VERNON — The Vermont Yankee nuclear power station reduced power to 46 percent this week after one of the two recirculation pumps malfunctioned.
According to VY spokesperson Larry Smith, on Sunday at 11 p.m., the motor generator unit stopped working in a reactor recirculation pump (RRP), called “Bravo” by engineers.
The failure had “no health, safety, or nuclear implications,” said Smith.
Recirculation pumps are critical for electricity production, said U.S. Nuclear Regulatory Commission spokesperson Neil Sheehan, adding VY is “compliant at this point” with NRC regulations.
Boiling-Water Reactors (BWR) like VY's circulate large quantities of water through the reactor as part of power production, said Sheehan. The motor generator units provide the horsepower to move the water.
Sheehan said the RRPs are part of electricity production and separate from the safety-related cooling water system.
The water circulated by the pumps belongs to a “completely separate” closed-loop system. It does not draw from, or deposit water into, the river, he added.
By virtue of its contact with the fuel rods, Sheehan said, the water circulated by the RRPs is “slightly radioactive.”
According to Sheehan, engineers worked Monday and Tuesday to shift the double-loop circulation system to a “single-loop operation.”
“Entergy made a decision to place the plant into single-loop operation, using one reactor [“Alpha”] recirculation pump, while repairs to the “B” reactor recirculation pump continue,” said Sheehan.
To shift to a single loop mode, said Sheehan, VY engineers had to complete a number of steps checking safety limits and settings within 12 hours.
The NRC has lifted the 24-hour limited conditions of operation (LCO) it placed on VY after the plant reported the Bravo pump had stopped working, said Sheehan.
This means VY had until 11 p.m. on Monday night to fix the pump, or shift to a single-loop operation. If plant engineers had not managed to make repairs by the deadline, said Sheehan, the plant would have been shut down until engineers completed repairs.
Working together, the Bravo and Alpha pumps keep the plant generating electricity at 100 percent, said Smith. So when one pump ceases operating, the plant's electrical generation capacity drops to 50 percent or less.
According to Smith, engineers lowered power production further while working on the Bravo pump. As of Tuesday afternoon, he said, VY was generating at 47 percent power.
Plant employees are “investigating the cause of the problem,” said Smith, adding that he did not have a timeline for how long the pump would remain down.
Sheehan said NRC resident inspectors continue to monitor the repair process.
The NRC assigns different time frames to LCOs, depending on the safety significance of the affected plant components. Time frames range from 24 hours to 14 days.
Prior to the RRP issue, VY's power production had dropped to 90 percent, said Smith.
With the October scheduled refueling around the corner, VY had already entered a “coast down” phase, said Sheehan and Smith.
Coasting down occurs as a result of the reactor slowly running out of fuel, said Smith.