05000261/LER-2010-004
| Docket Number | |
| 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)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2612010004R00 - NRC Website | |
I. DESCRIPTION OF EVENT
On April 26, 2010, with H. B. Robinson, Unit No. 2, shutdown in Mode 6 during Refueling Outage 26 (RO-26), it was determined that a condition existed that could have prevented the fulfillment of the safety function needed to mitigate the consequences of an accident.
From April 18 to April 26, 2010, an equipment clearance had inadvertently rendered Emergency Diesel Generator (EDG) 'A' [EK] inoperable by removing power from the 'A' Train Blackout Sequencer [EK:2]. During this time period, on April 24, 2010, EDG 'B' was taken out of service at 1343 hours0.0155 days <br />0.373 hours <br />0.00222 weeks <br />5.110115e-4 months <br /> for required surveillance testing in accordance with OST-401-2, "EDG B Slow Speed Start." Technical Specification (TS) 3.8.2 requires one diesel generator capable of supplying one train of the onsite AC electric power distribution subsystem(s) when in Mode 6. TS 3.8.2, Condition B, states that if one required EDG is inoperable initiate action to restore an EDG to operable status immediately. The condition where EDG 'B' was out of service for surveillance testing while EDG 'A' was inadvertently inoperable resulted in a failure to meet the required action associated with TS Action Statement 3.8.2.B.4.
This condition is reportable under 10 CFR 50.73(a)(2)(i)(B), "Any operation or condition which was prohibited by the plant's Technical Specifications." In addition, the combined inoperability of EDG 'A' and 'B' for approximately 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 22 minutes, is a condition which could have prevented the fulfillment of a safety function and is reportable under 10 CFR 50.73(a)(2)(v)(D), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
II. CAUSE OF EVENT
The cause of the event was determined to be insufficient procedural guidance. Guidance was not available to provide a risk assessment or an additional supervisory review of a clearance following the completion of the pre-outage clearance window. Additional weaknesses were identified in operator knowledge in the areas of; control wiring diagrams, EDG Blackout Sequencer, and Technical Specifications.
III. ANALYSIS OF EVENT
The conditions described in this Licensee Event Report are reportable in accordance with 10 CFR 50.73(a)(2)(i)(B), Any operation or condition which was prohibited by plant's Technical Specifications," and 10 CFR 50.73(a)(2)(v)(D), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
This event was investigated in accordance with the HBRSEP, Unit No. 2, Corrective Action Program (CAP) and documented in Significant Adverse Nuclear Condition Report 395800. This reportable event and the associated significant adverse condition investigation was reviewed by the Plant Nuclear Safety Committee on June 8, 2010. The investigation for the inoperability of EDG 'A' due to the clearance error identified that insufficient procedure guidance was the cause.
Updated Final Safety Analysis Report, Section 8.3.1.1.5.3, provides the description of the expected plant response to a loss of power and the blackout load sequence. The automatic sequence upon undervoltage on an emergency bus is described as follows:
1. All motor feeder breakers, the main supply and the tie breakers which are on the affected bus are tripped, except Motor Control Center (MCC)-5 and MCC-16 2.. The diesel generator is started 3.After the unit comes up to voltage, the emergency generator breaker is automatically closed and the electrically driven auxiliary feedwater, service water, and component cooling water pumps connected to the bus automatically start (the automatic starting of these loads via the blackout loading sequencer was inoperable due to the clearance error) 4.Other auxiliaries are manually started as required for safe plant operation.
In this event, EDG 'A' was capable of automatically starting and providing power to the associated bus. The automatic loading of the blackout sequence loads was inoperable.
Therefore, manual operation of the required loads would have been required if a loss of power had occurred. The service water pumps are needed to maintain cooling water flow to the EDG.
The investigation determined that EDG 'A' was considered available during the period from April 18 to April 26, 2010. Operator action would have been required to restore service water via the control room in accordance with procedure, AOP-020, "Loss of Residual Heat Removal.
AOP-020 is a direct entry procedure for a loss of Residual Heat Removal and includes steps to restore service water. A validation of operator actions was conducted with two Nuclear Shift Managers. The operators validated the procedural guidance based on conditions similar to the event and it was concluded that it would take approximately 25 minutes to restore the required components that would normally be started via the 'A' Train Blackout Sequencer. The EDG can operate without service water under station blackout loads for approximately 40 minutes.
An extent of condition was performed as part of the investigation. No conditions were identified during RO-23, RO-24, and RO-25 where control power was removed from the blackout sequencer while the EDG was required for operability.
IV. CORRECTIVE ACTIONS
Completed Corrective Actions:
- At the time of discovery, EDG 'A' was declared inoperable and actions were taken to remove the clearance to restore EDG 'A' to operable status. EDG 'A' was restored to operable status at 1743 hours0.0202 days <br />0.484 hours <br />0.00288 weeks <br />6.632115e-4 months <br /> on April 26, 2010.
- Active clearances were reviewed and evaluated independently for impact to component operability and a standing instruction was provided on clearance preparation and review.
- Procedure, OWP-015, "Reactor Protection and Safeguards," was revised to include information on the relevant Technical Specifications associated with emergency diesel generators.
- Procedure, APP-002, "Engineering Safeguards," was revised to include information on the relevant Technical Specifications associated with emergency diesel generators.
Planned Corrective Action:
- Procedure, OMM-001-18, "Outages," is scheduled to be revised by June 29, 2010, to include requirements for additional reviews for clearances developed following completion of the clearance development window.
- Additional operator training on control wiring diagrams and Technical Specifications is scheduled for completion by August 10, 2010.
V. ADDITIONAL INFORMATION
Previous Similar Events:
Licensee Event Reports (LERs) for HBRSEP, Unit No. 2, were reviewed from the past 10 years.
There were no similar events identified.