Preventive measures for the recurrence of control rod insertion failure in Japan Research Reactor No.4 (JRR-4)

JRR-4 Operation Division; Research Reactor Utilization Division

Japan Research Reactor No.4 (JRR-4) was shut down manually, due to the control rod insertion failure occurred during the rated power (3,500kW) operation on June 10, 2005. It became evident by the investigation that a screw bolt at the control rod support got loose and blocked the control rod insertion. The failure was recovered through replacement with the new screw bolt. Considering the importance of this event, we decided to inspect all screw bolts over the core that may cause a control rod insertion failure. Furthermore, we decided to carry out periodical inspection about these screw bolts whether they were tightened enough or not. This report describes the result of inspection carried out as the preventive measures.