05000346/LER-2025-001, Required Shutdown for Steam Generator Low Level Instrumentation Reference Leg Steam Leak
| ML25282A050 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 10/09/2025 |
| From: | Craven R Vistra Operations Company |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| L-25-166 LER 2025-001-00 | |
| Download: ML25282A050 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3462025001R00 - NRC Website | |
text
L-25-166 October 9, 2025 A TIN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
Subject:
Davis-Besse Nuclear Power Station, Unit 1 Docket Number 50-346, License Number NPF-3 Licensee Event Report 2025-001-00 Davis-Besse Nuclear Power Station Robert B. Craven Site Vice President 5501 N. State Route 2 Oak Harbor, OH 43449 Office: 419.321.7676 10 CFR 50.73 Enclosed is Licensee Event Report (LER) 2025-001-00, "Required Shutdown for Steam Generator Low Level Instrumentation Reference Leg Steam Leak." This event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(A) and 10 CFR 50.73(a)(2)(i)(B).
There are no regulatory commitments contained in this letter or its enclosure. The actions described represent intended or planned actions and are described for information only. If there are any questions or if additional information is required, please contact Mr. Robert W.
Oesterle, Manager, Regulatory Compliance, at (419) 321-7462.
Sincerely, G:?L_
Robert B. Craven CDA Enclosure: LER 2025-001-00 cc: NRC Region III Administrator NRC Resident Inspector NRR Project Manager Utility Radiological Safety Board
Abstract
On August 12, 2025, at 1600 while operating in MODE 1 at 100% power, Davis-Besse Nuclear Power Station (DBNPS) operators entered Technical Specification (TS) 3.3.11 due to not meeting Steam Generator 1 Low Level Instrumentation channel requirements. DBNPS operators commenced a TS required shutdown at 1621 on August 12, 2025, and was reported as Event Number 57865. An uncomplicated shutdown was completed when DBNPS entered Mode 3 at 2121 on August 12, 2025. A common reference leg for two of the four Steam Generator Low Level Instrumentation channels developed a steam leak and degraded to where the associated channels were declared inoperable.
The reference leg steam leak was from a bonnet seal weld on an isolation valve. The valve was replaced during the subsequent plant shutdown. A past operability review of the event determined the level instrumentation was first inoperable greater than six hours prior to entering Mode 3.
This event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(A) as completion of a plant shutdown required by TS, and 10 CFR 50.73(a)(2)(i)(B) operating in a condition prohibited by TS.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
System Description
The Davis-Besse Nuclear Power Station (DBNPS) Steam Feed Rupture Control System [JB] designed safety function is to automatically start the Auxiliary Feedwater System [BA] in the event of a main steam line break, main feedwater line rupture, a low level in the steam generators (SG) [AB-SG], or a loss of all four reactor coolant pumps. The Steam Feed Rupture Control System Instrumentation consists of two independent and redundant actuation channels. The trip output of each complementary logic channel is combined in each actuation channel in a two-out-of-two logic, so the Steam Feed Rupture Control System will initiate an actuation channel trip when both complementary logic channels trip. Steam Generator Level-Low is monitored by four dedicated startup range level transmitters [AB-LT] per Steam Generator which are used to generate the signals for detection of low-level conditions within each Steam Generator. The associated Steam Feed Rupture Control System actuation channel will initiate a trip when two transmitters associated with the same Steam Generator sense a low-level condition.
The maximum level difference permitted between the four channels on a Steam Generator is 7.5 inches.
Technical Specifications:
DBNPS Technical Specification (TS) Limiting Condition for Operation (LCO) 3.3.11, Steam and Feedwater Rupture Control System (SFRCS) Instrumentation requires the SFRCS instrumentation channels for each Function in Table 3.3.11-1 be Operable as specified in the Table. For Function 3, Steam Generator Level - Low, the four required channels per each SG are required to be Operable while the plant is operating in Modes 1, 2, and 3 with an Allowable Value of 17.3 inches. With one or more Functions with one channel inoperable, TS LCO 3.3.11 Action A.1 requires the channel be placed in trip within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. With one or more Functions with two or more channels inoperable, or if the Required Action and associated Completion Time of Condition A not met, TS LCO 3.3.11 Action B.1 requires the plant be placed in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, and for Function 3 Action B.3 requires the plant be placed in Mode 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
DESCRIPTION OF EVENT
At approximately 1000 hours0.0116 days <br />0.278 hours <br />0.00165 weeks <br />3.805e-4 months <br /> on August 9, 2025, operators received an unexpected annunciator alarm for an Integrated Control System (ICS) Input Mismatch. Review of plant data on August 10, 2025, showed SFRCS Instrumentation SG 1 start-up level channels 1 and 3 rising and diverging from channels 2 and 4. One of the SG1 operate level instruments also showed a rising trend. Initial containment entries did not identify any failures on the accessible portions of the SG 1 level transmitters or sensing lines. Failure modes analysis identified the most likely cause of the instruments rising level as a leak on the shared reference leg. On August 12, 2025 at approximately 1300, footage from a remote drone inspection confirmed a steam leak from a reference leg isolation valve. At 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br /> on August 12, 2025, the two affected channels of SFRCS SG1 instrumentation were declared inoperable due to erratic readings decreasing margin to the permitted maximum level difference.
With two of the required four channels not operable for SFRCS function 3, LCO 3.3.11 was not met and conditions A & B were entered. At 1621, operators commenced a TS required shutdown which was completed when DBNPS entered Mode 3 at 2121 on August 12, 2025.
CAUSE OF EVENT
The DBNPS TS required shutdown was the result of two SFRCS SG 1 level transmitters declared inoperable for erratic readings. The indicated channel 1 and 3 erratic readings were due to a steam leak from their common reference leg. The steam leak in the common reference leg was from an isolation valve bonnet seal weld.
ANALYSIS OF EVENT
The two inoperable SFRCS SG 1 level instruments provide input to complementary SFRCS Logic Channels.
While a low level in SG 1 may not have been detected by SFRCS Logic Channels 1 and 3, a low SG 1 level would have been detected by SFRCS Logic Channels 2 and 4 to actuate SFRCS Actuation Channel 2. This would then initiate Auxiliary Feedwater flow to SG 2, trip the reactor via the Anticipatory Reactor Trip System (ARTS) and trip the main turbine. The four SFRCS level instruments for SG 2 were unaffected by this issue, so a low SG 2 level would result in initiation of Auxiliary Feedwater to both SGs along with tripping the reactor and main turbine. Therefore, no loss of safety function existed. Additionally, the SFRCS Feedwater/SG Differential Pressure-High trip remained available to sense a loss of Main Feedwater Pumps to isolate Main Feedwater flow to and Main Steam from both SGs and initiate Auxiliary Feedwater flow to both SGs. Since the SFRCS would have actuated for all designed initiating conditions, the plant risk associated with the leak on the SG 1 low level instrumentation reference leg was of very low safety significance.
Reportability Discussion:
The initiation of any nuclear plant shutdown required by the plants Technical Specifications is reportable within four hours of the event in accordance with 10 CFR 50.72(b)(2)(i). A shutdown commenced on August 12, 2025, at 1612 hours0.0187 days <br />0.448 hours <br />0.00267 weeks <br />6.13366e-4 months <br /> per TS LCO 3.3.11, this event was reported to the NRC Operations Center at 1758 hours0.0203 days <br />0.488 hours <br />0.00291 weeks <br />6.68919e-4 months <br /> as Event Number 57865. The plant completed the shutdown by entering Mode 3 at 2121 hours0.0245 days <br />0.589 hours <br />0.00351 weeks <br />8.070405e-4 months <br /> which is reportable per 10 CFR 50.73(a)(2)(i)(A) as completion of a plant shutdown required by the Technical Specifications.
A past operability review completed after the event determined the two SFRCS SG 1 level instruments first demonstrated inoperability at approximately 1042 on August 12, 2025. As a result, the instruments were inoperable for greater than six hours prior to entering Mode 3. Therefore, the plant operated in a condition prohibited by TS and is reportable per 10 CFR 50.73(a)(2)(i)(B).
CORRECTIVE ACTIONS
Completed Actions:
The leaking reference leg isolation valve was replaced during the subsequent plant shutdown, and the SG 1 level instruments were declared operable. Initial non-destructive examination identified a linear indication along the valves bonnet seal weld.
PREVIOUS SIMILAR EVENTS
There have been no DBNPS Licensee Event Reports related to a completion of a Technical Specification required plant shutdown within the past three years.
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