05000306/LER-2009-001
| Prairie Island Nuclear Generating Plant Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3062009001R00 - NRC Website | |
On February 16, 2009 preparations for planned maintenance on train A of Prairie Island Nuclear was at 100% power and the Unit 2 train B emergency diesel generator, including all required support equipment was operable as required by technical specifications (TS). In order to perform this maintenance activity safely, it was necessary to isolate control power in the circuit. This was accomplished by opening the man control power breaker for the out of service (00S) EDG at 0129 CST. At this time it was recognized that tagging out control power for the train A EDG would remove power from a variety of components including automatic start capability of the train A fuel transfer pumps2.
The following day at 1100 CST, during a walkdown of unrelated work activities, it was discovered that by opening the main control power breaker for the train A EDG, both manual and auto start capabilities of the train A fuel oil transfer pumps were disabled. In preparation of this isolation it was recognized that automatic start capability would be lost, but it was believed that manual start capability was still maintained. PINGP design and TS require that at least 65,000 gallons of fuel be available for the Unit 2 EDGs or they must be declared inoperable. This quantity of fuel oil can be met with any combination of three out of the four available fuel oil storage tanks (FOST) for Unit 2, each having up to 30,800 gallons of fuel oil available. However, by not having the train A fuel oil transfer pumps available to transfer fuel to the train B day tanks, train B EDG was required to be declared inoperable by TS.
Upon being notified of the preceding events, operations immediately entered the applicable TS Limiting Condition for Operation (LCO) Action Statement and commenced actions to restore the train A fuel oil transfer pumps. Approximately 45 minutes later at 1145 CST, both train A fuel oil transfer pumps were returned to service and train B EDG was once again declared operable.
PINGP TS require that upon discovery of having two EDGs inoperable, one of the EDGs must be restored to operable within two hours. If an EDG cannot or is not restored to operable, then the Unit must be in MODE 3 within an additional 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. This event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by TS for not being in MODE 3 within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> of having two EDGs inoperable. In this event, both trains of PINGP Unit 2 were inoperable for approximately 34 hours3.935185e-4 days <br />0.00944 hours <br />5.621693e-5 weeks <br />1.2937e-5 months <br /> and 19 minutes from February 16, 0129 CST when the control power breaker was opened until February 17, 1145 CST when it was subsequently closed.
EVENT ANALYSIS & SAFETY SIGNFICANCE
PINGP Unit 2 has two safety related EDGs. Each EDG is designed to start and carry the engineered safety feature's load required for the design basis accident (DBA) and a concurrent loss of offsite power (LOOP). In addition, in the event of a station blackout (SBO) condition, each EDG is capable EIIS System Identifier: EK 2 EIIS System Identifier: DC of sequentially starting and supplying the power requirements of the loads for the Unit in MODE 3, as well as the essential loads of the blacked-out unit, through the use of manual bus tie breakers.
To accomplish this mission, the Unit 2 emergency diesel generators are supported by two fuel oil day tanks, four FOSTs, and four fuel oil transfer pumps. One fuel oil day tank is dedicated to its respective EDG (either train A or B) and cannot supply fuel to the other EDG directly. However, the day tanks can be filled directly from the FOSTs of the other EDG by performing a valve lineup and using the fuel oil transfer pumps.
The storage capacity of fuel oil for PINGP Unit 2 is based on two EDGs operating for seven days requiring 69,400 gallons, or one EDG operating for 14 days requiring 69,300 gallons. Each FOST can have up to 30,800 gallons available. Therefore, any combination of three tanks is needed to meet the storage capacity. The 14 day storage capacity in the FOSTs required for operation of one EDG is based on allowing enough time to replenish the onsite supply from outside sources in the event of a design basis flood. An analysis performed by the PINGP engineering department determined that with the fuel oil available in the train B FOSTs during this event, the train B EDG would have been able to run for 10.2 days.
This event is considered to have minimal safety significance because had the Unit 2 train B EDG been called upon to operate, it would have been able to start and carry the loads as defined by PINGP's safety analysis report. If this event occurred during a design basis flooding scenario, PINGP would have relied on the two available FOSTs to provide the needed fuel. Although in the current configuration the complete 14 days was not available, the tagged out pump would have been discovered when PINGP operating procedures to transfer fuel from the train A FOST were initiated.
Based on a walkthrough of PINGP operating procedures, this condition would have reasonably been discovered in approximately 5 days when the first FOST was going empty and the need to transfer fuel arose. Therefore, PINGP would have still had approximately an additional 5 days to restore the train A fuel oil transfer pumps to make the rest of the fuel oil inventory available. Actual restoration of the fuel oil transfer pumps took approximately 45 minutes. Therefore, this event is not considered a safety system functional failure and is not reportable under 10 CFR 50.73(a)(2)(v).
This event had no effect on PINGP Unit 1 operation which remained at 100%.
CAUSE
During the planning phase for the train A EDG maintenance, it was identified that control power would need to be de-energized in order to provide adequate worker protection. Operations planning and maintenance planning personnel worked together to find a suitable isolation. Rather than removing various fuses, it was concluded that opening the main control power breaker would provide more adequate worker protection.
Operations planning personnel reviewed the schematics and the proposed clearance. It was readily apparent that opening the main control power breaker would remove the auto-start capability of the fuel oil transfer pumps for the train A EDG scheduled for maintenance. However, it was incorrectly concluded that the pumps could still be operated in manual by using the local control switches at the U.S. NUCLEAR REGULATORY COMMISSIONNRC FORM 366A� LICENSEE EVENT REPORT (LER)(9-2007)
CONTINUATION SHEET
bench board. In reality, by opening the main control power breaker, the ability to start the train A fuel transfer pumps was lost — both in manual and automatic.
A causal evaluation of this event is in progress. The preliminary cause of this event is attributed to the clearance order preparer and approver not requesting assistance from individuals knowledgeable with reading electrical drawings. A contributing factor was unfamiliarity on the part of the operations planner and supervisor in reviewing electrical drawings and that they possessed a less than adequate skill level for reading electrical drawings. At the completion of the causal evaluation, NSPM will supplement this LER if any significant additional information related to the cause is discovered.
CORRECTIVE ACTION
Operators returned train B EDG to service by restoring the train A fuel oil transfer pumps to service.
Interim corrective actions are in place to require all complex electrical isolations to have an independent review by only highly qualified individuals in the electrical maintenance or electrical engineering department.
Long term corrective actions to prevent recurrence will be to enhance the clearance order preparation process and to train personnel using the clearance process to recognize when additional verification and reviews are required. In addition, qualified personnel will be required to perform reviews and approvals of complex electrical clearances. If the causal evaluation identifies any additional significant corrective actions, NSPM will supplement this LER as required.
PREVIOUS SIMILAR EVENTS
A review of past operability for the Unit 2 EDGs identified that a similar condition as described in this LER existed from 2246 CDT February 18, 2007 thru 0051 CDT on February 21, 2007. This condition would also be reportable under 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by TS. PINGP Unit 2 TS require that if a condition exists where two EDGs are inoperable, then the unit must be in MODE 3 within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> and MODE 5 within 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br />. These requirements were not met since both train A and train B EDGs were inoperable for a period of approximately 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br /> and 5 minutes.
On February 26, 2008 an adverse trend in outage related isolations was identified and entered into PINGP's corrective action program. The root cause evaluation for this condition evaluated multiple instances of workers not being adequately protected by the clearance order. It was determined the root cause of these events was that workers involved with the planning, review, implementation, and use of clearance orders did not consistently understand and implement procedural requirements.
One of the corrective actions for this event was to increase the knowledge level of electrical print reading within the operations planning group. As a result, selected individuals attended two days of training in electrical print reading