05000323/LER-2010-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | 7-21-2008 |
|---|---|
| Report date: | 03-17-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 3232010001R00 - NRC Website | |
I. Plant Conditions
At the time of the event Unit 2 was in Mode 1 (Power Operation) at approximately 100 percent reactor power with normal operating reactor coolant temperature and pressure.
II. Description of Problem
A.�Background The DCPP Auxiliary Building Ventilation System (ABVS) exhaust fans provide a constant exhaust air flow from the DCPP auxiliary building. The DCPP ABVS exhaust fans also:
- Maintain a negative pressure in the DCPP auxiliary building.
- Limit the maximum design engineered safety feature (ESF) equipment room temperature to 104°F in the auxiliary building.
- Ensure radioactivity is filtered during accident conditions and released through the exhaust system and monitored at the plant vent.
The ABVS has several modes of operation: Building Only operation, Safeguards operation, and Building and Safeguards operation. A safety injection signal ('S' signal) will cause a nonoperating, stand-by ABVS exhaust fan to start and align to the plant vent. An operating ABVS exhaust fan experiencing low inlet flow will shutdown down automatically. Should a low flow event occur, a system reset is required for the ABVS prior to manually restarting or auto-restart via an 'S' signal.
B.�Event Description On July 21, 2008, 05:04 PDT, DCPP Unit 2 entered Technical Specification (TS) 3.0.3 when plant operators closed the suction dampers of the running exhaust fan E-1 prior to placing an 'S' signal into the system. This action resulted in tripping auxiliary building exhaust fan E-1 with auxiliary building exhaust fan E-2 cleared for maintenance. Placing a 'S' signal into the system would have opened an alternate flow path and would have prevented the trip of the running fan.
Failure to select the 'S' signal caused the only available exhaust fan to trip off on low flow, placing the plant in a one-hour TS action statement. This resulted in a loss of all DCPP Unit 2 ABVS exhaust.
On July 21, 2008, 0534 PDT, DCPP Unit 2 exited TS 3.0.3 when plant operators restarted auxiliary building exhaust fan E-1 by resetting the ABVS and inserting a 'S' signal.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) On January 28, 2010, PG&E conducted a review of past events for safety system functional failures. This review was conducted utilizing recently published U.S.
NRC training material on this subject. This review concluded that the simultaneous inoperability of the DCPP Unit 2 auxiliary building exhaust fans constituted a safety system functional failure and as such was reportable under 10 CFR 50.73(a)(2)(v).
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event None.
D. Other Systems or Secondary Functions Affected No additional safety systems were adversely affected by this event.
E. Method of Discovery PG&E conducted a review of past events for safety system functional failures.
This review was conducted utilizing recently published U.S. NRC training material on this subject. This review concluded that the simultaneous inoperability of the DCPP Unit 2 auxiliary building exhaust fans constituted a safety system functional failure.
F. Operator Actions None.
G. Safety System Responses Plant operators restarted DCPP Unit 2 auxiliary building exhaust fan E-1 in safeguards mode by initiating a system reset and inserting a 'S' signal.
III.�Cause of the Problem A.�Immediate Cause Plant operators incorrectly caused the operating auxiliary building exhaust fan E-1 to secure on low flow with auxiliary building exhaust fan E-2 cleared for maintenance.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) B.� Cause Operations personnel performing the work were using a partially marked up procedure and a clearance to take the E-2 Fan out of service to allow maintenance work on the system. The operators did not recognize the sequence of the process required the 'S' signal be selected prior to closing the dampers isolating flow to the fan.
IV.� Assessment of Safety Consequences Under 10 CFR 50.73(a)(2)(v), "Any event or condition that alone could have prevented the fulfillment of the safety function of structures or systems that are or systems that are needed 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 event was reviewed to determine if it meets the criteria for a safety system functional failure. Based on a review of the event, equipment in the same system, ABVS exhaust fans E-1 and E-2, were not operable and available to perform the required safety function. The design of these fans impacts subsections (C) and (D), regarding control the release of radioactivity by filtering the exhaust stream and mitigate the consequences of an accident by maintaining the ESF room temperatures below the design limits, as cited above.
Although both DCPP Unit 2 ABVS exhaust fans were not operating, placing the unit into TS 3.0.3, the allowed outage time for this specification was not exceeded and the ESF features for this system were fully capable of performing their design safety functions. Based on the foregoing, this event is not considered risk significant and did not adversely affect the health and safety of the public.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) V.ACorrective Actions A. Immediate Corrective Actions 1. DCPP Unit 2 exited TS 3.0.3 when plant operators restarted auxiliary building exhaust fan E-1 by initiating a 'S' signal.
B. Corrective Actions to Prevent Recurrence (CAPR) 1. ABVS exhaust fan clearances have been updated to clearly show that a 'S' signal must be initiated prior to removing the associated ABVS exhaust fan from service.
VI.AAdditional Information
A. Failed Components
None
B. Previous Similar Events
None.
C. Industry Reports None.