The Palisades power plant is proposing a new design that officials hope will help end a recurring problem.
The heat generated by its nuclear reactor is restrained in part by 45 control rods. The rod mechanisms at Palisades have an uncommon design (one of only two plants in the country) and have had a lot more problems than at other plants.
All the control rod drive mechanisms were replaced in 2001. But a leak from one of the rods in 2012 forced the plant to shut down. Soon after, federal regulators issued the plant a violation for failing to prevent a “reoccurrence of a significant condition adverse to quality.”
A recent inspection prompted by that shutdown unveiled flaws in more than one out of every three rods.
“It is not good that they found flaws but, in a way, it’s a good story in that the inspection and the oversight program that was set up after the last issue worked,” Nuclear Regulatory Commission spokeswoman Viktoria Mitlyng said.
During the shutdown in 2012, Palisades replaced the leaking control rod drive mechanism. It tested eight others, which showed no flaws at the time.
Mitlyng says during this latest inspection the plant tested all 45 rods, 17 of which showed flaws. Two of those flawed rods had been tested in 2012. This test was a different kind of test than the ones completed in 2012. The flaws are not visible to the naked eye.
Palisades spokeswoman Lindsay Rose says the plant will replace 38 of the control rod drive mechanisms.
“It is certainly a proactive measure to be replacing some of the housing that did meet our inspection criteria,” Rose said, “We want to remove them and replace them with the better design that we have.”
She says the new design includes a different kind of stainless steel with fewer welds. Mitlyng says the design appears to have a different overlay and is more uniform.
“It’s complicated metallurgical stuff because it has to do with the place on the mechanism with how much oxygen can be trapped. It’s a particular design of the control rod drive mechanism housings that Palisades has. There a lot of factors here,” Mitlyng said.
When asked why the plant wouldn’t replace all 45 control rod drives with the new design, Rose said the plant said the replacement of 38 was “all we have the ability to do with this outage.”
Mitlyng says the NRC hasn’t determined whether the new design will be an improvement. But regulators are pleased the plant will replace the flawed rods.
“If the proposed solution can address some of these concerns we will have higher confidence that the design itself will help them avoid the same kind of problems in the future,” she said.
In the past, the Union of Concerned Scientists has argued that Palisades needs to resolves the underlying problems with the control rod drives. The organization says the NRC needs to do more to make sure the root cause is identified and fixed.
The root cause of the 2012 through wall crack and the flaws discovered in the latest inspection have not been determined.