05000346/LER-2011-002
| Davis-Besse Nuclear Power Station | |
| Event date: | 11-30-2010 |
|---|---|
| Report date: | 03-23-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3462011002R00 - NRC Website | |
Energy Industry Identification System (EllS) codes are identified in the text as [XX].
The Davis-Besse Nuclear Power Station (DBNPS) was operating in Mode 1 at approximately 100 percent power during all events described below.
System Description:
The Containment Air Coolers (CACs) [BK-CLR] provide for containment atmosphere cooling during both normal operation and accident conditions. Each cooling train is equipped with cooling coils [CCL] and a fan [FAN] driven by a two speed electric motor [MO]. For both normal operation and accident conditions, two of three CACs are in operation in fast speed, and the third (spare) CAC is placed in standby. CAC 3 is designated as a swing component, as it can be manually aligned electrically and mechanically to either train 1 or train 2 of electrical power and the Service Water System (SW) [BI].
Key interlocks are used to ensure CAC 3 is not electrically aligned to both trains at the same time.
The CAC Outlet Temperature Control Valves [BK-TCV] are normally open on operating CACs to provide a flow path of SW from the operating CACs to the SW return header outside containment.
During normal operation, these valves provide temperature control for their respective operating CAC.
In the event of a design basis accident, a Safety Features Actuation System (SFAS) [JE] Level 2 signal will automatically place the two operating CAC fans in low speed to prevent motor overload from the higher density atmosphere. Fan operation in low speed initiates an electrical signal to fully open the Outlet Temperature Control Valves to allow maximum SW flow through the operating CACs. These temperature control valves are unusual in that they are fail-open air-operated Containment Isolation valves. Because the SW piping inside containment is a closed system, only one Containment Isolation valve is provided on the SW return header.
Technical Specification(s):
Technical Specification (TS) Limiting Condition for Operation (LCO) 3.6.3 requires each containment isolation valve be operable while the plant is operating in Modes 1, 2, 3, and 4. With a containment isolation valve inoperable in Modes 1 to 4 for a penetration flow path with only one containment isolation valve, TS LCO 3.6.3 Condition C requires the affected penetration flow path be isolated in 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and verified isolated once per 31 days. If this action and associated completion time cannot be met, then TS LCO 3.6.3 Condition F requires the plant be placed in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
TS LCO 3.6.6 requires two containment spray trains [BE] and two containment air cooling trains be operable while the plant is operating in Modes 1 to 4. With one containment air cooling train inoperable in Modes 1 to 4, TS LCO 3.6.6 Condition C requires the affected train be restored to operable status in 7 days. If this action and associated completion time cannot be met, then TS LCO 3.6.6 Condition F requires the plant be placed in Mode 3 in 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 5 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
DESCRIPTION OF EVENT:
On October 24, 2010, CAC 3 was placed in service aligned as train 1 as part of a routine equipment rotation. Approximately 34 seconds after being started in fast speed, the supply breaker tripped open on overcurrent. CAC 1 was placed back in service to meet TS requirements.
DESCRIPTION OF EVENT: (continued) Clearances were installed on October 24 and 25, 2010 to allow troubleshooting and other associated maintenance on the isolated CAC 3 fan motor. These clearances maintained power to the CAC 3 Outlet Temperature Control Valve (SW 1358), and this valve remained operable as demonstrated by testing on November 23, 2010. However, with the CAC 3 fan motor supply breakers open from both electrical train 1 (BE105) [EC-BKR] and from electrical train 2 (BF105), Control Room annunciators for both Bus El and Fl were brought into alarm.
On November 30, 2010 ("event date"), a revised clearance was installed for CAC 3 fan motor. This action supported the Control Room Annunciator black board philosophy to reduce distractions to the Control Room Operators. This clearance moved the clearance boundaries nearer to CAC 3 fan motor, tagging only the four individual breakers for the high and low speed motor windings for each electrical train (Breakers BEF151, BEF152, BEF153, and BEF154). This allowed supply breakers BE105 and BF105 to be reopened and clear the trouble annunciators for Bus El and Fl. This clearance inadvertently removed control power to the CAC 3 Outlet Temperature Control Valve (SW 1358) solenoid valve, preventing the valve from being closed to perform its containment isolation function.
On January 22, 2011, at 2145 hours0.0248 days <br />0.596 hours <br />0.00355 weeks <br />8.161725e-4 months <br /> ("discovery date"), during performance of procedure DB-PF-03020, Service Water Train 1 Valve Test, it was identified that the CAC 3 Outlet Temperature Control Valve could not be closed using the Control Room handswitch [HIS]. Follow-up investigation identified the problem with the clearance installed in November, so the clearance was removed and control power was restored to the Outlet Temperature Control Valve, allowing the valve to be properly stroked on January 23, 2011, at 0454 hours0.00525 days <br />0.126 hours <br />7.506614e-4 weeks <br />1.72747e-4 months <br />.
CAUSE OF EVENT:
The CAC outlet temperature control valves are unusual in that they are fail-open air-operated Containment Isolation valves. In addition, the temperature control valves are interlocked with the CAC fan motors that operate in multiple speeds with complicated logic schemes (including multiple inputs from the SFAS). The control logic for the CAC 3 outlet temperature control valve is further complicated by the fact that it can be powered from either train of electrical power.
The root cause of this event was that the elementary wiring diagram used to prepare the clearance did not reference an additional drawing to inform the reviewer of breaker contacts that impact the power supply to the CAC 3 Outlet Temperature Control Valve controls. The clearance preparer and reviewer failed to identify this breaker interlock, resulting in the CAC 3 Outlet Temperature Control Valve being rendered inoperable when the clearance was revised on November 30, 2010. The valve remained inoperable until discovered on January 22, 2011, during routine testing.
ANALYSIS OF EVENT:
CAC 3 was the designated spare train during the period of time that the outlet temperature control valve was inoperable. CAC 1 and 2 remained operable during this time period, and the DBNPS Probabilistic Risk Assessment (PRA) does not credit recovery of the spare CAC in the event of an accident.
Therefore, there was no impact on the calculated Core Damage Frequency (CDF) during this time period (November 30, 2010, to January 22, 2011). Because an additional failure of the closed SW piping inside containment would be required for a potential containment bypass pathway to exist, there was no impact on the Large Early Release Fraction (LERF) due to the containment isolation valve being de-energized in the open position. Therefore, this event was of very low safety significance.
Reportability Discussion:
With the CAC 3 outlet temperature control valve not capable of being closed, the valve was not capable of performing its design function of isolating containment as required by TS LCO 3.6.3. This valve was inoperable for approximately 53 days, which exceeded the Completion Time specified in TS LCO 3.6.3 to restore the valve to Operable status or shutdown the plant. Therefore, this issue is being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as it resulted in operation of the plant in a condition prohibited by the TS. Because an additional failure would be required for a potential containment bypass pathway to exist, and no other CAC was affected by the clearance that isolated electrical power to CAC 3, this issue did not result in a loss of safety function.
CORRECTIVE ACTIONS:
The clearance installed November 30, 2010, was removed and control power was restored to the CAC 3 Outlet Temperature Control Valve, allowing the valve to be properly stroked on January 23, 2011, at 0454 hours0.00525 days <br />0.126 hours <br />7.506614e-4 weeks <br />1.72747e-4 months <br />.
The elementary wiring diagrams (E-48B Sheets 33A and 33B, and E-58B Sheets 2A and 2C) associated with the CAC 3 fan motor and CAC 3 Outlet Temperature Control Valve will be revised to add a note/reference to refer to the opposite drawing to inform the reader of the associated interlocks between the equipment.
PREVIOUS SIMILAR EVENTS
There have been no Licensee Event Reports submitted for the DBNPS in the past three years where equipment was inadvertently rendered inoperable for longer than permitted by the TS as a result of a clearance issue.