05000323/LER-2002-003
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| 3232002003R02 - NRC Website | |
EXPECTED
SUBMISSION DATE (15) I I
MON DAY YR
On August 22, 2002, at 0332 PDT with Unit 2 in Mode 1 (Power Operation) at 100 percent power, Required Action A.1 for Technical Specification 3.7.7, Vital Component Cooling Water (CCW) System, was not met when the completion time was exceeded due to a cable fault.
On August 19, 2002, after receiving brief CCW Pump 2-3 ground alarms, operators started CCW Pump 2-1, shut down Pump 2-3, and declared CCW Pump 2-3 inoperable until the cause could be determined.
On August 21, 2002, in response to a request for enforcement discretion, the NRC granted verbal enforcement discretion for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> until the motor feeder cable replacement and post-maintenance testing could be completed. On August 23, 2002, at 1342 PDT, repairs and testing were completed, and CCW Pump 2-3 was declared operable.
The immediate cause of the event was a ground fault in the "C" phase power cable to CCW Pump 2-3, resulting in pump inoperability. Immediate corrective actions included replacing all three phases of the CCW Pump 2-3 cables. The root cause of the event was a manufacturing defect based on the presence of isolated contaminant particles in the insulation. Corrective actions to prevent recurrence include replacing 4kV cables in both units.
LICENSEE EVENT REPORT (LER) FACILITY NAME (1) Diablo Canyon Unit 2 0 5 DOCKET NUMBER (2) 0 300 2 3 2002 LER NUMBER (6) 300 20 2 OF 8 I.D Plant Conditions Unit 2 was in Mode 1 (Power Operation) at 100 percent power.
11.D Description of Problem
A. Background
The Component Cooling Water (CCW) System [CC] provides a heat sink for the removal of heat from safety related components during a Design Basis Accident (DBA) or transient. During normal operation, the CCW System provides this function for safety-related components, various nonessential components, and the spent fuel storage pool [DA].
The CCW system consists of three CCW pumps [P] powered by separate vital 4kV [EB] buses [BU], two CCW heat exchangers [HX], and a two-chamber CCW surge tank. The piping system consists of three parallel headers. The headers extend from the outlet of the heat exchangers through the header heat loads to the suction of the CCW pumps. Headers A and B supply cooling to safety-related components while header C supplies cooling to nonsafety-related components.
Normally, two CCW pumps are operating with the third CCW pump in standby.
Technical Specification (TS) 3.7.7, "Vital Component Cooling Water System," requires that two vital CCW loops shall be operable. TS 3.7.7, Bases, Limiting Condition for Operation (LCO), states that in the event of a design basis accident, one vital CCW loop is required to provide the minimum heat removal capability assumed in the safety analysis for the systems to which it supplies cooling water assuming occurrence of a single failure. To ensure this requirement is met, two vital loops of CCW must be operable. To meet the LCO on CCW loops, vital Headers A and B, both CCW heat exchangers, the surge tank, and all three CCW pumps must be operable.
B. Event Description
On August 18, 2002, at 2200 PDT, with Diablo Canyon Power Plant (DCPP) Unit 2 at 100 percent power and CCW Pumps 2-2 and 2-3 running, a brief CCW Pump 2-3 feeder ground alarm [VA] was received.
A walkdown of CCW Pump 2-3 and the associated breaker did not identify any problems. The CCW Pump 2-3 stator current and temperature were also normal.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) On August 18, 2002, at 2251 PDT and on August 19, 2002, at 0004 PDT, additional CCW Pump 2-3 feeder ground alarms were received. The duration of each alarm was less than 0.3 seconds. Following the alarm that occurred on August 19 at 0004 PDT, CCW Pump 2-1 was started and CCW Pump 2-3 was shut down. The CCW Pump 2-3 breaker cubicle was opened and inspected for abnormal indications. No abnormal odors or problems were noted in the breaker cubicle. A review of the plant drawings determined that Relay 50NHH12, which was the source of the alarm, senses a ground between the CCW Pump 2-3 motor [MO] and breaker, indicating an actual ground fault had occurred.
On August 19, 2002, at 0332 PDT, Operations declared CCW Pump 2-3 inoperable until investigations of the pump relay, motor, and cable [CBL5] could be completed.
On August 19, 2002, the feeder cable and motor were meggered at 2500V dc, and no problems were identified. The feeder cable and motor were subsequently tested using a high potential tester. They failed this test when ramping up from 2500V to 4000V. The motor and cables were then determinated and each was tested independently. The motor and feeder cable "A" and "B" phases tested satisfactorily at 2500V, but the "C" phase cable failed this test.
Following the identification of the ground in the CCW Pump 2-3 motor feeder cable, additional work was performed to develop the repair plan, repair schedule, work orders, and clearances required to support the removal of the cable for all three phases and the installation of replacement cables.
On August 21, 2002, PG&E submitted Letter DCL-02-100 to formally request enforcement discretion. On August 21, 2002, at 1320 PDT, PG&E telephoned the NRC to request enforcement discretion regarding compliance with TS 3.7.7, and at 1410 PDT, the NRC verbally granted enforcement discretion until August 25, 2002, at 0332 PDT (72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> past the required completion time).
On August 22, 2002, at 0332 PDT, Required Action A.1 of TS 3.7.7 was not met when the required completion time was exceeded.
On August 23, 2002, at 1342 PDT, CCW Pump 2-3 was declared operable following completion of cable replacement and post-maintenance testing, and on August 25, 2002, at 0332 PDT, the enforcement discretion expired.
CCW Pump 2-3 was inoperable for 106.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
LICENSEE EVENT REPORT (LER) FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event There were no other inoperable structures, systems, or components that contributed to the event.
D. Other Systems or Secondary Functions Affected To ensure personnel safety during the cable replacement, Safety Injection (SI) [BQ] Pump 2-2 and Residual Heat Removal (RHR) [BP] Pump 2-2 were cleared due to the proximity of their power cables to the CCW Pump 2-3 motor feeder cable.
E. Method of Discovery On-shift PG&E licensed operators received CCW Pump 2-3 feeder ground alarms in the control room during normal operation. After pump and breaker walkdowns, operators inspected the breaker cubicle and identified Relay 50NHH12 as the source of the alarms. High potential testing of the cables identified the ground fault.
F. Operator Actions After receiving a third brief ground alarm, operators started CCW Pump 2-1 and shut down CCW Pump 2-3 and declared it inoperable.
G. Safety System Responses None.
III.�Cause of the Problem A.�Immediate Cause TS 3.7.7 LCO and Required Action A.1 were not met because the "C" phase power cable to CCW Pump 2-3 shorted to ground and could not be replaced within the TS required completion time.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) B.� Root Cause All three phases of the feeder cable were sent to an offsite vendor for failure analysis. Evaluations performed by the offsite vendor included a detailed visual examination to locate the fault site and any other anomalies, characterization of the fault site (microscopic examination of thin cross sections, identification of possible water trees, and infrared spectroscopy to identify gross changes in the base compound), and testing of the tensile and elongation properties of the cable insulation and jacket.
The electrical evaluation of the cable included partial discharge inception (per ICEA T-24-380-1994) and AC breakdown of cable sections (per ASTM D 149, voltage values per IEEE Standard 4). Investigation of the cable environment identified no chemical contaminants. PG&E determined that the cable was not submerged in water and no water, excessive heat, or foreign agents were present in the cable conduit. In addition, installation damage, increased electrical operating stresses, thermal aging, and damage due to maintenance practices were ruled out.
Observation of the "C" phase fault site by the offsite vendor revealed an unusual square-shaped area in the insulation, suggesting the presence of a foreign particle. Further observation of the "B" phase AC breakdown site revealed a discrete contaminant particle in the insulation containing bonds similar to a cellulose-containing polymer, suggesting the molecule contained a bonded hydroxyl.
The root cause of this event was determined to be a manufacturing defect, based on the presence of isolated contaminant particles within the cable insulation, as identified by the offsite vendor. Over time, water (moisture) most likely diffused into the insulation and was attracted to the hydroxyl groups in the contaminant particle, thus changing the particle's dielectric properties and making it more conductive.
The CCW Pump 2-3 motor feeder cable failure is the first cable failure at DCPP in the last five years and not indicative of a negative trend. The potential presence of contaminant particles in the insulation of the remaining safety-related cables is not an immediate operability concern, although under the right conditions, it could lead to a reduction in the life of the cable.
LICENSEE EVENT REPORT (LER) FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Diablo Canyon Unit 2 0 5 0 0 0 3121 3 2002 0 0 3 0 2 6 OF 8
IV. Assessment of Safety Consequences
There were no actual safety consequences involved in this event since one vital CCW loop, which included two CCW pumps and one heat exchanger, remained operable after CCW Pump 2-3 was declared inoperable. In the event of a design basis accident, the CCW system serves as a mitigating system and one vital loop provides the minimum heat removal capability assumed in the safety analysis for the systems to which it supplies cooling water. The design basis of the CCW System is for one vital CCW loop to remove the post-DBA heat load without exceeding a CCW supply temperature of 120°F with an allowable transient not to exceed 140°F for more than 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
With one vital CCW loop inoperable, PG&E used the "at power" probabilistic risk assessment model to calculate the change in core damage frequency associated with increasing the completion time to 144 hours0.00167 days <br />0.04 hours <br />2.380952e-4 weeks <br />5.4792e-5 months <br /> (72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> beyond the 72-hour TS completion time). Three configurations were considered in the risk assessment:
1. CCW Pump 2-3 out of service.
2. The combination of CCW Pump 2-3 and Auxiliary Feedwater (AFW) Pump 2-1 out of service (for planned maintenance).
3. CCW Pump 2-3 out of service, with SI Pump 2-2 and RHR Pump 2-2 de-energized to ensure personnel safety of the maintenance workers.
The corresponding change in core damage frequency (ACDF) was calculated as 2.60E-7 and, therefore, not significantly sensitive to small changes in duration of each one of the configurations. Based on the above information and analyses, PG&E used the NRC's significance determination process and believes the event was very low risk.
The event did not adversely affect the health and safety of the public.
V.�Corrective Actions A.�Immediate Corrective Actions PG&E replaced all three phases of the CCW Pump 2-3 motor feeder cables with similar Okonite 5kV cable (red ethylene propylene rubber (EPR) insulation, copper foil shield, linear low density polyethylene (LLDPE) jacket). The pump was declared operable on August 23, 2002, at 1342 PDT and the failed cable was shipped to a laboratory for failure analysis.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) OF 8Diablo Canyon Unit 2 0 51 0 0 0 3 2 1 3 2002 -10 0 3 0 2 7 B.C Corrective Actions to Prevent Recurrence PG&E will replace safety-related 4kV cables manufactured prior to 1974 as a conservative corrective action to prevent recurrence. There is a possibility that cables manufactured prior to 1974 may have had contaminants introduced in the insulation due to the insulation extrusion process used at that time. This conclusion is based on the fact that the cable vendor's manufacturing process changed, in 1974, from an open bulk material handling system to a closed system, which controlled exposure of bulk materials to the environment.
During the Unit 1 thirteenth refueling outage (1R13), November 2005, HiPot testing was performed on three of the cables being removed. This testing has been successful in the past at identifying cable degradation prior to failure. The HiPot testing demonstrated these cables exceeded the minium maintenance test value voltage capability. A visual inspection of 4kV cables replaced in the Unit 2 twelfth refueling outage (2R12) and 1R13 did not identify degradation that would challenge the continued operability of the cable. Also, an assessment of the risk impact for the 4kV cable replacement schedule concluded that there was no significant nuclear risk increase in the following schedule.
PG&E will replace the suspect cables by 2009, targeting a safety-related bus each refueling outage. Cable replacements will include those cables associated with components such as RHR Pumps, Charging (CHG) Pumps, CCW Pumps, SI Pumps, AFW Pumps, and Diesel Generator (D/G) feeders. Cables associated with all auxiliary saltwater (ASW) pumps have been replaced. The schedule for replacements is as follows:
Unit 1 cable replacement schedule:
During 1R13, the D/G 1-1 feeder cable and the Bus "H" 4kV load cables were replaced for; AFW Pump 1-2, C/S Pump 1-2, RHR Pump 1-2, CCW Pump 1-3, SI Pump 1-2 and 480V Bus "H.
During the Unit 1 fourteenth refueling outage (1R14), currently scheduled to begin in April 2007, the D/G 1-3 feeder cable and the Bus "F" 4kV load cables will be replaced for; AFW PP 1-3, CHG Pump 1-1, CCW Pump 1-1, SI Pump 1-1, and 480V Bus "F.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) During the Unit 1 fifthteenth refueling outage (1R15), currently scheduled to begin in January 2009, the D/G 1-2 feeder cable and the Bus "G" 4kV load cables will be replaced for; C/S Pump 1-1, RHR Pump 1-1, CHG Pump 1-2, PDP 1-3, CCW PP 1-2, and 480V Bus "G.
Unit 2 replacement schedule:
During 2R12, (October 25, 2004, through December 16, 2004) the D/G 2-1 4kV feeder cable and the Bus "G" 4kV load cables were replaced for; containment spray (C/S) Pump 2-1, RHR Pump 2-1, CHG Pump 2-2, positive displacement pump (PDP) 2-3, CCW Pump 2-2, and 480V Bus "G." These Bus "G" components constitute a complete safe shutdown train for Unit 2.
During the Unit 2 thirteenth refueling outage (2R13), currently scheduled to begin April 2006, the D/G 2-2 feeder cable, and the 4KV Bus "H" load cables will be replaced for; AFW Pump 2-2, C/S Pump 2-2, RHR Pump 2-2, SI Pump 2-2, and 480V Bus "H".
During the Unit 2 fourteenth refueling outage (2R14), currently scheduled to begin February 2008, the Bus "F" 4kV load cables will be replaced for; AFW Pump 2-3, CHG Pump 2-1, CCW Pump 2-1, SI Pump 2-1, and 480V Bus "F". The Bus "F" 4kV feeder cable from D/G 2-3, was installed in May 1992 and therefore is not being replaced.
VI.�Additional Information
A. Failed Components
The CCW Pump 2-3 motor feeder cable was Okonite 5kV cable (copper conductor, black ethylene propylene rubber (EPR) insulation, copper foil shield, neoprene jacket).
B. Previous Similar Events
DCPP has experienced eight medium voltage cable failures since 1989.
Five of these cases involved water accumulation in the cable vaults and pull boxes. All of these failures were associated with cables exposed to an outside environment, but in all cases a root cause could not be determined. For the three cases in which water was not a factor, the CCW Pump 2-3 cable failure is similar in that the preliminary evaluation shows no evidence of installation damage, manufacturing defects, electrical operating stresses, inadequate maintenance practices, thermal aging, or degradation.