Nuclear Reactors 301 - The NRC Performs Properly In Dealing With A Problem With Critical Valves At Nuclear Power Plants - Part 2 of 2 Parts
Part 2 of 2 parts (Please read Part 1 first.)
On February 11 of this year, the Unit 2 reactor at LaSalle was shut down for refueling. The operators used this shutdown as an opportunity to inspect and maintain emergency systems such as the HPCS. They tried to fill the HPCS system with water but were unable to fill the piping that led from the HPCS valve to the reactor vessel. Upon inspection of the gate valve, they discovered that one of the disks in the gate valve had broken free from the stem of the valve and was blocking the flow path. This valve is normally closed in the standby mode and it only opens when the HPCS system is turned on in an emergency. The motor turns the stem which raises the disk to open the pipe and lowers the disk to close the pipe. The disk that broke loose was in the closed position so the motor was unable to open it and permit the flow of water. The workers replaced the internal parts of the gate valve and restarted the Unit 2 reactor when the refueling was finished.
In April of 2017, Exelon contacted the NRC and informed them of the problem with the HPCS gate valve. A Special Inspection Team (SIT) was dispatched to carry out an investigation of the cause of the gate valve failure and the response that followed the discovery of the problem.
The SIT agreed with the conclusions of the LaSalle operators. A part inside the gate valve had broken due to severe stress. The broken part allowed the connection of the stem to the disks to become so misaligned that the disk separate from the stem. Anchor Darling has redesigned the internal parts of the gate valve to prevent such occurrences in the future.
Exelon to the NRC in early June that it intended to fix sixteen other problems with Anchor Darling gate valves that may suffer the same sort of failure of the HPCS system for Unit 2 the next time they shut down the two reactors for refueling. The SIT considered the explanations given by Exelon for waiting until the next refueling outage to repair the valve problems at their plant. The SIT agreed with their reasoning in fifteen of the sixteen problems. Exelon said that the Unit 1 gate valve in the HPCS system was much newer than the gate valve for Unit 2 and they felt that it would be safe to wait until they shut down the plant for refueling to check on it. The SIT said that there were significant differences between the two gate valves including how they were originally tested, some differences in design, unknown material strength properties and differences in thread wear that caused them to reject the delay in replacing the gate valve for Unit 1. They said that they felt confident that the gate valve would fail and that it would be dangerous to delay replacement. Unit 1 at the LaSalle plant was shut down on June 22 to replace the internal components of the valve for the HPCS system.
The SIT said that Exelon had violated Criterion III, Design Control, of Appendix B to 10 CFR Part 50 that involved torque values for the motors of the HPCS gate valves. Exelon mistakenly assumed that the weakest part of the gate valve was the stem and made sure that the torque of the motor would not overstress the stem. However, it turned out that another part was the weakest and the permitted torque of the motor damaged that part, permitting the disk to separate from the stem. Although the NRC considered the Exelon mistake to be a violation of their regulations, they did not issue a citation because they decided that the problem with the valve was so subtle that Exelon could be forgiven for missing it.
Looking this sequence of events and responses to them, how did the system perform in the case of the problem with these valves? First of all, the TVA reported a serious problem that they found. The manufacture investigated and found design flaws. The NRC sent out an advisory to the nuclear industry members that operated nuclear power plants. The NRC subsequently sent out a special team to investigate. They concluded that the owner of the nuclear power plant had performed properly given the subtly of the problem.
