The U.S. Nuclear Regulatory Commission is continuing its heightened watch over nuclear power plants in the northeastern United States impacted by hurricane Sandy. Three reactors experienced shutdowns during the storm while another plant, Oyster Creek in New Jersey, remains on 'Alert' status owing to high water levels in its water intake structure.
The three reactors to experience trips or shutdowns during the storm are Nine Mile Point 1 in Scriba, N.Y., Indian Point 3 in Buchanan, N.Y.; and Salem Unit 1 in Hancocks Bridge, N.J.
Nine Mile Point 1 underwent an automatic shutdown at about 9 p.m. on Monday 29 October when an electrical fault occurred on power lines used to supply power to the grid. It is likely to have been a storm-related event, but the plant’s operators are still evaluating the cause. All plant safety systems responded as designed and the shutdown was safely carried out. Meanwhile, Nine Mile Point 2 experienced a loss of one of two incoming off-site power lines as a result of the fault. One of the plant’s emergency diesel generators started in response to generate power usually provided by the line. Nine Mile Point 2 remained at full power.
Indian Point 3 automatically shut down at about 10:40 p.m. on Monday 29 October in response to electrical grid disturbances caused by the storm. All safety systems responded as designed and the unit was placed in a safe shutdown condition.
Salem Unit 1 was manually shut down by plant operators at about 1:10 a.m. on the Tuesday as a result of circulating-water pumps being affected by high river level and debris in the waterway. The circulating-water system is used to cool down steam generated by the reactor; it is a closed system that does not come into contact with any radioactivity.
At Oyster Creek, the Alert was declared at approximately 8:45 p.m. An Alert is the second-lowest level of emergency classification used by the NRC. The Alert was preceded by an 'Unusual Event' at about 7 p.m. when the water level first reached a minimum high water level criterion. The water level rose owing to a combination of a rising tide, wind direction and storm surge. While the water level has dropped from its peak, the Alert will not be cancelled until the level is below the specific criteria for the intake structure, which is where water from an intake canal is pumped into the plant for cooling purposes. Oyster Creek was shut down for a refuelling and maintenance outage prior to the storm and the reactor remains out of service. As of 5pm on 30 October, water levels were beginning to subside to more normal levels, but the plant remains on Alert until there is enough confidence that levels will remain at more normal levels. Offsite power at the plant is in the process of being restored.
Meanwhile, three plants – Millstone 3, in Connecticut, Vermont Yankee, in Vermont, and Limerick, in Pennsylvania, – reduced power in advance of or in response to the storm. Millstone 3’s power was reduced to about 70 percent in advance of the storm to minimise potential impacts on its circulating water system due to the storm. Vermont Yankee reduced power to 89 percent in response to a request from the grid operator due to the loss of a transmission line in New Hampshire. Limerick Unit 1’s power was reduced to about 50 percent and Limerick Unit 2’s to about 25 percent in response to low electrical demands on the grid because of storm-related power outages.
The NRC stationed inspectors at all of the plants expected to experience the greatest effects of the storm. Those inspectors were tasked with independently verifying that operators were following relevant procedures to ensure plant safety before, during and after the storm. In addition, the NRC has been monitoring the storm from its emergency response centres.
Nuclear power plant procedures require that the facilities shut down under certain severe weather conditions. The plants’ emergency diesel generators are available if off-site power is lost during the storm. Also, all plants have flood protection above the predicted storm surge, and key components and systems are housed in watertight buildings capable of withstanding hurricane- force winds and flooding.
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