NRC raps Yankee on procedure
In August, one of the plant's 22 cooling cells collapsed. Less than 10 days later, a failure in a turbine stop valve caused an automatic shutdown of the plant.
The NRC was concerned because the lack of an adequate maintenance program for the boiling water reactor's turbine stop valves "has a cross-cutting aspect in the area of human performance," wrote the NRC.
In addition, the cooling tower collapse "has a cross-cutting aspect in the area of problem identification and resolution because Entergy did not implement and institutionalize operating experience through changes to station processes and procedures."
A "cross-cutting" condition is defined as a condition that affects many different aspects of a procedure or program. In essence, if there is a deficiency in inspection and maintenance programs for both the cooling towers and the turbine stop valves, the NRC is concerned that there be similar problems in other systems at Yankee.
"Neither the cooling tower cell that collapsed nor the turbine (and its stop valves) are considered nuclear safety-related," wrote NRC spokesman Neil Sheehan in an e-mail announcing the results. Nonetheless, wrote Sheehan, "the cross-cutting aspect will be considered in our next annual assessment for Vermont Yankee, which is due out next March."
The NRC also promised to review any corrective actions performed by Entergy technicians to remedy the deficiencies "during an upcoming inspection," wrote Sheehan.
"We agree with the NRC," said Rob Williams, spokesman for Vermont Yankee. "Although all of the findings are of low safety significance, we take them seriously and we are putting in place the corrective actions necessary to help insure that they do not happen again."
The NRC wrote that Entergy failed to integrate "readily available operating experience" into Yankee's cooling tower inspection process.
The NRC requires nuclear plant operators to circulate information on issues that arise at plants around the nation. Technicians at plants with similar cooling towers as those at Yankee had noticed that using remotely operated cameras to inspect the interiors of the structures was inadequate and operating experience reports recommended that technicians conduct hands-on visual inspections.
Entergy's failure to recognize the importance of conducting hands-on inspections of Vermont Yankee's cooling towers contributed to the collapse of one of the plant's 22 cooling cells, the NRC stated.
The cooling cell collapsed because vertical support columns failed for a number of reasons, wrote the NRC, including "a chemical iron-salt attack related to iron bolting used to connect the wooden columns, a biological fungal attack in areas that the wooden columns had been affected by the iron-salt and from overtightened bolts at spliced locations."
But all of that could have been prevented had Entergy technicians not relied on cameras to inspect the interior of the cooling cells. In addition, if Entergy had taken into account the operating experience of other plants with similar cooling towers, technicians could have prevented the failure.
"Although the operating experience had been received ... no actions had been developed for Entergy's cooling tower inspection programs," wrote the NRC in the inspection report. "Multiple opportunities existed for Entergy to identify and incorporate the recommendations."
The NRC advised Entergy to take corrective actions such as conducting a physical inspection of the cooling towers, repairing any deficient structures, incorporating operating experience into its preventive maintenance program and completing a review of the plant's corrective action procedures.
The inspection report also addressed a turbine stop valve failure that caused an automatic shutdown on Aug. 30.
In that case, wrote the NRC, Entergy failed to perform "adequate preventive maintenance" on the valve which should have included disassembly and inspection of the bell crank mechanism, which controls the opening and closing of the valve.
The NRC recommended that Entergy create a preventive maintenance program to address the turbine stop valve failure.
During troubleshooting of the turbine stop valve, technicians observed that the bell crank assembly was sticking. When the bell crank control linkage was manually operated, the assembly "suddenly freed, causing the turbine stop valve to open faster than normal."
This action caused a fluctuation in the hydraulic system controlling the four turbine stop valves, which in turn caused the valves to slam shut and resulted in the automatic shutdown.
Although maintenance was performed on the valves during a refueling outage in May, Entergy's failure to inspect, rebuild and lubricate the bell crank assembly "constituted a performance deficiency," wrote the NRC.
Both the cooling tower and turbine stop valve findings were classified "green," or a very low safety significance, for which Entergy received non-cited violations. A non-cited violation is not punishable nor requires follow-up by the NRC.
Nonetheless, both findings were of concern to the NRC because they were in contradiction to "the cornerstone objective of limiting the likelihood of those events that upset plant stability and challenge critical safety functions during shutdown as well as power operations."
Bob Audette can be reached at email@example.com or 802-254-2311, ext. 273.
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