Quake-hit Virginia nuclear reactors "in an unanalyzed condition that significantly degrades plant

Data shows quake may have exceeded design basis


[link to www.nrc.gov]


Power Reactor Event Number: 47201
Facility: NORTH ANNA
Region: 2 State: VA
[...]
Event Date: 08/26/2011
Event Time: 13:00 [EDT]
[...]
Event Text
UNANALYZED CONDITION BASED ON SEISMIC ACTIVITY THAT POTENTIALLY EXCEEDED DESIGN BASIS EARTHQUAKE MAGNITUDE

On August 23, 2011 at 1351 hours, North Anna Power Station experienced a seismic activity event which resulted in a loss of offsite power and automatic reactor trip of both units. At 1403 hours, an Alert was declared. based on Shift Manager judgment, due to significant seismic activity on the site. Subsequent to the earthquake, both units were stabilized and offsite power was restored. Following the event, seismic data was retrieved from the installed monitoring system and shipped to the vendor to determine the response spectrum for the event. On August 26, 2011 at 1340 hours, initial reviews of the data determined that the seismic activity potentially exceeded the Design Basis Earthquake magnitude value above 5 Hz. Therefore, this is reportable per 10CFR50.72(b)(3)(ii) (B) for the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety.

North Anna Unit 1 is currently in Cold Shutdown with the Residual Heat Removal System providing core cooling. North Anna Unit 2 is currently in Hot Shutdown and will be taken to Cold Shutdown with the Residual Heat Removal System providing core cooling. No significant equipment damage to Safety Related system (including Class 1 Structures) has been identified through site walk-downs nor has equipment degradation been detected through plant performance and surveillance testing following the earthquake. Therefore, there is reasonable assurance that the Safety Related systems are fully functional. [...]