Development of hydride absorber for fast reactor—application of hafnium hydride to control rod of large fast reactor—

The application of hafnium hydride (Hf-hydride) to a control rod for a large fast reactor where the B₄C control rod is originally employed is studied. Three types of Hf-hydride control rods are designed. The control rod worth and its change during the burnup are evaluated for different hydrogen-to-hafnium ratios and are compared with those of the original B₄C control rod. The result indicates that the worths of the Hfhydride and the ¹⁰B-enriched B₄C control rods are approximately the same, and the lifetime of the Hfhydride control rod is almost four times longer than that of the ¹⁰B-enriched B₄C control rod. The core performances of the shutdown margin, sodium void reactivity, Doppler reactivity coefficient, and breeding ratio are analyzed. It is indicated that those for the Hf-hydride control rod are almost the same as those for the original B₄C control rod. The behavior of neutrons moderated by the Hf-hydride control rod is analyzed. It is confirmed that the Hf-hydride control rod does not cause any thermal spike problems in the fast reactor core.