On May 10 2019, 10:31, during low power physics test to measure the control rods worth, reactor power rapidly increased up to 18.06%. Due to the heat transfer from the reactor coolant system, the level of SGs (steam generators) increased beyond the H-H level set-point and thus it caused generation of main feedwater isolation and all MFWPs trip signal. As the results, motor-driven AFWPs were actuated to provide feedwater to the SGs.
Followings are the detailed sequences of events. Firstly, one of the control rods was deviated from demand position due to the mis-operation of reactor operator. Secondly, during trouble shooting of the problem, one of the control rod assembly showed stuck phenomenon. Thirdly, a control bank was withdrawn carelessly, and fourthly, the reactor power rapidly increased up to 18.06%, beyond limiting value (5%) of TS (technical specification) LCO (Limiting condition for operation). In this case, the reactor should have been immediately shutdown according to the required action of corresponding article of TS LCO. But, recognizing the abnormal condition, the operator manually inserted the control rods instantly instead of opening the reactor trip breaker as required in TS LCO. The reactor power thus decreased to the zero power state. Approximately 11 hours after the transient, the reactor was manually shutdown for the inspection and investigation.
As results of the event investigation, it was confirmed that the step deviation of the rod was caused by operator’s mis-understanding of rod manipulation, the withdrawal of the control bank was caused due to the improper maintenance work process for the trouble shooting. The failure to meet the required action of TS LCO was caused by omitting pre-job meeting when the shifts turned over and insufficient understanding of the TS LCO during the test. It was assessed that temporary stuck of the M6 control rod resulted from latch jam or CRUD (Chalk River Unidentified Deposit).
With regard to the impact of the event, there was no adverse effect on the plant safety, no radiation exposure to the workers, or no release of radioactive materials to the environment.
Event date: Fri, 10-05-2019