05000282/LER-2011-003
| Prairie Island Nuclear Generating Plant Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2822011003R00 - NRC Website | |
During June and July, 2011, planning was conducted to clean and inspect Bus 3101. With Bus 310 de-energized, it was known that 11 Containment Sump A Pump2 and run time instrumentation3 were unavailable. It was unrecognized that Bus 310 also powered relay4 1— 9182xx that would block the start of 12 Containment Sump A Pump even though the pump was powered from bus 320. An erroneous conclusion was made that 12 Containment Sump A Pump would be operable with Bus 310 de-energized.
On Saturday, August 13, 2011, the 1R11 radionuclide monitors failed and was taken out of service for maintenance. As a result, both 11 and 12 Containment Sump A Pump breakers6 were protected to avoid entering Technical Specification (TS) 3.4.16, Condition (D).
On Thursday, August 18, 2011, with Unit 1 at approximately 100% power, work to isolate and de- energize Bus 310 was completed at approximately 09:00. At this time, 1R11 and both Containment Sump A Pumps were inoperable.
While the isolation was in progress, a control board indicator' stating "SUMP A DISCH AER SUMP TK CLOSED" was noted to be illuminated. The control room operator contacted the Work Control Center (WCC) operator in charge of the isolation. After reviewing the associated documentation, the WCC operator concluded that the indication was expected and no further investigation was conducted. The documentation incorrectly showed that MV-322288 (recirculation valve) would be affected by the isolation. The indicator is actually associated with MV-32229 (discharge valve) which had closed as a result of de-energizing relay 1-9182xx. This error was turned over to the night shift as an expected indication.
On August 19, 2011, at approximately 01:00 CDT, the night Shift Manager questioned the illuminated indicator light. It was discovered that MV-32229 (discharge valve) was actually closed. At 01:20 CDT on August 19, 2011, it was determined the site was in TS 3.0.3 due to not satisfying the requirements of TS 3.4.16. It was believed that with MV-32229 (discharge valve) closed the containment sump pump run time meter would not provide meaningful information. The crew re-opened MV-32229 (discharge valve) and exited T.S 3.0.3. It was later determined that the decision to exit TS 3.0.3 was faulted. TS 3.4.16 specified the run time meter. The run time meter is driven by the sump pump regardless of whether a discharge path is available or not.
On August 19, 2011, when day shift personnel arrived, the TS 3.0.3 entry was challenged again on the belief that 12 Containment Sump A Pump would start and run on a sump high level. A review of 1 EIIS System Code:
EC 2 EMS Component Identifier: P 3 El IS Component Identifier: KI 4 El IS Component Identifier: RLY 5 EMS System Code:
IJ 6 EllS Component Identifier: BKR 7 EllS Component Identifier: IL 8 EllS Component Identifier: V the start circuitry for 12 Containment Sump A Pump was conducted. During this review, the full implications of Bus 310 outage's impact on 12 Containment Sump A Pump were realized. This was reported to the Shift Manager (SM) and TS 3.0.3 was re-entered at 10:20 CDT. Bus 310 was re- energized at 12:30 CDT and TS 3.0.3 was exited at 12:55 CDT.
EVENT ANALYSIS
The Prairie Island Nuclear Generating Plant (PINGP) Unit 1 leakage detection system has the capability to detect significant reactor coolant pressure boundary (RCPB) degradation as soon after occurrence as practical to minimize the potential for propagation to a gross failure. Thus, an early indication or warning signal is necessary to permit proper evaluation of all unidentified LEAKAGE.
Industry practice has shown that water flow changes of 0.5 to 1.0 gallon per minute (gpm) can be readily detected in contained volumes by monitoring changes in water level, in flow rate, or in the operating frequency of a pump. The containment sump A pump run time instrumentation may be used to detect increases in unidentified LEAKAGE.
Containment Sump A has two sump pumps (#11 and #12). The sump pumps discharge through MV- 32229 (Unit 1 Containment Sump Discharge Motor Valve) to the Unit 2 Sump A holding tank located outside of Containment. The output of the sump pumps can also be directed back to Containment Sump A via MV-32228 (12 Annulus Sump Pump Discharge to Containment Sump A Motor Valve).
The Containment Sump A Pumps and valves, are powered from two separate sources (Bus 310 and Bus 320), but their operation is not electrically independent. Relay 1-9182xx is powered from Bus 310 but has contacts in the start circuitry for both Containment Sump A Pumps. This condition was noted in 2004 while performing work on breaker 312-12 for MV-32228.
The reactor coolant contains radioactivity that, when released to the containment, can be detected by radiation monitoring instrumentation. The containment atmosphere radiation monitoring channel, R- 11, normally provides the required monitoring.
TS Limiting Condition of Operation (LCO) 3.4.16 is satisfied when monitors of diverse measurement means are available to provide indication of Reactor Coolant System (RCS) leakage. Thus, the containment sump A monitor (pump run time instrumentation), in combination with a containment radionuclide monitor, provides an acceptable means of monitoring RCS leakage.
The full consequences of isolating Bus 310 were not recognized during the planning phase for the maintenance on Bus 310, even though it was previously identified in 2004. On Saturday, August 13, 2011, the 1R11 radionuclide monitor failed and was taken out of service for maintenance. On August 18, 2011, at approximately 09:00 CDT, when the work to isolate and de-energize Bus 310 was completed, 1R11 and both Containment Sump A Pumps were inoperable. Theses conditions required entry to TS 3.4.16, Condition D (all required monitors inoperable) which required action to enter TS, LCO 3.0.3 immediately. This condition is reportable under 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by TS (TS LCO 3.4.16, Condition D).
This condition is not reportable under 10 CFR 50.73(a)(2)(v) as an event or condition that could have prevented fulfillment of a safety function since the RCS Leakage Detection Instrumentation is for indication only and does not mitigate the consequences of an accident.
This condition is also not reportable under 10 CFR 50.73(a)(2)(vii) as a common-cause inoperability of independent trains or channels since the RCS Leakage Detection Instrumentation is for indication only and does not mitigate the consequences of an accident.
SAFETY SIGNIFICANCE
During the time that 1R11 and Containment Sump A Pumps were out of service, diverse methods of RCS leak detection were still available. These include other radiation detectors in containment, monitoring the primary coolant volume control system for imbalances, and monitoring Reactor Coolant Pressurizer level for changes. In addition, SP 1001AA (Daily Reactor Coolant System Leakage Test) is performed daily. Therefore this event does not represent a safety system functional failure for Unit 1. There were no nuclear, radiological, environmental, or industrial impacts associated with this event. Therefore, this event did not affect the health and safety of the public.
CAUSE
The causal evaluation determined that station processes for complex electrical clearance order development and approval do not preclude the use of logic diagrams as a sole and final source of information to determine effect on plant conditions.
CORRECTIVE ACTION
On August 19, 2011, Bus 310 was re-energized at 12:30 CDT and TS 3.0.3 was exited at 12:55 CDT.
The procedure (FP-OP-TAG-01) for complex isolation reviews will be revised to use form QF-1108.
The QF-1108 is an existing form that asks direct questions regarding worker safety and plant conditions. In addition, the procedure will be revised to prohibit use of logic diagrams as a sole and final source of information in the preparation and review of electrical isolations.
PREVIOUS SIMILAR EVENTS
A LER search was conducted and one similar event involving isolating and de-energizing electrical equipment was identified in the last three years at PINGP (LER 2-09-01, Clearance Order Renders Opposite Train Emergency Diesel Generator Inoperable (ML091100213)). The causal evaluation determined that the clearance order preparer and approver did not request assistance from individuals knowledgeable with reading electrical drawings.