05000298/LER-2024-003, High Pressure Coolant Injection Steam Leak Causes Condition That Could Have Prevented Fulfillment of a Safety Function and a Condition Prohibited by Technical Specifications
| ML24113A090 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 04/22/2024 |
| From: | Dia K Nebraska Public Power District (NPPD) |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| NLS2024029 LER 2024-003-00 | |
| Download: ML24113A090 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2982024003R00 - NRC Website | |
text
H Nebraska Public Power District "Always there when you need us" 10 CFR 50.73 NLS2024029 April 22, 2024 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Subject: Licensee Event Report No. 2024-003-00 Cooper Nuclear Station, Docket No. 50-298, DPR-46
Dear Sir or Madam:
The purpose of this correspondence is to forward Licensee Event Report 2024-003-00.
This letter does not contain regulatory commitments.
Sincerely,
. * ~
Khalil Dia Site Vice President
/bk
Attachment:
Licensee Event Report 2024-003-00 cc:
Regional Administrator w/attachment USNRC - Region IV Cooper Project Manager w/attachment USNRC - NRR Plant Licensing Branch IV Senior Resident Inspector w/attachment USNRC-CNS SRAB Administrator w/attachment NPG Distribution w/attachment INPO Records Center w/attachment via IRIS entry SORC Chairman w/attachment CNS Records w/attachment COOPER NUCLEAR STATION 72676 648A Ave/ P.O. Box 98 / Brownville, NE 68321 http://www.nppd.com
Abstract
On January 31, 2024, a steam leak was identified near the High Pressure Coolant Injection (HPCI) drain line steam trap. This issue was entered into the work management process. The steam leak was later noted as degraded on February 19, 2024, and during a walkdown on February 22, 2024, to support the work order for repair, a maintenance technician identified steam potentially coming from the piping between the HPCI steam trap and a HPCI downstream valve. As a result of this potential through-wall pipe leak, Operations declared HPCI inoperable at 1718 hours0.0199 days <br />0.477 hours <br />0.00284 weeks <br />6.53699e-4 months <br /> on February 22, 2024, and entered the applicable Technical Specifications Condition.
An eight-hour non-emergency event notification (EN 56989) was made to the Nuclear Regulatory Commission.
The HPCI trap assembly (steam trap, outlet valve, and piping) was replaced and HPCI was returned to operable status at 2101 on February 24, 2024. A failure analysis will be performed on the removed HPCI trap assembly to determine the cause of the failure. This report will be supplemented.
PLANT STATUS 05000-298 YEAR 2024 SEQUENTIAL NUMBER 003 REV NO.
00 At the time of the event on January 31, 2024, Cooper Nuclear Station (CNS) was in Mode 1, Power Operation, at 100 percent steady-state power.
BACKGROUND The High Pressure Coolant Injection (HPCI) system [EIIS:BJ] provides protection to the core in case of a small break in the reactor coolant pressure boundary which does not result in a rapid depressurization of the reactor vessel [EIIS:RPV]. The HPCI system permits the nuclear plant to be shutdown while maintaining sufficient reactor vessel water inventory until the reactor vessel is depressurized. HPCI continues to operate until reactor vessel pressure is below the pressure at which Low Pressure Coolant Injection system [EIIS:BO]
operation or Core Spray system [EIIS:BM] operation can be used to maintain core cooling.
HPCI consists of a steam turbine assembly [EIIS:TRB] driving a multi-stage booster and main pump assembly [EIIS:P] and system piping valves, controls and instrumentation. The HPCI turbine is driven by steam from the reactor which is generated by decay and residual heat.
The HPCI turbine drain line steam trap, HPCI-TP-S57, provides a drain path for the HPCI steam supply drip leg since the HPCI steam supply block valve is normally closed preventing steam from supplying the turbine.
Condensate forms from the steam in the drip leg. HPCI-TP-S57 allows the high pressure condensate to pass through it to the lower pressure drain line to the Main Condenser [EIIS:SG]. The trap is designed to prevent high pressure steam from passing through the drain with the condensate. HPCI-TP-S57 is manufactured by Yarway Corporation, model 515BSWR.
HPCI is a single train system and is required to be operable to satisfy the requirements of Technical Specifications (TS) 3.5.1.
EVENT DESCRIPTION
On January 31, 2024, a station operator identified a steam leak near HPCI-TP-S57. The steam leak could be heard once entering the HPCI room and an approximate six-inch steam plume was visible around HPCI-TP-S57. This issue was entered into the work management process for repair. On February 19, 2024, during a subsequent station operator tour, the steam leak had degraded. Condensation was no longer completely contained in the catch basin; floor troughs and drains had completely captured the remaining condensation.
The operability reviews for these conditions assessed the steam leak as coming from the body to bonnet gasket of HPCI-TP-S57 which is not considered American Society of Mechanical Engineers (ASME) code leakage (leakage from mechanical connections is classified as non-code leakage). Based on the leakage being non-code leakage, not impacting surrounding HPCI equipment, and HPCI steam pressure unaffected, HPCI remained operable. Page 2 of 4 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 04/30/2027 (04-02-2024)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET Cooper Nuclear Station 05000-298 YEAR 2024
- 3.
LER NUMBER SEQUENTIAL NUMBER 003 REV NO.
00 On February 22, 2024, during a walkdown performed prior to scheduled corrective maintenance for the HPCI steam leak, a maintenance technician discovered that the steam leak was potentially coming from drain line piping between HPCI-TP-S57 and down stream valve HPCI-V-130, HPCI steam line drip-leg drain trap outlet valve. HPCI was declared inoperable at 1718 hours0.0199 days <br />0.477 hours <br />0.00284 weeks <br />6.53699e-4 months <br /> on February 22, 2024, and Operations entered TS 3.5.1, Condition C.2 to restore HPCI to operable status within 14 days.
Following removal of piping insulation, it was determined the HPCI drain line piping had developed a through-wall leak located on the HPCI-TP-S57 downstream weld toe. The trap assembly (HPCI-TP-S57, HPCI-V-130, and drain line piping) was replaced and post-work testing completed. HPCI was declared operable at 2101 hours0.0243 days <br />0.584 hours <br />0.00347 weeks <br />7.994305e-4 months <br /> on February 24, 2024, and Operations exited TS 3.5.1, Condition C.
The removed piping was visually inspected by borescope and an ultrasonic test (UT) performed. The UT indicated significant loss of pipe thickness near HPCI-TP-S57. The HPCI drain line piping between HPCI-TP-S57 and HPCI-V-130 is one-inch, Class II carbon steel and is within the ASME Section XI Class 2 (high energy piping) boundary.
BASIS FOR REPORT Since HPCI is a single-train system, this event is being reported per 10 CFR 50.73(a)(2)(v)(D), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident."
An eight-hour non-emergency event notification (EN 56989) was also made to the Nuclear Regulatory Commission (NRC).
An engineering review was completed and concluded there was evidence HPCI was inoperable back to the date the steam leak was discovered; i.e., on January 31, 2024. Based on this information, HPCI was inoperable from January 31, 2024, until February 24, 2024. TS 3.5.1, Condition C.2, to restore HPCI system to operable status, applies which has a completion time of 14 days. Based on exceeding the TS completion time, this event is also reportable under 10 CFR 50.73(a)(2)(i)(B) as any operation or condition which was prohibited by TS.
SAFETY SIGNIFICANCE
Engineering evaluated the effects resulting from the HPCI steam line leak downstream of steam trap HPCI-TP-S57. The effects of the steam leak were determined for the identified through-wall hole size and conservatively assumed that HPCI-TP-S57 failed in the open position.
The results of this evaluation demonstrated that the temperatures and pressures remain acceptable for the equipment located in the affected spaces during the limiting conditions of a high energy line break and the post loss of coolant accident heat-up environment. The components in the affected areas are qualified for the Page 3 of 4 (04-02-2024)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 04/30/2027 LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/readinq-rm/doc-collections/nureqs/staff/sr1022/r3/)
- 3.
LER NUMBER Cooper Nuclear Station 05000-298 YEAR 2024 SEQUENTIAL NUMBER 003 REV NO.
00 maximum possible relative humidity and the environmental qualification program normal and accident total integrated doses remain bounding for the steam leak. The existing flooding analysis remains valid and bounding as the steam leak did not impact internal flood heights. Additionally, the steam diverted from the HPCI turbine by the steam leak does not impact HPCI system operation.
The evaluation concluded the HPCI system and components in the affected areas remained capable of performing their safety-related functions. Accordingly, this event will not count toward the NRC Safety System Functional Failure performance indicator.
There were no consequences to general safety of the public, nuclear safety, or radiological safety.
CAUSE AND CORRECTIVE ACTIONS
The most likely direct cause was steam trap leak-by induced pipe thinning downstream of HPCI-TP-S57 resulting in a through-wall leak. The HPCI trap assembly was replaced to correct the condition.
The root cause of this condition is pending awaiting destructive testing and analysis on the removed HPCI trap assembly. Following completion of the analysis, a supplement to this report will be submitted.
PREVIOUS EVENTS A review of CNS licensee event reports (LER) for the last three years was performed. There were no previous occurrences involving a similar failure to that discussed in this report. The following conditions related to HPCI were reported.
LER 2022-003 Inadvertent HPCI injection into the RPV during HPCI beginning of cycle surveillance testing. The injected water increased reactivity and caused a reactor scram [January 11, 2023].
LER 2021-003 During surveillance testing, HPCI flow indicating switch, HPCI-FIS-78, failed to trip prior to exceeding the TS 3.3.5.1 (Emergency Core Cooling System Instrumentation) function for HPCI pump discharge flow-low (bypass) [December 27, 2021]. Page 4 of 4