Development of advanced control rod of hafnium hydride for fast reactors

Boron carbide (B₄C) has been mainly used in fast reactors as a control rod. The life-time of B₄C control rods is limited by Pellet-Cladding Mechanical Interaction (PCMI) failure due to the swelling of B₄C pellet. Helium, which is generated by the nuclear reaction of B¹⁰, causes the pellet swelling. Application of hafnium (Hf) hydride as a control rod is discussed in this paper, since the generation of He 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, sincefast 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 ¹⁷⁷7(n,γ)Hf¹⁷⁸(n,γ)Hf¹⁷⁹. The Hf¹⁷⁸, which is generated by a neutron capture reaction of Hf ¹⁷⁷, has also a large neutron capture cross section. This implies that the Hf hydride control rod is superior to the B₄C 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 which 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.