The Nuclear Regulatory Commission (NRC) has begun a special inspection at the Callaway Nuclear Plant to review circumstances following a reactor shutdown involving the failure of three of four control valves that regulate water flow to the steam generators.
The plant is operated by Ameren Missouri.
Following a reactor trip on Aug. 11, all systems performed as expected, including the automatic start of a system that controls water flow to the steam generators. However, when operators tried to switch to the motor-driven water pumps, a control valve failed to operate. Internal circuitry in the control valve system had been previously modified in late 2014, apparently introducing a flaw in the design which resulted in the failure.
The NRC learned that another control valve in the system had been similarly modified and also experienced a failure in Dec. 2014. The affected valves were repaired and tested prior to the plant being restarted on Aug. 12. Additionally, the NRC is aware of a third unrelated control valve failure in the same system earlier this year that had already been corrected.
"The purpose of this special inspection is to better understand the circumstances surrounding the valve failures, determine if the licensee's extent of condition review was sufficiently comprehensive, and review the licensee's corrective actions to ensure that the causes of the failures have been effectively addressed," NRC Region IV Administrator Marc Dapas said.
NRC inspectors will spend about a week on site looking into outstanding questions with respect to the licensee's testing, maintenance, design change, and corrective action processes specific to these valves and the associated system. They will also evaluate the licensee's root cause analysis, extent of condition review, and the adequacy of corrective actions.
An inspection report documenting the team's findings will be publicly available within 45 days of the end of the inspection.