05000346/LER-2012-001
| Docket Numbersequential • Rev | |
| Event date: | 05-19-2012 |
|---|---|
| Report date: | 07-23-2012 |
| 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 |
| 3462012001R00 - NRC Website | |
Energy Industry Identification System (EllS) codes are identified in the text as [XX].
System Description:
The Davis-Besse Nuclear Power Station (DBNPS) onsite electric power system [EB] is powered by two separate and independent Emergency Diesel Generators (EDGs) [EK-DG]. These EDGs provide onsite standby electrical power sources to supply their respective 4160 Volt Alternating Current (AC) Essential Buses [EB-BU]. Additionally, one non-class 1 E diesel generator can be aligned to power either 4160 Volt Essential Bus in the event of a station blackout.
The DBNPS Direct Current (DC) electrical power system [El] provides a continuous power source to essential instruments, safety features systems, and necessary control systems regardless of the auxiliary electrical system conditions. The DC System is arranged in two completely independent load groups (trains). Each train consists of a power source consisting of two 125 VDC batteries [EJ-BTRY], one battery charger [EJ-BYC] for each battery along with a spare battery charger per train. Each train of distribution consists of a DC motor control center (DCMCC), two essential distribution panels, and two non-essential distribution panels.
Each essential distribution panel is normally supplied from one DCMCC, with a manually transferred alternate supply available from the opposite DCMCC for maintenance purposes when the plant is in cold shutdown. The manual switches and related fuses for transferring the essential distribution panel between the normal and alternate DCMCC supply are classified as administratively controlled circuits.
These administratively controlled circuits enable transferring the essential distribution panel between normal and alternate supply, thus allowing the battery charger and battery power sources from one train to be cross-connected to the essential distribution panels for both trains.
Technical Specification(s):
Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.2 requires one EDG be operable to supply one train of the onsite Class 1 E AC electrical power distribution subsystems while the plant is in Modes 5 and 6 and during movement of irradiated fuel assemblies. TS LCO 3.8.5 requires one train 1 or train 2 DC electrical power source be operable to support one train of the DC electrical power distribution system while the plant is in Modes 5 and 6 and during movement of irradiated fuel assemblies. TS LCO 3.8.10 requires the necessary portions of AC, DC, and AC vital bus electrical power distribution subsystems be operable to support equipment required to be operable while the plant is in Modes 5 and 6 and during movement of irradiated fuel assemblies. With any of this required equipment inoperable, the applicable TS LCO Conditions require movement of irradiated fuel assemblies be suspended immediately, any operations involving positive reactivity additions that could result in loss of shutdown margin or boron concentration be suspended immediately, and actions be immediately initiated to restore the required equipment to operable status.
DESCRIPTION OF EVENT:
On May 6, 2012, the DBNPS shutdown for refueling and maintenance activities. On May 7, 2012, the station entered Mode 5, and EDG 1 was declared inoperable for maintenance activities, including replacement of its exhaust piping. EDG 2 remained operable to meet the requirements of TS LCO 3.8.2 in Modes 5 and 6. On May 10, 2012, train 1 of the DC System was removed from service for maintenance activities, including replacement of cell 13 of Battery 1P. Train 2 of the DC System DESCRIPTION OF EVENT: (continued) remained operable to meet the requirements of TS LCO 3.8.5 in Modes 5 and 6. On May 11, 2012, after replacement of the battery cell was complete, Battery 1P was placed on an equalizing charge. A performance discharge test was conducted on Battery 1P on May 14, 2012, followed by recharging of the battery.
On May 17, 2012, after EDG 1 had been returned to service, EDG 2 was removed from service at 0206 hours0.00238 days <br />0.0572 hours <br />3.406085e-4 weeks <br />7.8383e-5 months <br /> for replacement of its exhaust piping along with other maintenance activities. On May 19, following completion of Battery 1P charging at 0445 hours0.00515 days <br />0.124 hours <br />7.357804e-4 weeks <br />1.693225e-4 months <br />, DC System Train 1 loads were returned to their normal lineups. This included a live transfer of essential DC distribution panel D1P at 2231 hours0.0258 days <br />0.62 hours <br />0.00369 weeks <br />8.488955e-4 months <br />.
Surveillance Testing was then conducted for Train 1 of the DC System.
In parallel with these activities, fuel handling activities commenced on May 18, 2012, with the core offload to the Spent Fuel Pool being completed on May 20, 2012, at 1548 hours0.0179 days <br />0.43 hours <br />0.00256 weeks <br />5.89014e-4 months <br />. On May 22, 2012, while making preparations for reloading the fuel into the reactor core, it was discovered that monthly surveillance tests for batteries 1N and 1P were overdue, potentially rendering Train 1 of the DC System inoperable. Subsequent reviews determined that DC System distribution panel D1P had been transferred from alternate to normal alignment prior to completion of required testing to demonstrate the power source was operable following maintenance. Following the transfer of panel D1P at 2231 hours0.0258 days <br />0.62 hours <br />0.00369 weeks <br />8.488955e-4 months <br /> on May 19, 2012, Battery 1P was functioning as part of the DC electrical power source for EDG 1 to provide the loss of power start function for EDG 1 while EDG 2 was inoperable for exhaust piping replacement. While Battery 1P was available and in service at this time, it was inoperable with the station in Mode 6 with fuel handling activities in progress, resulting in no operable EDG as required per TS LCO 3.8.2.
CAUSE OF EVENT:
On May 19, 2012, personnel in the Outage Control Center (OCC) heard reports that battery testing was field complete and that the battery met the test acceptance criteria; however, there was still paperwork to review. The mindset of station personnel was that the DC train 1 power source was functional, and that the transfer from alternate to normal alignment was part of restoration of the power source to a functional state after maintenance. Personnel knew the DC distribution trains were cross-tied to one power source, but did not realize that during fuel handling activities the cross-tie provided the Loss of Power Start function for the Operable EDG 1.
The cause of this event was determined to be less than adequate administrative controls for maintaining the DC System power source operability with the system cross-tied during shutdown conditions.
ANALYSIS OF EVENT:
For the time period when Battery 1P was functioning as part of the DC electrical power source for EDG 1 to provide the loss of power start function without the necessary surveillance testing being completed, the battery was in service and available. Testing completed approximately 1.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> after the transfer of panel D1P to its normal power source documented that Battery 1P had sufficient charge to perform all required functions. The battery had been on equalizing charge for over three days following the battery performance discharge test, and was removed from equalizing charge prior to the source transfer of panel D1 P to Battery 1 P. Completion of required surveillance testing on May 22, 2012, at 1144 hours0.0132 days <br />0.318 hours <br />0.00189 weeks <br />4.35292e-4 months <br /> confirmed all required equipment was operable. Additionally during this time, the Station Blackout Diesel Generator, which has its own DC system and does not rely on the station DC System for its loss of power start function, was available. Therefore this event was of very low safety significance.
Reportability Discussion:
From the time when panel DIP was transferred to its untested normal power source until completion of all required testing, it could not be confirmed that that Battery 1P was operable when D1P was transferred to the normal power supply. Therefore, EDG 1 was inoperable during this time period.
Because EDG 2 was also inoperable at this time, no EDGs were Operable as required by the Technical Specifications. TS LCO 3.8.2 Action B requires that with the one required EDG inoperable, movement of irradiated fuel assemblies shall be suspended immediately (any Modes). Because fuel movement continued during this timeframe, the plant operated in a condition prohibited by the Technical Specifications, which is reportable per 10 CFR 50.73(a)(2)(i)(B).
Also, with no EDGs operable during this timeframe, this represents a condition that could have prevented fulfillment of a safety function for a system needed to mitigate the consequences of an accident (fuel handling accident). This is also reportable per 10 CFR 50.73(a)(2)(v)(D). Because this condition was discovered while the plant was defueled and no fuel movement in progress, no immediate reporting per 10 CFR 50.72(b)(3)(v)(D) was required because the EDGs provided no safety function at the time of discovery.
CORRECTIVE ACTIONS:
Required surveillance testing was completed on May 22, 2012, to confirm train 1 of the DC System and EDG 1 were operable as required for the existing plant conditions.
Procedure DB-OP-06321, "250/125 VDC Station DC Switching Procedure," will be revised for the live transfer of essential distribution panels to add a prerequisite for ensuring operability of the DCMCC being transferred to, or to ensure both EDGs are operable. Procedure DB-OP-06316, "Diesel Generator Operating Procedure," will be revised to include the affect of the operability of the DC distribution subsystem on the operability of the EDG.
Procedures regarding returning systems and components to service will be revised to be applicable at all times when systems or components are required to be functional or operable.
Existing pre-job briefs will be revised and additional pre-job briefs will be created for the live transfer of DC distribution panels to alternate/normal feed so that the impact on the DC System operability is addressed and this event is identified as operating experience.
CORRECTIVE ACTIONS: (continued) The outage schedule will be enhanced by including an activity to verify operability of power sources / loads between surveillance testing and transfer of DC system distribution panels between normal and alternate alignments.
A case study will be disseminated to licensed operators on the intended restoration of the DC system during outages that highlights the problem, causes, and corrective actions identified for this event. The key aspects of this case study will be reinforcement of the expected behaviors during outages with respect to return to service guidance, tracking alternate configurations of key shutdown safety defense in depth systems, and use of the questioning attitude human performance event-free tools.
PREVIOUS SIMILAR EVENTS:
DBNPS Licensee Event Report 2005-002 documented the inoperability of a DC System Battery during a plant shutdown that was not detected during surveillance testing. This resulted in the station starting up without the required operable batteries, which was a condition prohibited by the Technical Specifications. The corrective actions for the 2005 event focused on improving test procedures to ensure all battery cells met operability requirements, and would not be expected to have prevented the current event that involved the interface between the DC System and its effect on the operability of the EDGs. There have been no Licensee Event Reports submitted for the DBNPS in the past three years regarding similar issues where an electrical system alignment was transferred from alternate to normal supply prior to surveillance test completion.