The Nuclear Regulatory Commission (NRC) announced on Sept. 21 that it has begun a special inspection at the Callaway nuclear plant in Fulton, Mo., to investigate the failure of three of four control valves that regulate water flow to the plant’s steam generators.

The review follows a reactor trip that occurred on August 11. According to the NRC, 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 that resulted in the failure.

The NRC learned that another control valve in the system had been similarly modified and also experienced a failure in December 2014. The affected valves were repaired and tested prior to the plant being restarted on August 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.

Callaway is a 1,190-MW station, operating a single-unit pressurized water reactor that went commercial in 1984. Ameren Missouri—owner and operator of the facility—scrapped plans for a second unit at the location in early August.

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. The inspectors will also evaluate the licensee’s root cause analysis and extent of condition review, and the adequacy of corrective actions.

Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)