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.

Control rod blades are comprised of a stainless steel sheath, which contains neutron absorber tubes (filled with boron carbide powder). During decommissioning, the first stage of size reduction consists of cutting the connector (bottom portion) of the control rod, while the second stage consists of separating the blades of the control rod by cutting through the tie rod. The last stage consists of segmenting the control rod blades by cutting through absorber tubes. In this study, the control rod blades segmentation (last stage of size reduction) is investigated using an actual control rod (unused). During the experiments, we used a forming press on the cut locations followed by a plasma arc cutting underwater. The purpose of this cutting technique is to minimize the scattering of boron carbides into water by using the stainless sheath melt to seal the absorber tubes. After the segmentation, we confirmed the sealing of the absorber tubes by visually examining the cut cross-sections. The water analysis showed that the boron carbide scattering was relatively low (only 0.07% of the total boron carbides was scattered). Finally, we confirmed that the off-gas emission is considerably reduced by using Argon plasma instead of Argon-Hydrogen plasma.