05000458/LER-2022-004-01, High Pressure Core Spray Inoperable Due to Transformer Failure
| ML23319A436 | |
| Person / Time | |
|---|---|
| Site: | River Bend  |
| Issue date: | 11/15/2023 |
| From: | Crawford R Entergy Operations |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| RBG-48261 LER 2022-004-01 | |
| Download: ML23319A436 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4582022004R01 - NRC Website | |
text
entergy RBG-48261 November 15, 2023 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Randy Crawford Manager Regulatory Assurance 225-381-4177 10 CFR 50.73
Subject:
Licensee Event Report 50-458 / 2022-004-01, High Pressure Core Spray Inoperable due to Transformer Failure River Bend Station - Unit 1 NRC Docket Nos. 50-458 Renewed Facility Operating License No. NPF-47 On November 17, 2022, Licensee Event Report 50-458 / 2022-004-00, High Pressure Core Spray Inoperable due to Transformer Failure (ML22321A306) was submitted by Entergy Nuclear Operating Inc. This letter is a supplement to the report to provide the updated results of the causal evaluation. The updated information is denoted by revision bars located in the right hand margin. The report is submitted in accordance with 10 CFR 50.73.
This document contains no commitments.
Should you have any questions, please contact Mr. Randy Crawford, Regulatory Assurance Manager, at 225-381-4177.
Respectfully, Randy Crawford RC/twf
Enclosure:
cc:
Licensee Event Report 50-458 / 2022-004-01, High Pressure Core Spray Inoperable due to Transformer Failure NRC Region IV Regional Administrator - Region IV NRC Senior Resident Inspector - River Bend Station Entergy Operations, Inc., 5485 U.S. Highway 61 N. St. Francisville, LA 70775
Enclosure RBG-48261 Licensee Event Report 50-458 / 2022-004-01, High Pressure Core Spray Inoperable due to Transformer Failure
Abstract
At 0132 CDT on September 19, 2022, River Bend Station (RBS) was operating at 100% power when the High Pressure Core Spray System (HPCS) was declared inoperable due to a transformer fault that occurred during Division Ill Standby Diesel Generator monthly surveillance testing. RBS entered Technical Specification 3.8.9, Condition E and declared HPCS and Standby Service Water System Pump 2C (SWP-P2C) inoperable. Reactor Core Isolation Cooling (RCIC) was verified operable. HPCS is a single train system at RBS, therefore an 8-hour report was submitted in accordance with 10 CFR 50.72(b)(3)(v)(D) as an event or condition that could have prevented the fulfilment of a safety function (EN 56116). This event is also being reported in accordance with 10 CFR 50. 73(a)(2)(v)(D) as an event or condition that could have prevented the fulfilment of a safety function.
The condition was corrected by replacing the transformer on September 30, 2022. HPCS and SWP-P2C were restored to operable at 0921 CDT on September 30, 2022.
EVENT DESCRIPTION
050 052
- 2. DOCKET NUMBER
- 3. LER NUMBER YEAR SEQUENTIAL REV NUMBER NO.
458 2022
- - 004
- - 01 At 0132 CDT on September 19, 2022, River Bend Station (RBS) was operating at 100% power. There were no other inoperable structures, systems, or components (SSCs) that contributed to this event. The Division Ill Diesel Generator [DG] was started per STP-309-0203, DIVISION Ill DIESEL GENERATOR OPERABILITY TEST. Approximately 5 seconds later the Main Control Room received multiple alarms on Division 3 components. The field operator performed a visual inspection of the Division Ill switchgear and discovered that the E22-S003, HPCS TRANSFORMER [XFMR]
was damaged. The Division Ill Diesel Generator was secured and placed in maintenance mode.
The transformer failure caused a loss of power to E22-S002, Division Ill 480 VAC switchgear [SWGR]. E22-S002, Division Ill 480 VAC switchgear supplies power to HPCS motor operated valves that are required to align flow to the reactor pressure vessel following system initiation. RBS entered Technical Specification (TS) 3.8.9, Condition E and immediately declared the High Pressure Core Spray System (HPCS) [BG] and Standby Service Water System Pump 2C (SWP-P2C) [Pl inoperable. This also placed RBS in a 14-day TS LCO shutdown action statement due to the inoperability of HPCS. Reactor Core Isolation Cooling (RCIC) [BN] was verified operable.
On June 21, 2022, Preventive Work Order 53003640 was performed on the transformer. No signs of degradation were noted, and all acceptance criteria were met satisfactory. Following the completion of the preventive work and prior to the transformer fault, STP-309-0203, DIVISION Ill DIESEL GENERATOR OPERABILITY TEST was successfully completed on July 25, 2022, and again on August 22, 2022.
Prior to this event HPCS and the Division 3 Transformer power source were operable. At time of discovery, the HPCS lost its initiation capability because the E22-S002, Division Ill 480 VAC switchgear would not have been powered from the Division Ill Standby Diesel Generator on any HPCS initiation signal. HPCS was inoperable and incapable of performing its design function during this event. RBS is reporting the event in accordance with 10 CFR 50.73(a)(2)(v)(D) as an event or condition that could have prevented the fulfilment of a safety function.
SAFETY ASSESMENT The RBS UFSAR states that in the event of a HPCS system failure such that HPCS cannot maintain reactor water level, the Automatic Depressurization System (ADS), which is independent of any other Emergency Core Cooling System (ECCS), reduces the reactor pressure so that flow from Low Pressure Coolant Injection (LPCI) [BO] and Low Pressure Core Spray (LPCS) [BM] systems enters the reactor vessel in time to cool the core and limit fuel cladding temperature.
Additionally, high pressure makeup was available from RCIC. Two of three divisions of ECCS are required for the RBS loss of coolant accident analysis. While HPCS was out of service, Division 1 ECCS and Division 2 ECCS systems and ADS were operable and would have met the ECCS performance criteria of 1 0CFR50.46. The out of service time was maintained within the limits of RBS Technical Specifications.
There were no actual Nuclear or Radiological safety consequences due to this event. This event was of minimal significance to the health and safety of the public.
EVENT CAUSE
050 052
- 2. DOCKET NUMBER
- 3. LER NUMBER YEAR SEQUENTIAL REV NUMBER NO.
458 2022
- - 004
- - 01 A Root Cause Evaluation (RCE) and an Equipment Failure Evaluation determined that the direct cause for the E22-S003 HPCS Transformer failure was an open circuit fault initiated by separation of the threaded stud at the secondary bus brazed connection, requiring replacement of the transformer.
The RCE determined the root cause for the HPCS Transformer failure was a cable to bus bar connection design that was susceptible to degradation from routine maintenance.
The HPCS Transformer Low Side (secondary) connections to the bus bars were made by brazing threaded brass studs to the bus bars. The bus bars were located behind insulated material. The threaded brass studs protruded through holes in the insulation material and each phase was connected by a bolted arrangement.
The brazed connections to the bus bar were degraded over time during routine maintenance testing activities that disassembled the bolted connections. The configuration did not provide a provision for installing a backing wrench to prevent the threaded studs from rotating when the connection nuts are being loosened or tightened. Torsional forces were being applied to the brazed stud connections each time the connection was disassembled and reassembled.
CORRECTIVE ACTIONS
Complete:
Replacement transformer was installed with a Temporary Modification Engineering Change. The bus connection for the temporary replacement transformer is bolted.
Planned actions tracked in the Corrective Action Program:
Complete the procurement and installation of the permanent replacement HPCS transformer.
Update the Design Specification to include transformer's termination of connectors for leads on low and high voltage sides should be bolted and accessible for maintenance.
PREVIOUS SIMILAR EVENTS
None NOTE:
Energy Industry Identification System and Component Function Identification codes are identified in the text of this report following the first mention of the system or component as [XX]. Page _3_ of _ 3