05000278/LER-2012-003
| Peach Bottom Atomic Power Station (Pbaps) Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| ENS 48376 | 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 2782012003R00 - NRC Website | |
Unit Conditions Prior to the Event The Main Control Room Emergency Ventilation (MCREV) System is common to both Units 2 and 3.
Unit 2 was shut down for a refueling outage and Unit 3 was in Mode 1, operating at 100% of rated thermal power when this event occurred. MCREV was not required to be operable for Unit 2 when this event occurred due to Unit 2 being in a de-fueled condition and no fuel handling operations in progress.
Reportabilitv This report is being submitted pursuant to:
10CFR 50.73(a)(2)(v)(D) — Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
The MCREV system is a standby system common to both Units 2 and 3 that provides a protected environment from which operators can control the unit following an uncontrolled release of radioactivity.
Two redundant subsystems of the MCREV system are required to be operable, so that in the event of a single active failure, one subsystem is available to perform the design function of limiting the dose to the occupants during a design basis event.
Description of the Event
On October 4, 2012, surveillance testing of the 4 kV emergency bus E-42 was being performed. Control room ventilation was being provided by the normal supply fans with both the "A" and "B" MCREV fans in standby. Operations personnel were in the process of placing the MCREV in service using the "A" fan in accordance with the surveillance procedure for testing a loss of power to the emergency bus. At approximately 9:59 am, when an initiation signal was generated with the "A" MCREV fan control switch positioned in 'Auto', the fan did not start as expected. The "B" MCREV fan started shortly afterward, as designed for the fan in standby. The "A" MCREV control switch had most recently been manipulated near the end of the previous shift after a "A' MCREV shutdown following a successful auto initiation to support testing. This indicates that the "A" MCREV control switch had been functioning properly at that time.
The E-42 bus, which powers the "B" MCREV train, was energized at this time, but was considered inoperable due to testing and other work being performed. Therefore, the "Er MCREV train was also inoperable. With both MCREV subsystems inoperable, Technical Specification (TS) 3.7.4 Condition E was entered, which requires the plant to be in Mode 3 in 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
Troubleshooting found that, although the "A" fan control switch indicated it was in the "Auto" position, the switch was positioned such that the switch contacts were still open. Voltage readings were taken as the switch was operated through all positions. Voltage readings were correct (i.e., contacts were closed) including when the switch was returned to the 'Auto' position. The switch was manipulated to successfully start the fan in the 'Run' and 'Auto' position. System operating procedures were performed to restore normal and emergency ventilation lineup. The "A" MCREV was restored to an operable status at 12:37 pm and the TS condition was exited.
An 8-hour report to the NRC was made via the Emergency Notification System at 4:37 pm (EN#48376) in accordance with 10CFR50.72(b)(3)(v)(D).
Analysis of the Event
The MCREV system provides a protected environment for operators in the Control Room Envelope (CRE) to control the unit following an uncontrolled release of radioactivity. The MCREV system consists of two independent and redundant high efficiency air filtration subsystems and two 100% capacity emergency ventilation supply fans which supply and provide emergency treatment of outside supply air and a CRE boundary that limits the in-leakage of unfiltered air. Upon receipt of the initiation signal (indicative of conditions that could result in radiation exposure to CRE occupants), the MCREV system automatically starts and pressurizes the CRE to minimize infiltration of contaminated air.
The loss of the MCREV safety function existed for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 38 minutes. In the unlikely event that a design basis event had occurred during this brief period of exposure, it is probable that the 'B' MCREV fan would have started and performed the design function of the MCREV or the 'A' MCREV control switch would have been re-positioned. For the DBA-LOCA event, MCREV operation is not credited for the first 30 minutes of the event.
The switch for the "A" MCREV fan is a GE model CR2940UB203W four position manual control switch.
This specific model switch is based on a standard model CR2940 switch with a specific contact configuration. Variations of the model CR2940 switch are widely used in the nuclear industry. PBAPS has over 1,900 switches that are variations of this base model switch. The performance history of the general population of the various models of this switch has been very good with an average of less than two failures per year out of the 1,900 switches.
Maintenance for this switch is based on a performance centered maintenance (PCM) template. This type of manual switch is considered to be an inherently reliable component, with a duty cycle of 10,000 operations. Therefore, there is no replacement or other preventative maintenance performed unless there is a history of failure.
Previous Similar Occurances The hand switch for the "A" MCREV fan was replaced in 2004 because it did not feel to be fully entering the stop at the 'Auto' position. In 2001, the hand switch for the "B" MCREV fan was replaced due to it sticking when going from 'Standby to 'Auto'. In 1999 an operator rotated a similar switch past its 'Run' position because the 'Run' label for the switch was not lined up with the run position.
Cause of the Event
The cause of this event was that, although the control switch indicated it was in the "Auto" position, the switch was positioned such that the switch contacts were still open, which prevented a start of the "A" MCREV fan. The switch position indication is not precise enough to ensure the switch contact block is set for a given indicated position. Unlike a majority of applications for these switches, this switch does not always provide direct operator feedback when a position is selected. With most switches a resulting action is observed, such as a motor start or an indicating light changing state.
Corrective Actions
The switch was repositioned, normal system lineup was restored and the system was returned to an operable status. This event has been placed in the station's corrective action program to further evaluate and track additional actions.