05000423/LER-2006-001
| Docket Number | |
| Event date: | 04-04-2006 |
|---|---|
| Report date: | 10-04-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4232006001R01 - NRC Website | |
1. Event Description The Millstone Power Station Unit 3 (MPS 3) control room outside air isolation valves, 3HVC*A0V25 and 3HVC*A0V26, are air-operated isolation valves, installed in series with each other. These valves have a dual safety function. First, they are designed to automatically close on a control building isolation (CBI) signal so the control room can be pressurized with bottled air from the Control Room Envelope Pressurization System (CREPS). The second safety function requires the valves to open to divert outside air through the control room emergency ventilation system (CREVS)[VI], which includes the control room emergency ventilation filters, for continued pressurization of the control room after the air bottles are exhausted. For this second function the valves are opened either remote manually from control panel VP1 in the control room or locally using the manual jackscrew operator.
On two different occasions just over a month apart, scheduled maintenance activities were initiated on each of the two in series air inlet isolation valves. The maintenance activities involved isolation of the inlet duct by closing the valve under maintenance, with subsequent removal, overhaul, and reinstallation of the actuator.
Removal of the actuator renders both trains of CREVS inoperable. On March 1, 2006, the maintenance activity was completed and both trains of CREVS were inoperable for a period of approximately 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> and 40 minutes. On April 4, 2006, maintenance was terminated prior to removal of the actuator when questions were raised regarding Technical Specification compliance. It was this latter instance that prompted a historical review of maintenance activities on these components and the identification of the loss of function on March 1, 2006.
The CREVS is relied upon in the plant safety analysis to mitigate the consequences of an accident by limiting the radiological exposure to the control room operators from a Loss of Coolant Accident (LOCA). For this reason the loss of safety function of both trains of the CREVS is considered reportable under the provisions of 10 CFR 50.73(a)(2)(v)(D), as an event or condition that could have prevented fulfillment of a safety function of structures or systems needed to mitigate the consequences of an accident.
2. Cause The Root Cause of this event was a failure to recognize and correct an operating practice associated with an allowed mode of operation (isolated filtered recirculation, IFR) after it was removed from Unit 3 Technical Specification (TS) 3.7.7 in 2002. When the TS was revised to remove the IFR mode of operation, Operator training was not effective in changing the mindset of the Operators. The practice of using the IFR mode prior to 2002 resulted in the Operation's mindset that pressurizing the control room was only associated with the air bottles. The condition was exacerbated by incomplete implementation of the TS change, specifically references to the IFR mode of operation were not effectively purged from the TS bases and the existing work management program documents.
3. Assessment of Safety Consequences The radiological exposure to the MPS 3 Control Room operators from a MPS 3 LOCA due to the condition described above was evaluated and determined to be less than the exposure identified in the bounding analysis of record for MPS 3. This is due to the conservative assumptions in the safety analysis with regard to control room unfiltered inleakage during periods of neutral pressure and during the time that the CREVS is in operation in response to a LOCA. The identified condition resulted in a potential delay of placing CREVS in operation.
When CREVS is operating, the control room is pressurized and the amount of unfiltered inleakage of contaminated outside air is reduced. The result of the evaluation, which assumed 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> to place CREVS in service and utilized actual measured unfiltered inleakage amounts, was a control room dose less than the dose identified in the bounding analysis of record.
4. Corrective Action An investigation was conducted and appropriate corrective actions are being addressed in accordance with the Millstone Corrective Action Program.
The corrective actions to prevent recurrence of this condition were determined to be
- Address the correct use of the term "filtered pressurization" in TS 3.7.7.
- Provide additional training for Unit 3 licensed operators.
- Provide guidance for Unit 3 Operations on reportability criteria for a single-failure that defeats the safety functions of a redundant system. A loss of a single component in these circumstances would prevent the fulfillment of the safety function of the system and therefore is reportable even though the plant TS may allow such a condition to exist for a limited time.
5. Previous Occurrences No previous similar events/conditions were identified.
Energy Industry Identification System (EMS) codes are identified in the text as [XX].