05000301/LER-2011-003
| Point Beach Nuclear Plant, Unit 2 | |
| Event date: | 04-08-2011 |
|---|---|
| Report date: | 06-07-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3012011003R00 - NRC Website | |
Event Description
During the ongoing Unit 2 Refueling 31 outage, with Unit 2 in MODE 5 and Unit 1 in MODE 1 (100% power), clearance orders were developed to support termination of wiring for the Unit 2 Train A and Train B safeguards buses. The clearance orders opened the primary input DC breaker to its respective Unit 2 Train A and Train B safeguards relay circuitry. The clearance orders were hung on April 6 and 7, 2011.
On April 8, 2011 at 20:31 CST, Train B emergency diesel generator (EDG) G-04 [EK], was removed from service for its monthly Technical Specification (TS) test. Preparations for the Unit 2 containment integrated leak rate test were being performed concurrently with the G-04 monthly surveillance. One of the required actions was to manually test the Unit 2 containment isolation (Cl) function [JM]. At 23:36 CST, operators were unable to initiate a Cl signal. While investigation of this issue was in progress, the G-04 surveillance test was completed on April 9, 2011, at 00:36 CST and the EDG was returned to service.
Removal of the G-04 EDG from service for surveillance testing at 20:31 CST resulted in an unplanned and unidentified TSAC entry into TS 3.8.1.G for Unit 1 and TS 3.8.2.B for Unit 2 that rendered standby emergency power to 4160 V buses 2A-05 and 2A-06 and 480 V buses 2B-03 and 2B-04 inoperable for approximately four (4) hours. The Completion Time for TS 3.8.2.B is Immediately. Accordingly, this resulted in a condition prohibited by Technical Specifications and is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B).
The Completion Time for TS 3.8.1.G is two (2) hours, with subsequent Condition H requiring that the affected unit be in MODE 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. EDG G-04 was returned to service approximately two hours into the Condition H specification.
The event does not constitute a safety system functional failure.
Analysis of the Event
The clearance orders included opening of the primary DC input breaker and removing actuation power to the A or B train-specific safeguards relay rack. One of the clearance orders removed power to the Train A service water (SW) pump auto-start relay associated with the G-02 EDG , while the second removed power to the Train B SW pump auto-start relays. Technical information used to develop the clearance orders superficially appeared to be similar for both the A and B trains. However, the A Train G-01 and G-02 EDG associated SW pump auto-start feature information provided in the Bases for TS 3.8.1 was not clearly delineated in the technical information used to develop the clearance orders.
The Bases for TS 3.8.1 state the Train A SW pump auto-start feature is required for operability of the unit's 4160 V standby emergency power sources (1A-05 or 2A-05). The A train EDGs (G-01 and G-02) are dependent upon SW for cooling, while the B train EDGs (G-03 and G-04) are not. Therefore, the auto-start signals are not required to support operability of the 1A-06 or 2A-06 4160 V standby emergency sources supplied by G-03 and G-04. The clearance order did not affect the SW pump auto-start feature for EDG G-01.
Therefore, EDG G-01 was operable during this event.
With EDG G-04 out of service for surveillance testing and G-02 EDG out of service due to the SW auto-start signal being defeated, standby emergency power was not available for 4160 V buses 2A-05 and 2A-06.
These buses feed 480 V buses 2B-03 and 2-B-04, respectively. Therefore, the loss of standby emergency power to 480 V buses 2B-03 and 2B-04 for approximately four (4) hours should have resulted in an entry into TS 3.8.2.B for Unit 2 with a Required Action B.1 Completion Time of Immediately. As previously noted, standby emergency power to buses 2B-03 and 2B-04 was restored when EDG G-04 was returned to service following completion of a successful surveillance test, and prior to a determination of the cause of this event.
Cause of the Event
The validity of the underlying assumptions contained in the TS was not reviewed during the review and approval of the clearance order. Personnel trusted their knowledge of the TS rather than verifying the assumptions of the clearance order with the TS Bases.
Safety Significance
The EDG configuration consists of four shared EDGs. Each EDG is capable of sequentially starting and supplying the power requirements for one complete set of safeguards equipment for one reactor unit in the event of a design basis accident (DBA) and providing sufficient power to allow the second reactor unit to be placed in a safe shutdown condition. Each EDG provides the necessary power to cool the core and maintain containment pressure within the design value for a loss of coolant accident, coincident with a loss of offsite power, in addition to supplying sufficient power to shut down the unaffected unit.
At no point were all four EDGs inoperable. EDGs G-01 and G-03 were operable throughout the approximate four (4) hour event. Therefore, the ability to mitigate the consequences of a DBA on the operating unit and to maintain the opposite unit in the safe shutdown condition was not compromised. Normal power to buses 2A-05, 2A-06, 2B-03, and 2B-04 was provided for the duration of the event. Accordingly, the nuclear safety significance of this event was low.
Corrective Actions
A review of current and pending clearance orders was performed. This review found no currently hanging or planned clearance orders that have an adverse/unanticipated plant impact. Additional actions, including revision of procedure guidance, are being tracked to completion in the corrective action program.
Similar Events None
Failed Components
None