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| Entered date | Event description | |
|---|---|---|
| ENS 54977 | 1 November 2020 11:04:00 | At 0556 EST on 11/01/2020, Sequoyah received unexpected alarms for seismological recording initiated and (Units) 1/2 Safe Shutdown Earthquake response spectra exceeded. No seismic event was felt on site, the National Earthquake Information Center was contacted to confirm there was no seismic activity, and this was also confirmed on the U.S. Geological Survey website. The alarms were determined to be invalid, and they occurred due to a failure in the seismic monitoring system. This failure results in loss of ability to assess the Emergency Action Level for Initiating Condition HU2 'Seismic event greater than Operating Basis Earthquake (OBE) levels' per procedure EPIP-1. If an actual seismic event occurred, HU2 could not be assessed. However, compensatory measures have been implemented and include assessing OBE criteria based on alternative criteria contained in procedure AOP-N.05 'Earthquake' which provides conservative guidance when seismic instruments are unavailable. This is an eight hour, non-emergency notification for an event resulting in a major loss of Emergency Assessment Capability. This event is reportable in accordance with 10 CFR 50.72(b)(3)(xiii). There is no impact on the health and safety of the public or plant personnel. The NRC Resident Inspector has been notified. |