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The following information was provided by the licensee via phone and email: On November 10, 2024, at 0337 CST, Grand Gulf Nuclear Station (GGNS) was operating in mode 1 at 100 percent power when a manual scram was initiated due to degrading main condenser vacuum. The cause of the degrading main condenser vacuum is not known at this time and is being investigated. All control rods fully inserted and there were no complications. Reactor pressure was initially maintained with main turbine bypass valves. Reactor water level was initially maintained with main feedwater and condensate. At 0457, operators transitioned pressure control to safety relief valves and began using reactor core isolation cooling (RCIC) to maintain reactor water level. This was performed using plant procedures due to degrading vacuum. GGNS is currently in mode 3. Reactor level is being maintained with RCIC and pressure is being maintained using the safety relief valves. The manual reactor protection system (RPS) actuation is being reported in accordance with 10 CFR 50.72(b)(2)(iv)(B) and the RCIC actuation is being reported in accordance with 10 CFR 50.72(b)(3)(iv)(A). The NRC Senior Resident Inspector was notified. The following additional information was obtained from the licensee in accordance with Headquarters Operations Officers Report Guidance: At the time of the notification, main steam isolation valves had shut on low vacuum.

• * * UPDATE ON 11/10/24 AT 1236 CST FROM MELISSA LIMBECK TO KERBY SCALES * * *

The following update was provided by the licensee via phone and email: This update is being made to report the following occurrences which took place after the scram reported in event number 57418. On November 10, 2024, at 0545 CST, a group 1 containment isolation signal resulted in the closure of all MSIVs. The signal was due to continued degradation of condenser vacuum post-trip. At 0620, an automatic RPS actuation occurred when reactor water level lowered to level 3. This RPS actuation occurred with all control rods fully inserted. Reactor water level lowered following closure of an open safety relief valve and was recovered to within the established band. The events are being reported as specified system actuations in accordance with 10 CFR 50.72(b)(3)(iv)(A). The NRC Senior Resident Inspector has been informed of the update. Notified R4DO (Dixon)

The following information was provided by the licensee via phone and email: At 1822 CST on September 27, 2024, Grand Gulf Nuclear Station (GGNS) was conducting surveillance testing on the high pressure core spray (HPCS) division Ill diesel generator. Following initiation of the test signal, the HPCS pump room cooler start time exceeded the surveillance procedure allowance of less than or equal to 20 seconds. The HPCS pump room cooler started in 26.2 seconds. HPCS was already inoperable for performance of the surveillance testing. The event is being reported in accordance with 10 CFR 50.72(b)(3)(v)(D) as an event or condition which could have prevented the fulfillment of a safety function. Troubleshooting is in progress. HPCS, a single-train system, will remain inoperable until the condition is corrected. All sources of offsite power are available. No other safety systems are inoperable. Reactor core isolation cooling was verified to be operable per GGNS technical specification 3.5.1.B.1. The NRC Senior Resident Inspector has been notified.

• * * RETRACTION ON 11/25/2024 AT 1333 EST FROM JEFF HARDY TO IAN HOWARD * * *

Investigation of the delayed start time of the HPCS pump room cooler indicated that the condition would not have challenged the ability of the room cooler to maintain temperatures less than the temperature limit of 150 degrees Fahrenheit. As a result, HPCS remained capable of fulfilling its safety function. Therefore, EN 57349 is being retracted. The NRC senior resident inspector has been notified of this retraction. Notified R4DO (O'Keefe)

The following information was provided by the licensee via email: At 0146 CDT on June 7, 2024, River Bend Station (RBS) was operating at 100 percent power when a loss of control room envelope (CRE) was declared due to failing to meet Technical Specification (TS) 3.7.2, Surveillance Requirement (SR) 3.7.2.4, during surveillance testing. Mitigating actions were established which included the ability to issue potassium iodide to control room staff. With mitigating actions in place, the dose consequence to control room staff continued to be less than the regulatory limit of 5 rem total effective dose equivalent for the duration of a design basis event. The CRE is considered a single train system at RBS, therefore, this event is being reported in accordance with 10 CFR 50.72(b)(3)(v)(D) as an event or condition that could have prevented the fulfillment of a safety function. 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 failed surveillance (SR 3.7.2.4) was for unfiltered air in-leakage greater than 300 cubic feet per minute.

• * * RETRACTION ON 08/05/2024 AT 1456 EDT FROM DARREN FARTHING TO ROBERT THOMPSON * * *

This event was initially reported under 10 CFR 50.72(b)(3)(v)(D) as an event or condition that could have prevented the fulfillment of a safety function. The licensee determined in a subsequent engineering evaluation of the conditions that existed at the time, that there was no adverse impact on the control room emergency ventilation system or the control room envelope (CRE) boundary's ability to perform its safety function. The CRE would not have been challenged to meet the regulatory limit of 5 rem total effective dose equivalent for the duration of a design basis event. Consequently, this condition is not reportable as an event or condition that could have prevented the fulfillment of a safety function. The NRC resident inspector has been notified. Notified R4DO (Vossmar).

The following information was provided by the licensee via fax or email: At 2342 CDT on August 19, 2022, with Grand Gulf Nuclear Station in Mode 1 and at 40 percent power, the station initiated a normal shutdown to comply with its Technical Specifications (TS). The station entered Mode 3 at 0000 CDT August 20, 2022 to comply with (LCO) 3.5.1 Condition G Action G.1 due to the condition reported to NRC previously (EN 56058). This event is being reported under 10 CFR 50.72(b)(2)(i) as a shutdown required by the plant's technical specifications. The NRC Senior Resident Inspector was notified. The following additional information was obtained from the licensee in accordance with Headquarters Operations Officers Report Guidance: The plant is now in a 36-hour LCO to be in Mode 4 due to Low Low Set Valves inoperability per TS 3.6.1.6.

• * * RETRACTION ON 10/14/2022 AT 1311 FROM JEFF HARDY TO LAUREN BRYSON * * *

Grand Gulf Nuclear Station (GGNS) is performing this notification to retract event EN 56059 that was reported on August 20, 2022. Previously, GGNS notified the NRC that it had initiated a shutdown required by Technical Specifications to comply with Limiting Condition of Operation (LCO) 3.5.1 Condition G.1 due to the inoperability of four Automatic Depressurization System (ADS) valves. Following the shutdown, GGNS completed walkdowns and determined that the condition affected only one ADS valve. As a result, the shutdown to satisfy the required actions of TS LCO 3.5.1 Condition G.1 was not required. The NRC Resident Inspector has been notified of the retraction. R4DO (Kellar) was notified.