Millstone Unit 2 is facing citations for three “very low safety significance” violations that occurred Oct. 4 when a relief valve on the Waterford, CT, plant cooling system remained stuck in the open position and caused a leak of 16,570 gallons of reactor coolant.
A Nuclear Regulatory Commission (NRC) special inspection team issued its report last week after it visited the plant after the Oct. 4 incident.
The violations all rated as “green,” the lowest of the four categories of violation severity used by the NRC.
Three violations from a single event are “within the normal range of findings from special inspections” and all relate to failure to follow proper procedures, said Neil Sheehan, spokesman for the NRC.
“We did find some weaknesses, but they did not rise to the level where the safety of the plant was at risk,” he said.
The NRC declared the Oct. 4 event an “Unusual Event,” the lowest of four levels of emergency classification used by the NRC.
The event occurred as Unit 2 was entering a refueling and maintenance outage.
Sheehan said a valve that circulates borated water used to cool the reactor during an outage would not close, causing the water to pool in a tank.
The water is slightly radioactive, Sheehan said. He said none of the water spilled outside the plant, and there was no risk to plant workers or the public.
“The valve opened, and they were not able to get their hands around why it opened and how much spilled quickly enough,” he said. “They should have identified where it was coming from and taken steps to halt it sooner.”
Plant owner Dominion and NRC inspectors determined a gas void beneath the valve caused the malfunction. Workers have since replaced the valve.
The three violations were:
• Dominion’s failure to implement procedural steps to stop the leak sooner.
• The failure of the shift technical adviser to provide an “independent, objective and technical assessment of plant conditions” as a result of the incident. Sheehan said the adviser should have given advice about how to respond to the incident and did not do so.
• Dominion did not conduct verification testing before putting the shutdown cooling system into service. “This was the likely cause for the relief valve opening,” the NRC said.
Ken Holt, spokesman for Millstone, said Dominion entered a corrective action program with the NRC.
“Nuclear safety is our top priority, and we take the NRC’s conclusions very seriously,” he said. “We conducted a thorough root cause investigation to understand what happened, and we’re putting actions in place to make sure we don’t repeat it. The lessons learned have been incorporated into our operator training programs.”
Staff involved in the incident, including the shift technical advisor, were all “removed from duty” immediately after the Oct. 4 event and received additional training, Holt said. He declined to comment on their current status.
During 2015, Millstone Unit 2 ended up cited for three “green” violations by the NRC, while Millstone Unit 3 had three “green” violations and one “white” violation, the second-lowest level of the four categories.