05000397/LER-2011-002
| Columbia Generating Station | |
| Event date: | 08-27-2011 |
|---|---|
| Report date: | 10-25-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 47209 | 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 3972011002R00 - NRC Website | |
CONTINUATION SHEET
Plant Conditions
At the time of the event, the plant was in Mode 4 and 144 days into refueling outage 20 (R-20). Shutdown cooling was provided by Loop B of the shutdown cooling (SDC) [BO] system. Reactor temperature was approximately 107 degrees Fahrenheit and RPV Level was in the 80 - 100 inch range as indicated on shutdown range instrumentation. There was no equipment that was inoperable at the time of the event which contributed to the event.
Event Description
At 2021 hours0.0234 days <br />0.561 hours <br />0.00334 weeks <br />7.689905e-4 months <br /> on August 27, 2011, the Reactor Protection System (RPS) electrical power system bus "B" (RPS-B) [EC] was lost resulting in isolation of the active loop of shutdown cooling . Field survey noted that both Electrical Protection Assembly (EPA). circuit breakers [52] providing normal power supply for RPS-B were in the trip free position. The upstream EPA breaker (RPS-EPA-3B) had undervoltage and power in lights illuminated. No indicating lights were present, on the downstream breaker (RPS-EPA-3D). Immediately following the event, the motor generator [MG] set supplying RPS-B was noted to be running at 120 V with approximately 0 amps indicated current. Loss of RPS-B results in isolation of primary containment isolation valves in multiple systems including closing the common shutdown cooling suction line rendering both shutdown cooling loops inoperable. Due. to both shutdown cooling. loops being inoperable, Technical Specification Action Statement 3.4.10.A was entered.
All expected isolations were validated to have occurred due to the loss of RPS-B, including isolation of containment isolation valves in Groups 2, 4, 6, & 7 including the residual heat removal, reactor water cleanup [CE], reactor recirculation [AD], radiological drain [Wk], control rod drive, [AA] and traversing incore probe [IG] systems. Immediate investigations confirmed that the loss of RPS-B was not caused by ongoing work, or by manual action directly on the circuit breaker. This event is reportable as an event that could have prevented fulfillment of a safety function needed to remove residual heat in accordance with 10 CFR 50.73(a)(2)(v)(B), as well as an invalid system actuation resulting in containment isolation in more than one system. A 10 CFR 50.72(b)(3)(v)(B) notification was made via Event Number 47209.
Immediate Corrective Actions
RPS-B was repowered from the alternate power supply, the valve isolations were reset per plant procedures, and the technical specification action statement was exited.
Following the event, a faulted logic board (GE Model 147D8652G007) [ECBD] was replaced with a vendor recommended newer replacement model. The logic board on the other in-line EPA circuit breaker for RPS-B normal power supply was also replaced with a newer model board.
Assessment of Safety Consequences
The total time without shutdown cooling was 34 minutes and the reactor coolant system (RCS) temperature rose approximately 4 degrees Fahrenheit. The calculated time to reach. 200 degrees Fahrenheit was approximately 11 hours1.273148e-4 days <br />0.00306 hours <br />1.818783e-5 weeks <br />4.1855e-6 months <br />. An extended loss of shutdown cooling could have led to excessive fuel temperatures 26158 R5 and ultimately fuel damage. However, the actual significance was limited due to the short duration of the SDC outage and minimal temperature rise with substantial margin to 200 degrees.
If the event had occurred earlier in the refueling outage shortly after shutdown, the significance would have been greater since the time to 200 degrees F is much shorter and could have resulted in an unintended and unplanned MODE change from MODE 4 to MODE 3.
Had the logic card failure occurred in Modes 1 or 2, the same isolations and actuations would occur but would not be expected to result in a plant trip. The resulting isolations would require operator intervention to mitigate, but all response actions are covered by existing station procedures.
Cause of Event
The reason for the loss of RPS-B was due to a spUrious undervoltage trip signal from a logic board.which is part of the EPA circuit breaker for RPS-B. The spurious undervoltage signal was caused by a logic board fault, which has been identified as the direct cause of the'eveni. The root cause is that Energy Northwest was not proactive in replacing older, obsolete logic boards.with new models recommended by the vendor.
Similar Events No previous occurrences involving a logic board failure leading to an EPA breaker trip have been identified at Columbia.
Further Corrective Actions At the time of the event, a total of six of the GE Mbdel-147D8652G007:logic boards' were installed in the plant.
Two were replaced as part of the irritnediate corrective actions and the remaining fOurjwill be replaced as part of the corrective actions to prevent recurrence. Analysis of the logic board by the vendor was inconclusive.
Energy Industry Identification System (EllS) Information codes from IEEE Standards 805-1984 and 803-1983 are represented in brackets as [XX], [XXX] and [XXXX] throughout the body of the narrative.
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