05000316/LER-2021-002, Re Manual Reactor Trip Due to an Unisolable Steam Leak
| ML21230A251 | |
| Person / Time | |
|---|---|
| Site: | Cook  |
| Issue date: | 08/18/2021 |
| From: | Lies Q American Electric Power Co, Indiana Michigan Power Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| AEP-NRC-2021-52 LER 2021-002-00 | |
| Download: ML21230A251 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3162021002R00 - NRC Website | |
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A unit of American Electric Power August18,2021 Docket No.: 50-316 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk 11555 Rockville Pike Rockville, MD 20852 Donald C. Cook Nuclear Plant Unit 2 LICENSEE EVENT REPORT 316/2021-002-00 Indiana Michigan Power Cook Nuclear Plant One Cook Place Bridgman, Ml 49106 lndianaMichiganPower.com AEP-NRC-2021-52 10 CFR 50.73 MANUAL REACTOR TRIP DUE TO AN UNISOLABLE STEAM LEAK In accordance with 10 CFR 50. 73, Licensee Event Report (LER) System, Indiana Michigan Power Company, the licensee for Donald C. Cook Nuclear Plant Unit 2, is submitting as an enclosure to this letter the following report:
LER 316/2021-002-00: MANUAL REACTOR TRIP DUE TO AN UNISOLABLE STEAM LEAK There are no commitments contained in this submittal.
Should you have any questions, please contact Mr. Michael K. Scarpello, Regulatory Affairs Director, at (269) 466-2649.
Sincerely, al~J. ~
Site Vice President SJM/mll
Enclosure:
Licensee Event Report 316/2021-002-00: Manual Reactor Trip Due To An Unisolable Steam Leak
U. S. Nuclear Regulatory Commission Page 2 c:
R. J. Ancona - MPSC EGLE - RMD/RPS J. B. Giessner - NRC Region Ill NRC Resident Inspector R. M. Sistevaris - AEP Ft. Wayne J.E. Walcutt-AEP Ft. Wayne S. P. Wall - NRC, Washington D.C.
A. J. Williamson - AEP Ft. Wayne AEP-NRC-2021-52
Enclosure to AEP-NRC-2021-52 Licensee Event Report 316/2021-002-00
Abstract
On June 22, 2021, Donald C. Cook Unit 2, was operating at 100 percent power. At approximately 2305 hours0.0267 days <br />0.64 hours <br />0.00381 weeks <br />8.770525e-4 months <br />, equipment operators reported a steam leak in a crossover pipe at the Right Moisture Separator Reheater (MSR) joint that was unisolable.
Operators manually tripped the reactor at 2331 hours0.027 days <br />0.648 hours <br />0.00385 weeks <br />8.869455e-4 months <br />. All safety systems and Engineered Safety Features equipment operated as expected.
Prior to the reactor trip, operators closed the Low Pressure Turbine (LPT) 'B' Left Reheat Steam Intercept Valve at 1040 hours0.012 days <br />0.289 hours <br />0.00172 weeks <br />3.9572e-4 months <br />, due to an electro-hydraulic control (EHC) fluid leak from the valve's hydraulic actuator. Following closure of the intercept valve, the EHC fluid leak from the hydraulic actuator was manually isolated. No abnormal system conditions were noted following closure of the valve. This valve closure is an abnormal system alignment.
A failure investigation determined that the steam leak occurred at a right MSR crossover piping expansion joint. The expansion joint protective liner was displaced downstream due to a failed weld that exposed the bellows to direct steam flow.
The event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), System Actuation, due to the valid actuation of the Reactor Protection System (RPS) and the Auxiliary Feedwater System, as a result of the manual reactor trip.
EVENT DESCRIPTION
On June 22, 2021, Donald C. Cook (CNP) Unit 2, was operating at 100 percent power. At approximately 2305 hours0.0267 days <br />0.64 hours <br />0.00381 weeks <br />8.770525e-4 months <br />, excessive steam noise was noted on the Unit 2 Turbine Building [NM] 633' elevation. Equipment operators were dispatched to investigate and reported a steam leak in a crossover pipe at 2-OME-80R, Right Moisture Separator Reheater (MSR) [SB][MSR] that was unisolable. Operators manually tripped the reactor [RCT] at 2331 hours0.027 days <br />0.648 hours <br />0.00385 weeks <br />8.869455e-4 months <br />. All safety systems and Engineered Safety Features equipment operated as expected.
Prior to the reactor trip, operators closed 2-OME-93-BL, Low Pressure Turbine (LPT) 'B' Left Reheat Steam Intercept Valve [SB]M at 1040 hours0.012 days <br />0.289 hours <br />0.00172 weeks <br />3.9572e-4 months <br />, due to an electro-hydraulic control (EHC) fluid leak from the valve's hydraulic actuator. Following closure of the intercept valve, the EHC fluid leak from the hydraulic actuator was manually isolated. No abnormal system conditions were noted following closure of 2-OME-93-BL and isolation of EHC supply to the hydraulic actuator. This valve closure is an abnormal system alignment.
Event Notification 55322 was submitted in accordance with 10 CFR 50.72(b)(2)(iv)(B), Reactor Protection System (RPS) [JG] actuation as a four (4) hour non-emergency report, and under 10 CFR 50.72(b)(3)(iv)(A), specified system actuation of the Auxiliary Feedwater System [BA], as an eight (8) hour non-emergency report.
A failure investigation was conducted following the reactor trip and determined that the steam leak occurred at 2-XJ-113-5, Right MSR OME-80R Reheat Steam to LPT 'B' Expansion Joint [SB][EXJ]. Upon disassembly, a circumferential weld attaching the liner which protects the convolutions ( or bellows) from direct steam flow had failed. The failed weld allowed the liner to displace downstream which exposed the bellows to direct steam flow.
The direct exposure of steam flow to the bellows resulted in increased local turbulence and vibration, leading to high cycle fatigue of the bellows, and the eventual steam leak.
During the failure investigation and extent of condition evaluation, the design of the expansion joint liner was reviewed. A design change was implemented to increase the thickness of the inner liner from 1/8 inch to 1/4 inch.
Additionally, a thicker weld was specified to attach the liner to the replacement expansion joint. The LPT 'A' Left,
'B' Right, and 'C' Left expansion joints were replaced that included the liners and welds with the revised specifications.
Following completion of repairs, Unit 2 was returned to service when the main generator [TB][GEN] was synchronized to the electrical grid at 2328 hours0.0269 days <br />0.647 hours <br />0.00385 weeks <br />8.85804e-4 months <br /> on July 8, 2021.
The event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A), System Actuation, due to the valid actuation of the Reactor Protection System (RPS) and the Auxiliary Feedwater System, as a result of the manual reactor trip.
COMPONENT 2-XJ-113-5, Right MSR OME-80R Reheat Steam to LPT 'B' Expansion Joint.
CAUSE OF THE EVENT
SEQUENTIAL NUMBER 002 REV NO.
00 A circumferential weld attaching the liner which protects the bellows from direct steam flow within the expansion joint had failed, allowing the liner to displace downstream and subject the bellows to direct steam flow. The direct exposure of steam flow to the bellows resulted in increased local turbulence and vibration, leading to high cycle fatigue of the bellows. The resultant steam leak was unisolable which required a manual reactor trip.
A causal evaluation is ongoing at the time of this report. A supplement to this LER will be provided if the results of the evaluation significantly change the perception of the course, significance, implications, or consequences of the event or if it results in substantial changes in corrective actions planned.
CORRECTIVE ACTIONS
Prompt Corrective Actions A design change was completed that increased the thickness of the inner liner from 1/8 inch to 1/4 inch, and specified a thicker weld to attach the liner to the replacement expansion joint.
The LPT 'A' Left, 'B' Right, and 'C' Left expansion joints were replaced that included the liners and welds with the revised specifications contained within the design change.
Plant procedures 1-OHP-5030-050-001 and 2-OHP-5030-050-006, Main Turbine and Feed Pump Turbine Valve Functional Checks, were placed on hold until new guidance is provided to revise the procedures for valve closure at power to preclude any potential impacts to the system.
Planned Corrective Actions
Revise plant procedures 1-OHP-5030-050-001 and 2-OHP-5030-050-006, Main Turbine and Feed Pump Turbine Valve Functional Checks.
ASSESSMENT OF SAFETY CONSEQUENCES
NUCLEAR SAFETY There was no actual or potential nuclear safety hazard resulting from the unisolable steam leak and manual reactor trip.
INDUSTRIAL SAFETY There was no actual safety hazard resulting from the unisolable steam leak and manual reactor trip.
Potential industrial hazards from the unisolable steam leak were recognized and mitigated by evacuating the Turbine Building 633' elevation to protect personnel and by tripping the reactor.
RADIOLOGICAL SAFETY There was no actual or potential radiological safety hazard resulting from the unisolable steam leak and manual reactor trip PROBABILISTIC RISK ASSESSMENT (PRA)
A PRA review of the manual reactor trip was performed to characterize the safety significance of the event. This review was performed by adjusting the CNP Internal Events PRA model to reflect the plant configuration at the time, and then using the model to calculate the estimated Conditional Core Damage Probability (CCDP) and Conditional Large Early Release Probability (CLERP). A comparison of the CCDP and CLERP estimates to regulatory guidance resulted in the conclusion that this event had very low safety significance.
PREVIOUS SIMILAR EVENTS
Licensee Event Report 316\\2016-001-01, Manual Reactor Trip Due To Moisture Separator Heater Expansion Joint Failure.
The Root Cause was determined to be an organizational failure to recognize the risk significance of, and to adequately correct or mitigate, previously identified vibration issues with the Unit 2 'B' right MSR crossover expansion joint tie rod and bellows in a timely fashion. Contributing causes included tack welds that were inappropriately applied to nuts on the tie rod areas under tensile load. Page 4 of 4