05000293/LER-2016-002
| Pilgriin Nuclear Power Station | |
| Event date: | 04-19-2016 |
|---|---|
| Report date: | 08-18-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| Initial Reporting | |
| 2932016002R01 - NRC Website | |
| ML16174A285 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim, Pilgriin Nuclear Power Station |
| Issue date: | 06/20/2016 |
| From: | Perkins E P Entergy Nuclear Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 2.16.034 LER 16-002-00 | |
| Download: ML16174A285 (11) | |
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BACKGROUND
There are six 4,160 volt alternating current (AC) buses (Al, A2, A3, A4, A5, and A6) in the station auxiliary power distribution system. The six buses are divided into emergency service and normal service buses. The two emergency service buses, A5 and A6, supply power to essential loads required during abnormal operational transients and accidents. The four normal service buses, A1, A2, A3, and A4, supply power to other station auxiliaries requiring AC power during planned operations. The maintenance activity had no effect on the normal service buses.
When the Pilgrim Nuclear Power Station (PNPS) is on line and buses A5 and A6 are powered from the Unit Auxiliary Transformer, on a turbine trip or reactor scram, Buses A5 and A6 should fast transfer to the startup transformer or dead bus transfer to EDGs if the startup transformer is unavailable or dead bus transfer to the shutdown transformer if both the startup transformer and EDGs are unavailable.
Another 4160 volt bus, Bus A8, is powered from either the shutdown transformer through Breaker A802 or the Station Blackout Diesel Generator (SBODG) through Breaker A801. Bus A8 provides power to Emergency Bus A5 through Breakers A600 and A501 and to Emergency Bus A6 through Breaker A600 and A601. Relays in the Bus A8 to Bus A5 and Bus A8 to Bus A6 control circuits are tested while the plant is online, at a two year interval, in accordance with Procedure 3.M.3-1, A5/A6 Buses 4kV Protective Relay Calibration/Functional Test and Annunciator Verification, Attachment 10, Bus A8 to A5 & A6 Relays. See Figure 1 for bus and breaker arrangement.
Procedure 3.M.3-1, Revision 136, step 6.2[5] and Attachment 10, provide a caution that during the performance of Attachment 10, with breakers 152-501 and 152-601 in the TEST position and CLOSED, in the presence of a plant Scram neither A5 nor A6 would automatically transfer to the Startup Transformer rendering both buses unavailable. Based upon this caution, activity risk compensatory measures for the August 2014 maintenance activity included a compensatory measure for operators to be briefed such that in the event of a plant trip the 152-501 and152-601 breakers would be tripped to restore the automatic load transfer function. The compensatory measure is a simple single control switch manipulation for each of the two buses.
Procedure 3.M.3-29, Shutdown Transformer and 23kV Relay Calibration and Functional Test, Revision 23, included, Prerequisite step 7.1[4], which states: "With permission from the Shift Manager (depending on weather conditions), the relays in Attachment 1 Step [2] may be calibrated with the Shutdown Transformer on line. The relays are to be calibrated one at a time. If plant electrical distribution will require the Shutdown Transformer to be loaded, the relay will be installed in an expeditious manner." Consistent with this prerequisite, a Control Room Narrative Log entry on February 20, 2014, at 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br /> 17 seconds, indicates that a Pre-Evolution Brief (PEB) was conducted for 3.M.3-29 to bring attention to the following: the limiting condition for operation (LCO) for shutdown transformer (SDT) will be entered but SDT will remain available; and per procedure, the associated protection relays will be removed from service for functional test and calibration, one at a time, with procedures in place to return to service expeditiously in the event that plant electrical distribution requires the SDT to be loaded; and that E-lab personnel have been briefed and will remain on-site with no concurrent duties while the relays are out of service (00S).
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For general information, when a 4160 volt breaker is required to be in TEST position, the breaker shall have been racked down, test position spindle inserted, test position cable assembly in place, 125 volt direct current (DC) control knife blade disconnect in closed position, and 4160 volt breaker springs charged.
As specified under Limiting Conditions for Operations (LCO) Action Statement 3.9.6.2, "From and after the date that incoming power is not available from both startup and shutdown transformers, continued operation is permissible, provided both diesel generators and associated emergency buses remain operable, all core and containment cooling systems are operable, and reactor power level is reduced to 25 percent of design".
EVENT DESCRIPTION
On April 19, 2016, at approximately 1450 hours0.0168 days <br />0.403 hours <br />0.0024 weeks <br />5.51725e-4 months <br />, it was discovered, and documented in the Corrective Action Program under Condition Report CR-PNP-2016-02735, that maintenance activities performed on protective relays between 2010 hours0.0233 days <br />0.558 hours <br />0.00332 weeks <br />7.64805e-4 months <br /> on August 26, 2014 and 0143 hours0.00166 days <br />0.0397 hours <br />2.364418e-4 weeks <br />5.44115e-5 months <br /> on August 27, 2014, had rendered the Startup Transformer (X4) and the standby EDGs (X-107A&B) unable to automatically supply power to Buses A5 and A6, due to the breaker interlock that prevents Startup Transformer breakers (152-504 and 152-604) and Standby EDG breakers (152-509 and 152-609) from closing when Bus A5 breaker (152-501) and Bus A6 breaker (52-601) are in the TEST position and CLOSED: With incoming power unavailable from the shutdown transformer and power not automatically available from the startup transformer or the standby EDGs, the Limiting Conditions for Operation (LCO) 3.9.112 was not met. See Figure 1 for bus and breaker configuration.
During functional testing of negative sequence relays 146-600/A and B undervoltage relays 127-600A/1 and 2 and 127-600B/1 and 2, breakers 152-501 (shutdown transformer supply to Bus A5) and 152-601 (shutdown transformer supply to A6) are closed briefly and tripped on three different occasions in accordance with Procedure 3.M.3-1, Attachment 10. Based on a review of associated alarm data, operator logs, and interviews with maintenance personnel, the longest single duration when the buses were not operable was approximately 13 minutes; the combined duration is estimated to be approximately 33 minutes.
Since this event occurred in the past, no immediate actions were required. Work Order (WO-52581,885) for the next scheduled functional test has been placed on administrative hold pending implementation of associated corrective actions.
CAUSE OF THE EVENT
While performing the extent of condition review for this event it was discovered, and entered into the Corrective Action Program under CR-PNP-2016-04139, that surveillance procedure 3.M.3-29, Shutdown Transformer and 23kV Relay Calibration and Functional Test, places the 4160V AC System in similar test configurations and operating conditions as procedure 3.M.3-1, Attachment 10. When the four year interval functional test required by procedure 3.M.3-29 was performed in February of 2014 it placed the plant in a configuration where both 4kV Emergency Buses A5 and A6 would have been prevented from automatically transferring to back-up power sources for approximately 26 minutes. Accordingly, the scope of the causal evaluation addresses the surveillance test configurations established by procedure 3.M.3-1, Attachment 10, and procedure 3.M.3-29.
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The root cause for placing the plant in a configuration where both Emergency Buses A5 and A6 would have been prevented from automatically transferring to back-up power sources is that the decision to perform 3.M.3-1, Attachment 10, and 3.M.3-29 online, instead of during cold shutdown, lacked sufficient rigor to ensure compliance with Technical Specifications. A contributing cause of this condition is corrective actions were ineffective in resolving identified risks with the online performance of 3.M.3-1 Attachment 10 and 3.M.3-29 protective relay functional tests. Another contributing cause is that Senior Reactor Operators have less than adequate task specific knowledge of Limited Conditions for Operation.
CORRECTIVE ACTIONS
The corrective action to prevent recurrence is to establish and institutionalize expectations and accompanying accountability for station leadership (i.e., supervisor and above) regarding consequence-biased decision- making and effective risk management.
Other correction actions include:
- Revising procedures 3.M.3-1, Attachment 10, and 3.M.3-29 so that they'may only be performed while in the cold shutdown condition
- Providing training to personnel on condition report initiation (Problem Identification) and corrective action closure quality (Evaluation and Resolution)
- Developing and presenting training to SROs on integrated knowledge of operability, availability, risk mitigation, and LCO requirements
SAFETY CONSEQUENCES
The actual consequences of the procedure 3.M.3-1, Attachment 10, and procedure 3.M.3-29 surveillance test configurations placed the plant in a condition not allowed by Technical Specifications. As there was no initiating event, there were no actual safety consequences.
The performance of procedure 3.M.3-1 and procedure 3.M.3-29 surveillance testing placed the plant in a configuration where both 4KV Emergency Buses A5 & A6 would have been prevented from automatically transferring to back-up power sources upon a plant scram. This testing placed the plant in a condition not allowed by Technical Specifications.
Had there been an initiating event (plant scram) during the performance of procedure 3.M.3-1, both Emergency Buses A5 and A6 would be inoperable immediately and until the recovery action (tripping of breakers 152-501 and 152-601) took place. The recovery action is a simple action, capable of being performed in the main control room or locally, and was part of the pre-job brief. Because the test configuration was brief and because qualified, dedicated operators were briefed and available to perform simple compensatory measures, the potential safety consequence was considered minimal.
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The potential safety consequence of this event if the "operator" barrier was removed is a station blackout condition. Had the resulting consequence of the August 2014 maintenance testing been a station blackout, PNPS Procedure 5.3.31, Station Blackout, would have been entered and the prescribed immediate and subsequent actions taken until the preferred (345 kV Offsite) power source or the standby (onsite) EDG power source was restored.
Had there been an initiating event (plant scram) during the performance of procedure 3.M.3-29, both Emergency Buses A5 and A6 would be inoperable immediately and until the recovery actions (tripping of breakers 152-501 and 152-601) took place. Unlike procedure 3.M.3-1, procedure 3.M.3-29 does not provide the following caution: "When breakers 152-501 and 152-601 are placed in the TEST position and CLOSED, in the presence of a plant Scram neither A5 nor A6 would automatically transfer to the Startup Transformer rendering both buses unavailable". In addition, unlike the testing performed under 3.M.3-1, it does not appear that risk activity compensatory measures were identified to direct specific operator actions in the event of a plant scram. However, Bus A5 and A6 recovery actions in the event of a plant scram while performing procedure 3.M.3-29 testing would be identical to recovery actions identified for procedure 3.M.3-1., Attachment 10.
The potential safety consequence of this event if response (recovery) actions were delayed is the possibility of losing 4kV Buses A5 & A6 for an extended period of time, resulting in a prolonged station blackout condition.
Based on the anticipated success of operator action, the risk is considered Low.
REPORTABILITY
This report is submitted in accordance with 10 CFR 50.73(a)(2)(i)(B) as any operation or condition prohibited by the plant's Technical Specifications and; 10 CFR 50.73(a)(2)(v)(A), 50.73(a)(2)(v)(B), 50.73(a)(2)(v)(C) and 50.73(a)(2)(v)(D) as an event or condition that could have prevented the fulfillment of the safety function of a system needed to shut down the reactor and maintain it in a safe shutdown condition, remove residual heat, control the release of radioactive material; or mitigate the consequences of an accident and; 10 CFR 50.73(a)(2)(vii) as any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to: (A) Shut down the reactor and maintain it in a safe shutdown condition; (B) Remove residual heat; (C) Control the release of radioactive material; or (D) Mitigate the consequences of an accident. This condition is also reportable under 10 CFR 50.73(a)(2)(ix)(A) as any event or condition that as a result of a single cause could have prevented the fulfillment of a safety function for two or more trains or channels in different systems that are needed to (1) Shut down the reactor and maintain it in a safe shutdown condition; (2) Remove residual heat; (3) Control the release of radioactive material; or (4) Mitigate the consequences of an accident.
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
PREVIOUS EVENTS
None ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) CODES The EIIS codes for Components and Systems referenced in this report are as follows:
COMPONENTS: N/A CODES: N/A SYSTEMS: EA CODES: N/A
REFERENCES
- Procedure 3.M.3-1, A5/A6 4kV Protective Relay Calibration/Functional Test and Annunciator Verification, Revision 136
- Procedure 3.M.3-29, Shutdown Transformer and 23kV Relay Calibration and Functional Test, Revision 23
- Condition Report CR-PNP-2016-02735
- Condition Report CR-PNP-2016-04139
- Work Order (VVO-52581885) comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53). U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
3. LER NUMBER