A potential scenario has been identified that has not been analyzed in the Comanche Peak Nuclear Power Plant Fire
Safe Shutdown Analysis (
FSSA). This situation is described below for Unit 1, but also applies to Unit 2. Listed below is the configuration for 1EA1. The basic configuration is typical for 1EA2, 2EA1 and 2EA2 as well (ref. E1-0001)
- Safeguard Bus 1EA1 is a 6.9 kV switchgear with a Main-Tie-Main configuration.
- The normal lineup has one feeder breaker closed, the other feeder breaker open and the tie breaker closed.
- 1EA1 receives normal power from the secondary side of Startup Transformer XST2 through breaker 1EA1-1.
- 1EA1 receives alternate power from the secondary side of Startup Transformer XST1 through breaker 1EA1-2.
- 1EA1 receives emergency power from diesel generator 1EG1 through breaker 1EG1.
- An alternate source of power for 1EA1 is also available from Train C through breaker 1EA1-3.
The control wiring for the 1EA1-1 circuit breaker contains the following attributes that are important to understand the issue:
- Switch 43/1EA1-1, located in the Shutdown Transfer Panel (STP) is used to transfer control of 1EA1-1 from the Control Room (1-CB-11 switch CS-1 EA1-1) to the Hot Shutdown Panel (HSP) switch CS-1 EA-1 L. (Ref. E1-0031-01&02)
- There is a trip circuit fuse located in the 6.9 kV switchgear compartment for 1EA1-1 for control of the trip circuit when the breaker is controlled at 1CB-11 and a separate fuse for the trip circuit when the breaker is controlled by the HSP.
- The trip circuit for 1EA1-1 has a contact routed through the Control Room to the Solid State Protection System (SSPS) Cabinet. This contact is in the trip circuit when control is from the Control Room or when control is from the HSP. (Ref. E1-0031-01)
The scenario is based on a fire in the Control Room. If the fire in the Control Room causes a ground in the wiring routed in the Control Room to the SSPS cabinet, the fuse for the 1E1-1 trip circuit would open. In the event of a fire in the Control Room, control of the plant is transferred to the HSP. When the control of breaker 1EA1-1 is transferred to the HSP and the ground condition in the SSPS wiring still exists, the second 1EA1-1 trip circuit would open. At this time there would be no way to remotely trip open 1EA1-1. The breaker could still be tripped mechanically at the breaker. Therefore, if 1EA1-1 is closed, 1EA1 could remain energized if off-site power is available. If off-site power is not available, but 1EA1-1 remains closed, bus 1EA1 would remain electrically connected to XST2. As part of the transfer of control from the Control Room to the HSP, operators start up the diesel generator and close 1EG1 to place 1EA1 loads on the generator. If 1EG1 is closed and 1EA1-1 breaker is still closed with off-site power available, the generator will be immediately connected to grid power through XST2 without synchronizing. If 1EG1 is closed and 1EA1-1 breaker is still closed with off-site power not available, the generator will be immediately connected to XST2 and attempt to energize XST2. This large current draw associated with energizing XST2 would likely stall and damage diesel generator 1EG1.
Compensatory Action is being implemented through procedure revisions to preclude damage to the diesel generator during this scenario.
The licensee will notify the NRC Resident Inspector.
- * * RETRACTION FROM TOM RUCKER TO PETE SNYDER AT 1713 ON 9/8/11 * * *
At 1939 central daylight time on July 21, 2011, Luminant Power notified the NRC (Event No. 47086) of a Unanalyzed Condition per 50.72(b)(3)(ii)(B) regarding the vulnerability from a potential control room fire on 'A' Safeguards bus. The event report described a portion of a cable running from the Hot Shutdown Panel (HSP) to the Solid State Protection System (SSPS) cabinet in the Control Room (CR) that was not protected from a CR fire scenario for which a worst case Control Room/Cable Spreading Room fire induced short could result in the 1EA1-1 circuit breaker, connecting the 345 kV Startup Transformer to the grid, not tripping and damaging the EDG.
Upon further review it has been determined that the Fire Safe Shutdown Analysis (FSSA) modeled that cable in the analysis since CPNPP began commercial operation. Additionally, the FSSA accounted for the fire-induced circuit ground on this cable and one of the specified manual actions is to trip the 1EA1-1 circuit breaker to assure the EDG would load the 'A' Safeguard bus to support the required fire scenario. The EDG output breaker is designed to auto close once the 1EA1-1 circuit breaker is tripped open. The previous version (prior to compensatory actions) of ABN-803A/B, the procedure used should a fire occur in the Control Room, directed the Reactor Operator to trip 1EA1-1 circuit breaker once the diesel was verified running, then ensure the EDG breaker closed. There is no procedural step directing personnel to manually close the EDG breaker. Since, in this scenario, indication for the 1EA1-1 circuit breaker at the Hot Shutdown Panel would be lost, it would be expected that the RO would direct the RRO to verify 1EA1-1 circuit breaker position.
Based on the above, Luminant Power has concluded that the FSSA adequately modeled the plant and procedures were written which would not direct any action that would have caused the condition stated in the CR description. Based on the above, the conclusion is that CPNPP did not have an unanalyzed condition that significantly degraded plant safety per 50.72 (b)(3)(ii)(B) regarding the vulnerability from a potential control room fire on 'A' Safeguards bus. Therefore, this event is retracted.
The licensee will notify the NRC Resident Inspector.
Notified R4DO (Lantz).
