U.S. Nuclear Regulatory Commission
Event Reports For
03/09/2010 - 03/10/2010
** EVENT NUMBERS **
|Power Reactor ||Event Number: 45754 |
| Facility: DIABLO CANYON |
Region: 4 State: CA
Unit:   [ ]
RX Type:  W-4-LP, W-4-LP
NRC Notified By: JOHN WHETSLER
HQ OPS Officer: CHARLES TEAL
| Notification Date: 03/09/2010 |
Notification Time: 23:39 [ET]
Event Date: 03/09/2010
Event Time: 19:16 [PST]
Last Update Date: 03/09/2010
| Emergency Class: NON EMERGENCY |
10 CFR Section:
50.72(b)(3)(ii)(B) - UNANALYZED CONDITION
| Person (Organization): |
MICHAEL HAY (R4DO)
|Unit ||SCRAM Code ||RX CRIT ||Initial PWR ||Initial RX Mode ||Current PWR ||Current RX Mode |
|1 ||N ||Y ||100 ||Power Operation ||100 ||Power Operation |
|2 ||N ||Y ||100 ||Power Operation ||100 ||Power Operation |
|POTENTIAL LOSS OF 1E PUMPS DUE TO DEGRADED VOLTAGE DURING POSTULATED ACCIDENTS |
"While attempting to analyze the consequences of scenarios (loss of coolant accident (LOCA) and loss of offsite power (LOOP)) postulated during the Component Design Basis Inspection (CDBI), PG&E concluded both units were in an Unanalyzed Condition that significantly degrades plant safety, and therefore, reportable under 10 CFR 50.72(b)(3)(ii)(B).
"The postulated scenario could result in permanently connected class 1E pump motors tripping over current relays due to sustained degraded voltage on the startup offsite power source.
"The scenario starts with a safety injection (SI) signal transferring 4160 volt vital buses from auxiliary power to the startup offsite power source. Following bus transfer, the startup source is postulated to degrade to a level above the first level under voltage relay (FLUR) setting of equal to or greater than 2583 volts and below the second level under voltage relay (SLUR) setting of equal to or greater than 3785 volts. Permanently connected class 1E pump motors would experience this degraded voltage for up to the SLUR time delay relay setting of equal to or less than 20 seconds. Prior to reaching the SLUR time delay relay setpoint and transferring loads to the onsite emergency diesel generators (EDGs), operating motors (e.g. auxiliary saltwater (ASW) pump and component cooling water (CCW) pumps) could trip on over current. Operators would be directed by procedures to re-start these motors on an operable power source (EDG). However, since this scenario has not been previously analyzed and could reasonably challenge containment pressure or peak clad temperature limits, it is considered unanalyzed.
"The postulated scenario had previously been considered non-credible, and therefore, had not been analyzed.
"PG&E considers this a nonconservative technical specification (TS) since the values in TS surveillance requirement (SR) 188.8.131.52 would not protect permanently connected class 1E loads from damage during degraded voltage conditions."
The licensee has notified the NRC Resident Inspector. The condition was discovered by NRC inspectors while performing a CDBI inspection.