05000410/LER-2004-001
| Event date: | |
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| 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 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4102004001R00 - NRC Website | |
I. Description of Event
On January 28, 2004, while performing surveillance testing on the Division II Control Room Outside Air Special Filter Train (CROASFT), the required flow to establish test conditions could not be achieved. Initial investigation revealed that backdraft damper 2HVC*DMP93, on the inlet ductwork to the Division II CROASFT, was stuck shut and could not be opened by hand. This resulted in the Division II Control Room Envelope Filtration (CREF) subsystem being inoperable. The plant entered Action A of Technical Specification 3.7.2, "Control Room Envelope Filtration (CREF) System," which requires restoration of the inoperable CREF subsystem within 7 days.
Damper 2HVC*DMP93, as installed, was determined to have a manual, door latch style, blade lock. The blade lock, when engaged, is designed to lock the damper in the closed position. To disengage the blade lock, the device is pulled down out of the way and secured by setscrews. To manually engage the blade lock, the setscrews are loosened, which allows the blade lock's internal spring to drive the lock into position. There is no automatic engagement of the blade lock. Further examination of damper 2HVC*DMP93 determined that the blade lock's setscrews were not holding the blade lock securely In the disengaged position. Based on a review of plant computer data and monthly surveillance test records for the Division II CREF subsystem, it was concluded that damper 2HVP*DMP93 had become locked in the closed position sometime between September 11, 2003 and October 11, 2003.
An inspection of the corresponding backdraft damper (2HVC*DMP94) in the redundant Division I CROASFT, and a review of the Division I CREF subsystem monthly surveillance test records, were performed. The inspection and review concluded that the blade lock associated with damper 2HVP*DMP94 was not interfering with the ability of the damper to open as designed. Therefore, the operability of the Division I CREF subsystem was confirmed.
The blade lock for damper 2HVP*DMP93 was restored to the disengaged position, and the damper was successfully tested and returned to service. The Division II CREF subsystem was declared operable on January 29, 2004 at approximately 1634 hours0.0189 days <br />0.454 hours <br />0.0027 weeks <br />6.21737e-4 months <br />, and Technical Specification 3.7.2, Action A was exited.
II. Cause of Event
The apparent cause of this event was a deficiency in the manufacturing, installation and/or construction of the manual blade lock associated with damper 2HVC*DMP93. The original design drawing for the manual blade lock showed a release pin installed through the blade lock's pull-down rod. However, the actual installed design uses two setscrews to maintain the device in the disengaged position. The first setscrew is tightened against the blade lock pull-down rod.
The second setscrew is used to jam the first setscrew to prevent it from loosening. No records were found indicating that the blade locks were replaced or ever used. As such, the blade lock appears to be original construction.
Although the installed design worked successfully for 18 years of operation, the friction connection between the pull- down rod and the first setscrew is not optimal due to system vibration caused by the damper closing and opening.
A contributing cause of this event was inadequate acceptance criteria in the CREF system monthly surveillance test procedure. Although total flow and various differential pressure readings were taken during monthly flow tests for each CROASFT, the procedure did not state any expected ranges or values for the readings taken. The lack of an acceptable flow range resulted in the operators not recognizing that the low air flow through the Division II CROASFT was indicative of damper 2HVC*DMP93 being closed.
Ill. Analysis of Event This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as 'Any operation or condition which was prohibited by the plant's Technical Specifications? The Division II CREF subsystem was inoperable as a result of damper 2HVC*DMP93 being locked dosed from sometime between September 11, 2003 and October 11, 2003, to January 29, 2004, which exceeded the Technical Specification 3.7.2 allowed outage time of 7 days for one CREF subsystem inoperable.
A review of the control room logs was conducted to determine if the Division I CREF subsystem had been inoperable at any time during the period that the Division II CREF was inoperable due to the locked closed damper. Three separate occurrences were identified during which both CREF subsystems had been simultaneously inoperable:
once in September 2003 (total duration of approximately 60.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />), once in October 2003 (total duration of approximately 11.1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />), and once in January 2004 (total duration of approximately 0.25 hours2.893519e-4 days <br />0.00694 hours <br />4.133598e-5 weeks <br />9.5125e-6 months <br />). The first two occurrences represent another condition that is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B). Limiting Condition for Operation (LCO) 3.0.3 was not immediately entered as required by Technical Specification 3.7.2, Action E, for two CREF subsystems inoperable with safety function not maintained, and a plant shutdown was not initiated within one hour as required by LCO 3.0.3.
This event is also reportable in accordance with 10 CFR 50.73(a)(2)(v), "Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to: (D) Mitigate the consequences of an accident." As identified above, there were three occurrences during which both the Division I and Division II CREF subsystems were inoperable.
The CREF system provides a radiologically controlled environment from which the unit can be safely operated following a Design Basis Accident (DBA). The safety related function of the CREF system used to control radiation exposure consists of two independent and redundant high efficiency air filtration subsystems for treatment of recirculated air and outside supply air. Each subsystem includes a CROASFT which is normally in standby. The remaining portions of the CREF system are operated to maintain the control room envelope environment during normal operation. Upon receipt of the initiation signal(s) (indicative of conditions that could result in radiation exposure to control room envelope personnel), the CREF system automatically switches to the emergency pressurization mode of operation to prevent infiltration of contaminated air into the control room envelope.
The CREF system is designed to maintain the control room envelope environment for a 30-day continuous occupancy after a DBA, while limiting the dosage to personnel to not more than 5 rem whole body or its equivalent to any part of the body.
For the event described in this LER, one of the two independent and redundant CROASFTs was inoperable due to damper 2HVP*DMP93 being in the locked closed position for approximately 4.5 months. There were no conditions or events requiring actuation of the CREF system during that 4.5 month period. Had an actual event occurred, for nearly all of the 4.5 month period, the other CROASFT (Division I) was operable and available to fulfill the CREF system safety function. For the three occurrences during which both CREF subsystems were inoperable (a total time duration of approximately 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />), the following were considered in assessing the safety significance:
Ill. Analysis of Event (Cont'cll
- The probability of a core damage event occurring during the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> period during which both CREF subsystems were inoperable was determined to be 4.4E-7. The damper position and CROASFT inoperability do not increase the probability of a core damage event.
- In the event of a core damage event, the Division I CREF subsystem could have been returned to an operable status within a reasonable amount of time.
- Even with damper 2HVP*DMP93 locked closed, approximately one half of the normal required flow was measured through the Division II CROASFT (the blade lock does not hold the damper tightly closed). With this intake flow rate, control room pressure was observed to remain positive relative to the outside atmosphere, but was less than the 1/8 inch water gauge required by Technical Specification Surveillance Requirement 3.7.2.4.
Assuming zero unfiltered leakage into the control room envelope, the reduced outside air intake flow rate would result in control room doses that would not exceed the limits of 10 CFR 50, Appendix A, General Design Criterion (GDC) 19. Any increase in post-accident control room dose due to unfiltered inleakage at the lower observed control room pressure would be at least partially offset by the reduction in dose due to the reduced outside air intake flow rate.
- In the event of a core damage event, radiological conditions in the control room would be monitored. Actions described in emergency plan implementing procedures would be taken, as necessary, to limit the thyroid dose to control room personnel. These actions could include ingestion of potassium iodide pills and the use of respiratory protection (e.g., self-contained breathing apparatus).
Based on the above considerations, the event is considered of low safety significance and did not pose a threat to the health and safety of the public or site personnel.
IV. Corrective Actions
1. To provide additional assurance that the blade lock will not inadvertently engage, a dimple was drilled in the blade lock's pull-down rod for both dampers 2HVP*DMP93 (Division II CREF) and 2HVP*DMP94 (Division I CREF). With the blade lock in the disengaged position, the setscrews were installed with loctite and tightened into the dimple to secure the blade lock in the disengaged position. The dampers were subsequently tested and returned to service.
2. The CREF system damper drawing was reviewed, and a walkdown of the CREF system was conducted. Only dampers 2HVC*DMP93 and 2HVC*DMP94 were identified as having the manual blade lock.
3. The CREF system monthly flow surveillance test procedure has been revised to provide guide values for acceptable flow and differential pressure conditions.
4. Other ventilation system surveillance test procedures that are similar to the CREF system monthly flow surveillance test procedure were reviewed, and the acceptance criteria associated with these procedures were determined to be adequate to prevent similar occurrences of this event.
V. Additional Information
1. Failed Components:
Backdraft damper, mark no. 2HVC*DMP93, manufactured by Pacific Air Products Co., Model SL100 2. Previous similar events: None 3. Identification of components referred to in this Licensee Event Report:
Components IEEE 805 System ID � IEEE 803A Function Control Room Envelope � NA � N/A Control Room Envelope Filtration System � VI � N/A Control Room Outside Air Special Filter Train � VI � N/A Damper � VI � UDMP Filter � VI � FLT 5