Event Notification Report for April 28, 2022

AGREEMENT STATE REPORT - SHUTTER MALFUNCTION

The following report was received via e-mail from the Texas Department of State Health Services (the Agency):

"On April 28, 2022, the licensee notified the Agency that the shutter handle on one of its Ohmart Vega model SH-F2 gauges, containing a 200 milliCurie cesium-137 source, had malfunctioned. A service company was preparing the gauge for removal and had opened and closed the shutter. The shutter closed completely (verified by survey), but they could not lock the shutter handle. A repair kit will be ordered and upon receipt the service company will make the repair and complete the removal. The gauge is in an area of the plant that is no longer operational in a location that is inaccessible without scaffolding. The scaffolding being used at the time of this event will be removed until time for repair.

"There were no exposures as a result of this event. More information will be provided as it is obtained in accordance with SA-300."

Texas Incident Number: I-9927

EN Revision Imported Date: 5/3/2022

EN Revision Text: NOTICE OF UNUSUAL EVENT

The following is a summary of information provided by the licensee via telephone:

On 04/28/22, at 2355 EDT, with both Sequoyah Unit 1 and 2 in Mode-1, 100 percent, a Notice of Unusual Event was declared due to receiving two seismic alarms and security feeling ground movement. Additionally, security in a tower heard an explosion. Both units remain in Mode-1, 100 percent and they are investigating the validity of the seismic alarms before proceeding with the Abnormal Operating Procedure required shutdown.

The following additional information was obtained from the licensee in accordance with Headquarters Operations Officers Report Guidance:

The licensee will notify the NRC Resident Inspector. The state of Tennessee and the Tennessee Valley Authority were notified.

Notified DHS SWO, FEMA Operations Center, CISA Central, FEMA NWC (email), DHS NRCC THD Desk(email), and DHS Nuclear SSA (email).

* * * UPDATE ON 04/29/2022 AT 0410 EDT FROM BRIAN KLEIN TO OSSY FONT * * *

The following is a summary of information provided by the licensee via telephone:

On 4/29/22, at 0406 EDT, Sequoyah Unit 1 and Unit 2 terminated the Notice of Unusual Event. The Civil Engineers determined that the alarms were due to a failed seismic indicator channel. Through interviews, only one security officer felt ground movement for a couple of seconds and heard a faint rumbling sound.

The following additional information was obtained from the licensee in accordance with Headquarters Operations Officers Report Guidance:

The licensee will notify the NRC Resident Inspector. The state of Tennessee and the Tennessee Valley Authority were notified.

Notified R2DO (Miller), NRR EO (Miller), and IR MOC (Gott) via email.

Additionally, notified DHS SWO, FEMA Operations Center, CISA Central, FEMA NWC (email), DHS NRCC THD Desk(email), and DHS Nuclear SSA (email).

* * * RETRACTION ON 05/02/2022 AT 2118 EDT FROM SCOTT SEAL TO LLOYD DESOTELL * * *

The following information was provided by the licensee via email:

"SQN [Sequoyah Nuclear Plant] is retracting the previous NOUE [Notice of Unusual Event] declaration made on 4/28/22 at 2355 [EDT] based on Emergency Action Level HU2 for a seismic event greater than Operating Basis Earthquake levels. Following the declaration of the NOUE, the station reviewed all available indications and determined that a seismic event had not occurred. The instrumentation failure was documented under Event Notification #55867."

Notified R2DO (Miller), and IR MOC (Gott), NRR EO (Miller) via email.

LOSS OF ASSESSMENT CAPABILITY

The following information was provided by the licensee via fax:

"On 4/28/2022, at 2338 EDT, Sequoyah received an unexpected alarm for seismological recording initiated. At 2341 EDT, unexpected alarm 1/2 Safe Shutdown Earthquake response spectra exceeded was received. 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, `Emergency Plan Classification Matrix.'

"If an actual seismic event had 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."

The following additional information was obtained from the licensee in accordance with Headquarters Operations Officers Report Guidance:

The faulty detector was removed from service, so the remaining detector provides conservative detection as the only source to make-up the logic for a seismological alarm.