Development of a technique preventing B₄C diffusion during size reduction of spent control rods

Abstract

The spent control rod assemblies are to be processed as a waste for an underground disposal (depth≥70 m). The spent control rod assemblies (4 m in length) need to be cut into 4 sections in order to be stored in the storage containers (1.6 m x 1.6 m x 1.6 m). When the control rod is cut, the B4C powder contained inside the rod is leaked into the water. Contaminating the surrounding water would increase the secondary treatment cost, and consequently the overall processing cost. In this study, we examined a cutting technology that combines press working and underwater plasma cutting, in order to seal the cut control rods and prevent the B4C from being scattered into the water. We cut the control rods underwater in a vertical upright position, and confirmed the effects of B4C scattering. After cutting, the water ingress of the neutron absorbers was 52.8 mg per control rod. The B4C leaked during cutting was 3.6 g. The ratio diffuse of B4C from per control rod was 0.05 %. In conclusion, the results showed that the cutting technology used in this study is effective, and the B4C can be reduced considerably during processing.