05000323/LER-2006-004
| Event date: | |
|---|---|
| Report date: | |
| Initial Reporting | |
| ENS 43047 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(a)(1)(i), Emergency Class Declaration, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3232006004R00 - NRC Website | |
EXPECTED
SUBMISSION DATE (15) I I I
MON DAY YR
On December 12, 2006, at 1322 PST, with Unit 2 at approximately 25 percent power, an electrical failure occurred in the circulating water pump (CWP) motor enclosure for CWP 2-1. An Emergency Notification System (ENS) Notice of Unusual Event (NUE), Number 23, Confirmed Explosion Onsite, was reported in accordance with 10 CFR 50.72 (a)(1)(i) at 1359 PST. The NUE was terminated at 1430 PST following confirmation that the initial report was due to a CWP 2-1 electrical discharge and fire, that was extinguished.
On December 12, 2006, at 1541 PST, ENS Event Number 43047 was updated to include a 4-hour report in accordance with 10 CFR 50.72 (b)(2)(iv)(B) due to a Reactor Protection System actuation while critical, and an 8-hour notification in accordance with 10 CFR 50.72 (b)(3)(iv)(A) due to the actuation of the auxiliary feedwater system. All equipment responded as designed.
The CWP 2-1 surge capacitor was replaced and the associated electrical system was repaired. The cause of the failure was a single random failure of the surge capacitor. Corrective actions will include replacing the 12kV three phase surge capacitors with more reliable single phase surge capacitors and enhancing the preventive maintenance program.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) R LER NUMBER (6) PAGE (3)
I. Plant Conditions
Pacific Gas and Electric Company (PG&E) Unit 2 was in Mode 1 (Power Operation) at approximately 25 percent reactor power prior to the event.
II. Description of Problem
A. Background
The circulating water system [KE] provides a continuous ocean water cooling supply to the main condenser [COND], condensate cooler [CLR], service water cooling system [KG], and intake cooling system EKE]. Each unit has two single-stage circulating water pumps (CWP). The CWPs are nominal 12kV motors with local surge capacitors installed to modify steep fronted electrical waves and prevent damage to the turn-to-turn insulation of the motor. The locally mounted surge capacitors are routinely inspected each refueling outage. The surge capacitors are replaced only when found damaged or leaking.
Each unit has two non-vital 12kV Buses. Non-vital Bus D provides power to CWP 2-1 and Reactor Coolant Pumps (RCPs) 2-2 and 2-4. A reactor
B. Event Description
On December 12, 2006, at 1322 PST, while conducting power ascension operations with Unit 2 at approximately 25 percent power, an electrical failure occurred in the circulating water pump (CWP 2-1) 12 kV motor enclosure. A loud bang and explosion was reported to licensed plant operators in the Unit 2 control room. The electrical transient experienced on the 12 kV non-vital bus D actuated an undervoltage protection relay tripping the load breakers for CWP 2-1, as well as RCP 2-2 and RCP 2-4.
The reactor trip signal was initiated when 2 out of 4 RCP motor breakers opened. All control rods fully inserted in response to the reactor trip and all plant systems functioned as required. The auxiliary feedwater system for Unit 2 was manually actuated per plant procedures, before an auto start signal for this system was generated.
On December 12, 2006, at 1340 PST, licensed plant operators declared a Notice of Unusual Event (NUE), Number 23, Confirmed Explosion Onsite.
On December 12, 2006, at 1356 PST, the Diablo Canyon Power Plant (DCPP) Fire Department first responders reported that the fire was out.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2006 0 OF On December 12, 2006, at 1359 PST, an Emergency Notification System (ENS) report was made in accordance with 10 CFR 50.72 (a)(1)(i) as Event Number43047.
Decay heat was removed via auxiliary feedwater to the steam generators steaming to the atmospheric dump valves. Emergency power was supplied to DCPP via offsite electrical transmission system power.
On December 12, 2006, at 1430 PST, DCPP terminated the NUE following confirmation that the initial report was due to a CWP 2-1 electrical discharge and fire, which was extinguished.
On December 12, 2006, at 1541 PST, ENS EN 43047 was updated to include a 4-hour report in accordance with 10 CFR 50.72 (b)(2)(iv)(B) due to a Reactor Protection System actuation while critical, and an 8-hour notification in accordance with 10 CFR 50.72 (b)(3)(iv)(A) due to the actuation of the auxiliary feedwater system.
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event None.
D. Other Systems or Secondary Functions Affected None.
E. Method of Discovery The automatic actuations were immediately known to licensed plant operators in the control room by alarms and indications. Onsite reports by operators and security personnel provided immediate reports of CWP 2-1 condition and the DCPP Fire Department lead provided detailed information regarding the fire suppression and termination.
F. Operator Actions Plant operators responded to alarms and indications provided in the control room, entered approved plant procedures, and initiated auxiliary feedwater prior to automatic actuation.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 3 2006 OF G.� Safety System Responses The reactor trip signal was initiated when 2 out of 4 RCP motor breakers opened. All control rods fully inserted in response to the reactor trip and all plant systems functioned as required. The auxiliary feedwater system was manually actuated per plant procedures, before an auto-start signal for this system was generated.
III.�Cause of the Problem
A. Immediate Cause
The CWP 2-1 surge capacitor failed causing a phase to phase electrical short resulting in a 12kV electrical discharge and localized fire.
B. Root Cause
The root cause of the surge capacitor phase-to-phase internal fault was an inservice insulation breakdown, a single random electrical failure. No adverse industry or site surge capacitor failure trend was identified.
C. Contributing Causes An engineering replacement part evaluation issued in 1993 allowed the use of a single three phase capacitor in lieu of the more reliable single phase capacitors. Also, the surge capacitor maintenance program was inadequate to ensure replacement consistent with testing or industry data.
IV.
� Assessment of Safety Consequences There were no actual safety consequences involved in this event since the nonsafety-related plant electrical systems responded as designed automatically terminating the 12kV power to the fault. All safety-related systems responded as designed by tripping the reactor and would have automatically started the auxiliary feedwater pumps, in the event the licensed plant operators did not manually initiate them via approved plant procedures.
Final Safety Analysis Report (FSAR) Update, Section 15.2.9, Loss of Offsite Power to the Station Auxiliaries, previously analyzed the total loss of offsite power, and bounds the partial loss of forced circulation experienced in this event.
The FSAR Update concluded that "for the loss.of offsite power to station auxiliaries event, all safety criteria are met.
5 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3 2 31 2006 0 0 4 0 0 5 OF This condition is not considered a Safety System Functional Failure as the loss of one of two nonsafety-related CWPs is a previously analyzed transient that does not create a significant condition adverse to safety.
Therefore, the event is not considered risk significant, and it did not adversely affect the health and safety of the public.
V. Corrective Actions
A. Immediate Corrective Actions The CWP 2-1 surge capacitor was replaced and the associated electrical system was repaired.
B. Corrective Actions
PG&E will replace the 12kV three phase surge capacitors with more reliable single phase surge capacitors.
PG&E will enhance the surge capacitor preventive maintenance program to include periodic replacement of the 12kV and 4kV surge capacitors based upon testing and/or industry data.
VI. Additional Information
A. Component:
The component is an ABB 3-phase 13.8 kV Capacitor, Part# ESG3180A39, and was last replaced in 1996.
B. Previous Similar Events
None.