The U.S. Nuclear Regulatory Commission said it had begun a special inspection of the Callaway nuclear power plant in Fulton, Mo., following a reactor shut down in August that involved the failure of three of four control valves that regulate water flow to the plant's steam generators.
The plant is operated by Ameren Missouri.
The events leading up to the inspection included a reactor trip on Aug. 11. During the event, all systems performed as expected, the NRC said. The system that controls water flow to the steam generators started automatically. However, when operators tried to switch on the motor-driven water pumps, a control valve failed to operate.
The problem, the NRC said, began with modifications to the control valve system's internal circuitry, dating back to 2014. That modification introduced a flaw in the design that caused the failure.
In addition, the regulator said that it had learned of another control valve in the system that had been similarly modified, which caused a failure in December 2014.
The affected valves were repaired and the plant resumed operations on Aug. 12. But the NRC said a third similar incident – and a subsequent correction – occurred, as well.
The comprehensive inspection underway, which will take about a week, is meant to “determine if the licensee’s extent of condition review was sufficiently comprehensive” and review steps Ameren Missouri undertook to ensure safe operations going forward.
The inspection findings will be made public in 45 days.