A safety evaluation review and decision are expected in November

epicenterThe NRC's review of the effects of the August 23 earthquake on Dominion's North Anna power station is coming to a logical and reasonable close. The agency says the staff is writing a safety evaluation review that could result in a re-start decision letter by mid-November.

At a meeting of the full commission on October 21, NRC staff presented the findings of an Advanced Inspection Team (AIT). That report was completed Oct 3.

The NRC wrote in its "FAQ" about the earthquake . . . "ground motion experienced by plant structures . . . may have exceeded design [but] does not appear to appreciably encroach on built-in seismic design margin of safety related structures and components."

The bottom line is that the plant responded well to the event with no apparent damage to safety significant systems nor to other systems and buildings generally.

NRC official Martin J. Virgilio told the Richmond Times Dispatch Oct 18, “The plant actually rode it [the earthquake] out pretty well.”

Not moving tons of paper

Dominion VP Eugene Grecheck told this blog in an exclusive interview immediately following the NRC’s Oct 21 meeting that "the utility is pleased we don't have to re-analyze the entire plant."

He added that the NRC commissioners asked good questions during the Oct 21 meeting focused on technical reports provided by staff.

NRC spokesman Scott Burnell confirmed to this blog Oct 21 that following a public meeting in Mineral, VA, on November 1 to review the agency's findings, that staff will write the safety evaluation review and prepare the decision letter. If all goes well, it seems reasonable to think North Anna could be back in the business of providing power to its customers by Thanksgiving.

seismograph2 The August 23 earthquake that shut down the nuclear reactors at Dominion's North Anna power station created questions throughout the nuclear industry about how the reactor would be re-started given the amount of ground acceleration it experienced from the event.

As it turns out, the seismic design basis for the plant assumes an event duration of 15-30 seconds when in fact the actual duration was less than five seconds.

See also Indiana UniversityEarthquake Measurement

The two nuclear reactors at North Anna Power Station in Mineral, Va., automatically shut down following a 5.8 magnitude earthquake in Central Virginia on Aug. 23, 2011, at 1:51 p.m. ET. The epicenter was approximately 11 miles west-southwest from the station.

The earthquake’s vibrations exceeded North Anna’s design analysis at some frequencies, but no significant damage has been seen. Analysis shows that the earthquake’s intensity and duration were not sufficient to cause significant damage at the station.

Reason the reactors shut down

Nuclear reactor simulator (ANL)So why did the reactor trip? Grecheck says that the reactors shut down because instruments at the plant detected a negative flux and a corresponding drop in power of greater than five percent. Vibration of the reactor core from the earthquake caused changes in the characteristics of the coolant and its moderation of the neutrons.

See also WNNReactor Core Simulation” [image right]

Assuming all goes well and the safety evaluation review and resulting decision by the NRC allow Dominion to re-start its reactors, what does the decision mean for the future of seismic information and the design basis for nuclear reactors in the U.S.?

For starters, the Electric Power Research Institute (EPRI) in cooperation with the U.S. Geologic Survey and the NRC are developing new models to assess earthquake effects on reactors especially those on the east coast. Grecheck points out that current models are mostly based on west coast seismic information. See also New York Times "After quake, Virginia nuclear plant takes stock"

"The east coast rock is different," Grecheck said, "It's tighter and the waves travel further from the center of a quake." See also CNN "Why quake rang like a bell"

earthquakeNRC spoksman Burnell says that a generic information document, GI-199 (NRC briefing slides) will incorporate the new EPRI models and be complete by the end of 2011.

The full technical document is out for public review. Grecheck said Dominion hasn't finished the calculations that are part of its technical assessment of the model yet so it is too early to comment on it.

The issue of how plants ride out seismic events is one of seven key issues for the NRC following its review of the Fukushima crisis. On October 20 the NRC staff provided a proposal to the Commission that selected seven recommendations as most appropriate for immediate action.

The recommendations cover issues including the loss of all A/C electrical power at a reactor (also called “station blackout”), reviews of seismic and flooding hazards, emergency equipment and plant staff training.

Speed over time

The most important point in Dominion's briefing of the NRC commissioners on Oct 21, Grecheck said, is that while measuring acceleration is important, duration must also be taken into account when assessing whether a seismic event exceeds a reactor's design basis.

Grecheck said he's not anticipating major changes in the instrumentation of plants to measure seismic effects.

"It would be helpful to get all the information in one place at one time more quickly," he said.

Power may be back on soon

Overall, Grecheck expressed a certain amount of relief that the end of the post earthquake review is in sight.

Dominion told the Richmond Times Dispatch Oct 21 it has spent about $21 million to ensure the North Anna nuclear power station is safe to operate again. The cost of replacement power has been in the range of $1-2 million/day.

North Anna generates 1,806 MW from its two units — enough electricity to power 450,000 homes. Unit 1 began commercial operation in June, 1978 and Unit 2 followed in December 1980.

Dominion submitted a Combined Operating License (COL) application to the Nuclear Regulatory Commission on Nov. 27, 2007. If approved, the COL will give the company permission to build and operate the new nuclear unit, called North Anna 3.

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