05000334/LER-2001-004
| Beaver Valley Power Station Unit No. 1 | |
| Event date: | 12-07-2001 |
|---|---|
| Report date: | 02-04-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 3342001004R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse-Pressurized Water Reactor (PWR) Station/Instrument Air System (LD)
CONDITIONS PRIOR TO OCCURRENCE
Unit 1: Mode 1 at 100 % power Station air compressor 1SA-C-1B was inoperable/unavailable due to maintenance and was therefore unavailable to mitigate the loss of station air pressure event. There were no other system, structures or components that were inoperable that contributed to the event.
DESCRIPTION OF EVENT
On December 7, 2001, Unit 1 station air compressor 1SA-C-1B was de-energized for preventive maintenance. Electricians were performing preventative maintenance tasks inside the control panel of station air compressor 1SA-C-1B when an uninsulated screwdriver shaft made contact with an energized circuit. An energized cross-tie circuit exists between the Unit 1 station air compressors 1SA-C-1A and 1SA-C-1B that provides a start signal to the standby compressor upon a trip of the operating compressor. The screwdriver contact created a short circuit and a blown fuse in the control circuitry, which resulted in a trip of the operating station air compressor 1SA-C-1A. With the loss of the operating compressor, the station air header pressure decreased due to normal station air demand. At 1353 hours0.0157 days <br />0.376 hours <br />0.00224 weeks <br />5.148165e-4 months <br /> the low station air pressure alarm was received in the control room. The on-shift operations crew responded per the appropriate alarm response procedure, and entered Abnormal Operating Procedure (AOP) 1.34.1, "Loss of Station Instrument Air". Operators were dispatched locally to station air compressor 1SA-C-1A, the compressors supply breaker and to the backup diesel air compressor 1SA-C-2. A local start of 1SA-C-1A was unsuccessful due to the blown control power fuse. The backup diesel air compressor was successfully started and placed in service; however, system pressure had already reached an abnormally low level and the backup diesel compressor was unable to restore system pressure due to insufficient capacity. At 1401 hours0.0162 days <br />0.389 hours <br />0.00232 weeks <br />5.330805e-4 months <br /> the low system air pressure caused Loop 1C main steam trip valve, TV-1MS-101C, to begin closing as indicated by alarm "Steamline Stop Valve Not Fully Open". AOP 1.34.1 directs the operators to manually trip the reactor upon receipt of indication that a main steam isolation (trip) valve (MSIV) begins to close in order to mitigate an MSIV closure transient from occurring.
Therefore, at 1401 hours0.0162 days <br />0.389 hours <br />0.00232 weeks <br />5.330805e-4 months <br />, operators manually tripped the Unit 1 reactor due to the loss of station air in accordance with AOP-1.34.1. All control rods fully inserted into the reactor core and all required safety systems operated as designed. Emergency Operating Procedure E-0 for Reactor Trip was performed and the plant was stabilized in Mode 3.
REPORTABILITY
The manual initiation of a reactor trip via the Reactor Protection System by the BVPS Unit 1 control room operator on December 7, 2001, was a valid reactor trip and was not part of a pre-planned sequence during testing or reactor operation. Therefore this event is reportable pursuant to 10 CFR 50.72 (b)(2)(iv)(B) and 10 CFR 50.73(a)(2)(iv)(A). The NRC was notified that a manual reactor trip occurred at BVPS Unit 1 pursuant to 10 CFR 50.72 (b)(2)(iv)(B) at 1623 hours0.0188 days <br />0.451 hours <br />0.00268 weeks <br />6.175515e-4 months <br /> on December 7, 2001 (EN# 38548).
CAUSE OF EVENT
The direct cause for the manual trip was a loss of station air due to the unavailability of both station air compressors. Operator action was taken to manually trip the reactor when the Loop 1C main steam trip valve, TV-1MS-101C, started to close as indicated by a alarm "Steamline Stop Valve Not Fully Open". The loss of station air resulted when 1SA-C-1A tripped due to a blown fuse in the control circuitry while 1SA-C-1B was de-energized for maintenance. The root cause of the event was determined to be the failure of the station work process to adequately plan, review, control, assign risk significance, and provide adequate oversight for work activities on important plant equipment.
The following six barriers were breached in the events that led up to the trip of the operating compressor (1SA-C-1A) on December 7, 2001:
1. Work Planning Barrier: Planning of work package was ineffective in identifying the presence of an energized cross-tie circuit in the Unit 1 station air compressors 1SA-C-1A and 1SA-C-1B.
Details of the work instructions for the preventative maintenance task was inadequate, drawing review/knowledge of staff was less than adequate, and assignment of proper risk to the work package was impeded by the lack of a critical component list.
2. Scheduling Barrier: Pre-approval of work packages well in advance of work being scheduled did not provide adequate opportunity to review the risk related to work being performed given specific plant conditions.
3. Clearance Barrier: Clearance preparation and review did not identify the presence of the energized crosstie circuit prior to hanging the clearance. The shared control circuitry drawing for compressors 1SA-C-1A and 1SA-C-1B made it difficult to identify the common circuit between the compressors.
4. Craft Barrier: Electricians did not identify the presence of cross-tie circuit in work package walkdown, briefing, or checks for energized circuits. Additionally, the electrician failed to use properly insulated tools to perform work per station expectations.
FACILITY NAME (1) PAGE (3) LER NUMBER (6) DOCKET (2) 5. Management Oversight Barrier: Morning managers meeting discussion of the station air compressor preventative maintenance program was ineffective in identifying the presence of the cross-tie circuit or true risk of performing the maintenance work as planned.
6. Corrective Action Process Barrier: Past condition and problem reports for the Unit 1 station air compressors could have provided an opportunity to identify the presence of the energized cross-tie circuit. None of these previous events resulted in a loss of the station air compressors, however; the circuit issues identified in these condition and problem reports could have raised the level of 'questioning attitude' related to how the circuit design was developed.
In addition, station personnel did not fully understand the limitations of the diesel air compressor 1SA-C-2, which did not have an automatic start feature on low air pressure or sufficient capacity to recover station air pressure to prevent the need for a manual reactor trip. This led to a false sense of security that 1SA-C-2 was an adequate contingency backup air supply during the time that that 1SA-C-1B was on clearance for preventative maintenance. A corrective action from the June 22, 2001 loss of instrument air event (BVPS Unit 1 LER 2001-001) was to install a larger capacity diesel driven air compressor with an automatic start feature. This air compressor was on site and scheduled for installation within the next several weeks of the December 7, 2001 trip; however, the implementation of this corrective action was not timely to prevent this trip.
SAFETY IMPLICATIONS
Unit 1 was at normal 100% power operations prior to the manual reactor trip. An evolution was in progress that contributed to the event and impacted plant response. The evolution was the maintenance being performed on station air compressor 1SA-C-1B. Station air compressor 1SA-C- 1B was inoperable/unavailable due to maintenance and was therefore unavailable to mitigate the loss of station air pressure event. All ESF systems functioned as expected for this event. The turbine tripped correctly as a result of the reactor trip and the main generator tripped correctly in conjunction with the turbine trip. All of the auxiliary feedwater pumps started correctly and all reactor control rods indicated on bottom following the reactor trip. Emergency Operating Procedure E-0 for Reactor Trip was performed and the plant was stabilized in Mode 3. A comparison of plant response to the Unit 1 UFSAR also found that the plant response was conservative with respect to assumptions modeled in the UFSAR analysis for a loss of station air.
The plant risk associated with the BVPS Unit 1 manual reactor trip on December 7, 2001, due to a loss of instrument air pressure is considered to be low. This is based on the conditional core damage probability for the event when considering the actual component unavailabilities that were present at the time of the trip.
Based on the above, the safety significance of the manual reactor trip on December 7, 2001 was low.
CORRECTIVE ACTIONS
1. Interim station guidance for increased review of work activities was implemented. These controls provide for a review of work activities to establish a risk level and increased management review and approval of the activity depending on the risk level of the activity.
The interim controls will remain in place until procedure changes reflecting the revised processes are implemented.
2. Warning labels were placed on the control cabinets for the 1SA-C-1A and 1SA-C-1B air compressors to warn personnel of circuit crosstie interface in the panels.
3. Separate drawings will be created for the control circuitry for compressors 1SA-C-1A and 1SA-C-1B.
4. An investigation will be performed for the feasibility of removing the crosstie interface circuit from the Unit 1 station air compressors.
5. An evaluation will be performed to improve the station planning process.
6. An evaluation will be performed to improve the station scheduling process.
7. A review will be performed of the causal factors related to electrician use of tools and skill of the craft practices related to working in energized or de-energized equipment.
8. A larger capacity diesel air compressor with an automatic start feature on low air pressure was installed in the Unit 1 station air system.
Completion of above corrective actions and resulting follow-up actions is being tracked through the Corrective Action Program.
PREVIOUS SIMILAR EVENTS
A review of past Beaver Valley Power Station Licensee Event Reports for the last five years found one similar event at BVPS Unit 1. A manual reactor trip due to loss of station instrument air occurred on June 22, 2001, and was reported by LER 2001-001 on August 13, 2001.