Development of Advanced Control Rod of Hafnium Hydride for Fast Reactors

Boron carbide (B{sub 4}C) has been mainly used in fast reactors as a control rod. The lifetime of B{sub 4}C control rods is limited by Pellet-Cladding Mechanical Interaction (PCMI) failure due to the swelling of B{sub 4}C pellet. Helium, which is generated by the nuclear reaction of B{sup 10}, causes the pellet swelling. Application of hafnium (Hf) hydride as a control rod is discussed in this paper, since the generation of He{sup 4} is not an issue in the nuclear reaction process of Hf with neutrons. A control rod of Hf hydride is considered for use in fast reactors ,since fast neutrons generated in the driver fuel region are moderated and are efficiently captured in the Hf-hydride region of a control rod assembly. The nuclear reaction process of Hf with neutrons is expressed by Hf{sup d77}(n,{gamma})Hf{sup d78}(n,{gamma})Hf{sup d79}. The Hf{sup d78}, which is generated by a neutron capture reaction of Hf{sup d77}, has also a large neutron capture cross section. This implies that the Hf hydride control rod is superior to the B{sub 4}C control rod in performance of long life use. The Hf-hydride control rod has been designed with the extended life of five to ten years in whichmore » the experimental data of the Zr-hydride were mostly referred due to the lack of data of Hf-hydride. The researches on Zr-hydride have been carried out much more than those on Hf-hydride since Zr-hydride is widely employed as a neutron moderator in nuclear reactors. (authors)« less