05000400/LER-2010-002
| Harris Nuclear Plant - Unit 1 | |
| Event date: | 11-15-2009 |
|---|---|
| Report date: | 06-07-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 4002010002R01 - NRC Website | |
Energy Industry Identification System (EMS) codes are identified in the text within brackets [ ].
I. DESCRIPTION OF EVENT
There were no structures, systems, or components that were inoperable at the start of the event which could have contributed to the event. On November 15, 2009, at 100% power, the Harris plant experienced a significant oil leak from the Hydrogen Seal Oil system [TI]. A non-licensed operator notified the Main Control Room (MCR) that the handle rotated each shift by Operations on the Air Side Seal Oil self-cleaning strainer became disconnected during manual rotation after approximately 1/4 turn, resulting in the oil leak. Based on this report and due to receipt of a "Turbine Lube Oil Reservoir Low Level" alarm, a decision was made to initiate a unit shutdown. The unit was manually tripped at 22:42 by the Main Control Room Operators. The plant promptly attained normal operation no-load temperature and pressure. Following the reactor trip, when the decision was made to break condenser vacuum, the "B" steam generator Main Steam Isolation Valve (MSIV) [SB] failed to fully close on demand from its control switch, but was closed due to field actions at 23:03 by locally isolating instrument air to the valve. The handle rotated each shift by Operations on the Air Side Seal Oil self-cleaning strainer [STR] became disconnected during manual rotation after approximately 1/4 turn, resulting in the oil leak. Aqueous foam was quickly sprayed on the oil to prevent a fire hazard. The loss of approximately 9,500 gallons of lube oil to the 261 ft elevation of the turbine building occurred with a small amount reaching the nearby gravel. No oil reached the Harris Lake. The oil was cleaned up and the Air Side Seal Oil strainer replaced. The Hydrogen Seal Oil/Lube Oil system was replenished and the unit was returned to service on November 20, 2010, at 2020.
II. CAUSE OF THE EVENT
The root cause of this failure was that during past maintenance on the Air Side Seal Oil strainer, the Graded Approach to Planning and Scheduling procedure (WCM-006) instructions and Decision Tree was too generic.
When considering level of planning detail, WCM-006 does not provide guidance on considering the vulnerabilities to the plant as a result of the work being performed on the component. This resulted in maintenance being performed on a critical component without the use of vendor drawings to validate that all parts are installed as designed. Troubleshooting indicated that the thrust collars that hold the filter handle in place were not installed during past maintenance, allowing the filter handle to eject.
The cause of the MSIV failing to fully close was determined to be two solenoid operated shuttle valves (SOVs) failing to fully realign to vent air after de-energizing. When both solenoids 1MS-82:007 SOV & 1MS-82:006 SOV are energized, both SOVs will open and supply air to the pilot operated 3-way valves. However, if either one of the two solenoids de-energizes (can be directed to do so manually from the MCB or automatically via RPS), the respective SOV will change state and exhaust the air supply from the pilot operators, closing the MSIV. 1MS- 82:007 SOV was found to be stuck in its energized position and 1MS-82:006 SOV was found in an intermediate position. Since neither valve fully positioned, a portion of the air was trapped in the valve actuator, preventing the valve from fully closing. After Operators isolated instrument air to the MSIV per plant procedures, the valve went fully shut.
Based on the observations made during the disassembly of the SOVs, there were several conditions observed with the "1MS-82:007" SOV that could have influenced its failure. First, it is believed that the piston assembly of the "1MS-82:007" SOV was stuck due to the amount of force required to move it. There were deposits present on the core assembly and the bore of the solenoid base subassembly that could have possibly caused the "1MS- 82:0007" SOV to not change state on demand. No obvious conditions were identified with the "1MS-82:006" SOV that would have resulted in its failure.
This event is being reported pursuant to 10 CFR 50.73(a)(2)(iv)(A), an event or condition that resulted in manual actuation of the Reactor Protection System. This event is also reportable pursuant to 10 CFR 50.73(a)(2)(i)(B), a condition prohibited by Technical Specifications since the MSIV was likely inoperable for a period of time longer than allowed by Technical Specifications, 10 CFR 50.73(a)(2)(v)(C), inability to isolate and mitigate a radioactive release, and 10 CFR 50.73(a)(2)(v)(D), a condition which could have prevented the fullfillment of a safety function to mitigate the consequences of an accident since closure of the valve would minimize a release. The actual consequences resulted in a manual reactor trip and spillage of large quantities of oil to the turbine building floor and some smaller amounts to the ground. No oil reached the Harris Lake. The manual reactor trip is bounded by the analysis in Chapter 15 of the Final Safety Analysis Report (FSAR). The operating staff performed the required actions for the trip and there were no adverse safety consequences.
Potential environmental consequences could have resulted if oil had reached the lake. Damage to plant equipment, and/or personal injury also could have occurred.
The MSIVs are designed to close within 5 seconds and performs a safety function to mitigate the consequences of a steam line break and a tube rupture. The failure of the MSIV to fully close occurred with the plant in Mode 3.
Failure of the B MSIV to fully shut would have prevented isolation of the B Steam Generator. During the inoperable period, there was no identified tube leakage.
Immediate corrective actions were to stabilize the plant, and secure the Turbine Seal Oil/Lube Oil systems to stop the leak. Aqueous foam was sprayed on the oil to prevent a fire hazard. The oil was cleaned up and the Air Side Seal Oil strainer replaced.
Planned corrective actions to prevent recurrence are to revise and implement WCM-006, Graded Approach to Planning and Scheduling such that Level "A" work activities (Quality Critical) are classified based on Equipment Classification that could cause a plant transient or shutdown and define other criteria for Level "A" (Quality Critical). Planning Level "A" is used for tasks that require additional rigor in preparation, oversight and implementation because the task can significantly impact plant operation or equipment.
Corrective actions for the MSIV failing to fully close were to replace the SOVs on 1MS-82. One additional SOV installed on 1MS-80 ("A" MSIV), 1MS-80:007, was found to be of the same manufacturing lot as the stuck SOV on 1MS-82. Although no defect was observed with this SOV, it was also replaced.
V. PREVIOUS SIMILAR EVENTS
A review of License Event Reports for the previous five years did not reveal any similar events at the Harris Plant.