|Monticello Nuclear Generating Plant|
|Reporting criterion:||10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident|
|ENS 52814||10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident|
|2632017004R00 - NRC Website|
|Person / Time|
|From:||Gardner P A|
Northern States Power Company, Minnesota, Xcel Energy Inc
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML17229B304 (4)|
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
Plant Conditions: Mode 1, 100% Power On June 19, 2017, following planned High Pressure Coolant Injection (HPCI) system [BJ] maintenance, a HPCI start attempt was performed per the quarterly test procedure. HPCI failed to start during the test due to the steam stop valve [SHV] HO-7 not opening caused by the HO-7 oil relay (manufacture Schutie and Koerting (S&K) Part Number 69-XC-103) not functioning properly.
Since the component was not the subject of the maintenance activity, the HPCI failure was reported to the NRC under Emergency Notification System, Event Number 52814.
HO-7, when closed, blocks reactor steam from reaching the HPCI turbine and thus when HO-7 is closed, HPCI cannot run. Valve HO-7 was last validated to open properly on June 15, 2017 while performing the Operations Weekly Control Room Checklist on the night shift. The HO-7 oil relay was repaired and the HPCI system returned to operable status at 13:30 on June 23, 2017.
The event was determined to be reportable in accordance with 10 CFR 50.73 (a)(2)(V)(D), "event or condition that could have prevented fulfillment of a safety function." This event is classified as a safety system functional failure.
There were minimal safety consequences associated with the condition. The condition existed for approximately 8 days (June 15-June 23), which was less than the Technical Specification limiting condition for operation of 14 days, and the Reactor Core Isolation Cooling system and Automatic Depressurization system were operable the entire period that HPCI was inoperable.
There are two piston rings and both were found to be pitted. One piston ring was pitted significantly and one piston ring was minimally pitted. The piston ring that was more pitted also had ring edge wear and was replaced. The second piston ring was determined to provide an acceptable fit and was not replaced.
004 - 00 APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2018 comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by interne( e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
The oil relay piston body and housing were cleaned and a single (one of two) piston ring, spring and gasket from the relay were replaced with new parts on June 21, 2017.
The direct cause of the HPCI failed start was the steam stop valve HO-7 not opening due to HO-7 oil relay not shuttling. The oil relay did not shuttle because of leakage past a piston ring resulting in reduced relay stroking force. The piston ring pitting and non-smooth relay internal bore surfaces are due to age related wear that occurred over the life of the HPCI turbine. Thus the direct causes are due to the piston ring not uniformly sealing resulting in reduced force and the wear particles/tight clearances causing the relay valve to not shuttle.
The apparent cause of the pitted piston rings and a non-smooth bore in the HO-7 oil relay is due to the lack of sufficient detail in the preventive maintenance procedure. HO-7 oil relay maintenance was performed in 2013. It was determined that the preventative maintenance procedure performed in 2013 lacked sufficient detail to ensure that adequate preventative maintenance was performed on the oil relay.
A contributing cause was a lack of trending information on HO-7 performance. Monitoring the opening times of the HO-7 valve may have provided some insight into the performance of the HO-7 oil relay.
The immediate corrective action was to clean the oil relay piston body and housing, and a single (one of two) piston ring, spring and gasket from the relay were replaced with new parts on June 21, 2017.
Long term corrective actions include the following:
- Provide additional steps in the HPCI preventive maintenance procedure to perform more direct inspection and maintenance of the HO-7 valve and associated oil relay components.
- Perform trending of HO-7 valve opening times to provide insight into the performance of the HO-7 oil relay.
PREVIOUS SIMILAR EVENTS
There were no previous similar licensee event reports in the past three years.
The Institute of Electrical and Electronics Engineer codes for equipment are denoted by [XX].