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Follow-up to Alert Emergency Action Level Declaration due to Loss of Offsite Power Resulting from a Seismic Event |
On August 23, 2011, an earthquake measuring 5.8 on the Richter scale with an epicenter about 14 miles (23 km) from the North Anna Power Station in Louisa, Virginia caused an automatic reactor trip of both units onsite. Units 1 and 2 entered cold shutdown conditions on August 24, 2011 and August 25, 2011, respectively. To demonstrate that no functional damage occurred as a result of the earthquake and that it was safe to operate the facility without undue risk to the health and safety of the public, the North Anna Power Station operator performed a number of inspections, tests, and analyses under the oversight of the United States Nuclear Regulatory Commission. On November 27, 2011 both units at the North Anna Power Station were returned to 100% power.
The North Anna Power Station, located in Louisa, Virginia, about 90 miles (145 km) southwest of Washington, D.C., declared an Alert, Emergency Action Level (EAL) due to a loss of offsite power following the earthquake. A number of other plants in the eastern United States declared a Notice of Unusual Event (NOUE) EAL due to seismic activity at their respective sites. Plants declaring NOUEs due to the potential for a decrease in plant safety included Peach Bottom, Three Mile Island, Susquehanna and Limerick in Pennsylvania; Salem, Hope Creek and Oyster Creek in New Jersey; Calvert Cliffs and a research reactor in Maryland; Surry in Virginia; Shearon Harris and a research reactor in North Carolina; and D.C. Cook and Palisades in Michigan. All of these plants exited their NOUE declarations after completing inspections and receiving confirmation of normal conditions.
Following the earthquake, both North Anna units experienced a complete loss of offsite alternating current (AC) power (LOOP) sources to emergency electrical buses due to the actuation of transformer pressure relays from the effects of seismic action. The four onsite emergency diesel generators (EDGs) started and powered the station emergency electrical buses after the LOOP. About 40 minutes after the EDGs started, one of the EDGs supplying power to the Unit 2 emergency buses, designated as the 2H EDG, was shut down by operators in response to a radiator leak. A backup, "station blackout" diesel generator was started to assume the loads that were being supplied by the 2H EDG. Normal offsite power was restored to the North Anna Power Station in the evening of August 23, 2011.
On the morning of August 24, 2011, North Anna downgraded from an Alert to an NOUE, the lowest of the four EALs, while the reactor cool-down and inspections of plant equipment and systems continued. The plant exited the NOUE on the afternoon of August 24 after completing all walkdown inspections of the equipment that is most susceptible to seismic activity. The inspections found that the equipment was in satisfactory condition. Due to additional seismic activity, North Anna declared an NOUE on two other occasions with the final one being declared on September 1, 2011. None of the aftershocks resulted in any impact to plant structures, systems, or components.
On August 29, the NRC dispatched an Augmented Inspection Team (AIT) to the North Anna Power Station to assess the circumstances surrounding the total loss of offsite power and dual unit reactor trip, the emergency diesel generator coolant leak and other plant equipment issues. North Anna Power Station AIT Report can be viewed at the following link:
http://adamswebsearch.nrc.gov/idmws/ViewDocByAccession.asp?AccessionNumber=ML113040031
Initial reviews conducted by the North Anna plant operator determined that the seismic activity exceeded the Design Basis Earthquake for the North Anna Power Station. Therefore, on August 26, the operator declared all safety-related SSCs of Units 1 and 2 inoperable for technical specifications purposes. Follow-up estimates of ground motion confirmed that the Safe Shutdown Earthquake (or Design Basis Earthquake) was exceeded for both units at the North Anna Power Station.
To demonstrate that no functional damage occurred as a result of the earthquake and that it was safe to operate the facility without undue risk to the health and safety of the public, the North Anna Power Station operator performed a number of inspections, tests, and analyses, consistent with EPRI NP-6695, "Guidelines for Nuclear Plant Response to an Earthquake." The plant operator also performed additional testing and inspections not included in the EPRI guidelines. The NRC staff's assessment employed the guidance provided in Regulatory Guide (RG) 1.167, "Restart of a Nuclear Power Plant Shut Down by a Seismic Event," which endorses, with exceptions, the Electric Power Research Institute's (EPRl's) NP-6695, "Guidelines for Nuclear Plant Response to an Earthquake." RG 1.167 can be viewed at the following link: http://pbadupws.nrc.gov/docs/ML0037/ML003740093.pdf.
The NRC issued a Confirmatory Action Letter (CAL) on September 30, 2011 restating the regulatory requirements associated with exceeding seismic design bases. The CAL confirmed the understanding that the plant operator was not to enter Modes 1 through 4 until the NRC has completed its safety evaluation and reported its conclusion to the plant operator in writing. The NRC technical evaluation of restart readiness can be viewed at the following links: http://adamswebsearch.nrc.gov/idmws/ViewDocByAccession.asp?AccessionNumber=ML11308B405; http://adamswebsearch.nrc.gov/idmws/ViewDocByAccession.asp?AccessionNumber=ML11308B406.
Amplifying information concerning the NRC actions in response to the August 23, 2011 earthquake in Virginia is available in the NRC's website at the following link: http://www.nrc.gov/about-nrc/emerg-preparedness/virginia-quake-info.html
The USNRC's participation in the International Nuclear and Radiological Event Scale is described in Information Notice 2009-27, dated November 13, 2009, Agencywide Documents Access and Management System Accession No. ML092510055. USNRC generic communications can be found on the USNRC public website http://www.nrc.gov, under NRC Library/Document Collections.
Event date: Tue, 23-08-2011
