hafnium specimens after these tests. The amount of hydrogen lost from the hydrides was negligibly small during both heat cycle and temperature transient tests. This indicated that the heat cycle and the temperature transient had negligible effects on the integrity of hafnium hydride.