05000413/LER-2002-006
| Catawba Nuclear Station, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4132002006R01 - NRC Website | |
BACKGROUND
Catawba Nuclear Station Unit 1 is a Westinghouse Pressurized Water Reactor [EIIS: RCT]. The onsite Class lE AC electrical power distribution system [EIIS: ES] is divided by train into two redundant and independent electrical power distribution subsystems.
The AC electrical power subsystem for each train consists of a primary Engineered Safety Feature (ESF) 4.16 kV bus [EIIS: BU] and secondary 600 volt buses, distribution panels [EIIS: FL], motor control centers (MCCs) and load centers. The 4.16 kV essential auxiliary power system physically consists of two independent and redundant 4.16 kV switchgear assemblies [EIIS: SWGR], designated ZETA and lETB. The 4160VAC Essential Auxiliary Power System supplies power to those Class lE loads required to safely shutdown the unit following a design basis accident. This system is also available to supply power to the 4160VAC Blackout Auxiliary Power System. The 4160 volt essential system is divided into two completely redundant and independent trains designated A and B, each consisting of one 4160 volt switchgear assembly, three 4160/600 volt transformers [EIIS: XFMR], two 600 volt load centers, and associated loads. Normally each Class IE 4160 volt switchgear is powered from its associated non-Class lE train of the 6900VAC Normal Auxiliary Power System. Additionally, an alternate source of power to each 4160 volt essential switchgear is provided from the 6900 volt system via two separate and independent 6900/4160 volt transformers. These transformers are shared between the Catawba units and provide the. capability to supply an alternate source of preferred power to each unit's 4160 volt essential switchgear from either unit's 6900 volt system. A key interlock scheme is provided to preclude the possibility of connecting the two units together at either the 6900 volt level or the 4160 volt level.
Each train of the 4160VAC Essential Auxiliary Power System is also provided with a separate and independent emergency diesel generator [EIIS: DG] to supply the Class lE loads required to safely shutdown the unit following a design basis accident. Additionally, each diesel generator is capable of supplying its associated 4160 volt blackout switchgear through a connection with the 4160 volt essential switchgear.
If the diesel generator is being tested (i.e., paralleled to the system) and a Safety Injection Actuation Signal is received by the sequencer, the diesel generator breaker is tripped and the diesel remains running in a standby mode. At this point, the sequencer automatically functions to apply the appropriate loads. Also, if the diesel generator is being tested and a loss of offsite power should occur, the diesel generator will attempt to pick up the load until an instantaneous overcurrent relay trips the diesel generator breaker. At this point the diesel generator will continue to run in a standby mode and the sequencer will initiate load shedding and automatically apply the appropriate loads. Since redundant diesel generators are not tested simultaneously, the other diesel generator would be started via its associated sequencer just as it would for any condition.
Technical Specification Limiting Condition for Operation (LCO) 3.8.1 requires two qualified circuits between the offsite transmission network and the Onsite Essential Auxiliary Power System and two diesel generators capable of supplying the Onsite Essential Auxiliary Power Systems. With one emergency diesel generator inoperable, the following actions are required:
1. Perform SR 3.8.1.1 for the offsite circuits within one hour and once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> thereafter. Surveillance Requirement (SR) 3.8.1.1 requires verification of correct breaker alignment and indicated power availability for each offsite circuit.
2. Declare the required feature(s) supported by the inoperable diesel generator inoperable when its required redundant feature(s) is inoperable 3. Determine that the operable diesel generator is not inoperable due to a common cause failure or perform SR 3.8.1.2 for the operable diesel generator. SR 3.8.1.2 requires verification that each diesel generator starts from standby conditions and achieves steady state voltage and frequency within prescribed limits.
4. Restore the diesel generator to operable status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and within 6 days from discovery of failure to meet the LCO.
This event is being reported pursuant to 10CFR 50.73(a)(2)(i)(B) (any operation or condition prohibited by the plant's Technical Specifications (TS)), and 10CFR50.36(c)(2)(i) (Limiting Condition for Operation (LCO) not met).
During the time of this event, Unit 1 was operating in Mode 1, Power Operation. With the exception of the diesel generator breaker discussed in this report, no structures, systems, or components were out of service at the time of this event that contributed to the event.
EVENT DESCRIPTION
(Dates and times are approximate) Date/Time � Event Description 6/24/02 2000 6/28/02 1000 6/28/02 1100 6/28/02 1622 6/28/02 2012 Twenty-four hour load test of diesel generator 1B was begun. The diesel generator output breaker lETB-18 was closed shortly thereafter.
This periodic test was completed satisfactorily. There were no alarms or indications of any unusual conditions related to the diesel generator output breaker.
Diesel generator 1B taken out of service for test PT/1/A/4350/15B, Diesel Generator 1B Periodic Test. This inoperability was entered in the Technical Specification Action Item Log.
During performance of PT/1/A/4350/15B, it was discovered that diesel generator 18 output breaker 1ETB-18 would not close onto the bus.
Visual inspection of the breaker did not show any obvious problems.
Engineering, Maintenance and Operations personnel developed a troubleshooting plan.
Further attempts to close the breaker remotely and locally were attempted and were unsuccessful.
lETB-18 was removed from the breaker cubicle.
It was determined that the charging motor toggle switch was faulty. There was a burnt wire and lug on one of the connections to the toggle switch. The spring charging motor was confirmed to be operating properly.
A new ring terminal and motor disconnect switch were installed on breaker 1ETB-18. lETB-18 was cycled successfully.
6/29/02 0125 � Diesel generator 1B was declared operable.
CAUSAL FACTORS
The only failed component found on breaker 1ETB-18 was the ring lug wiring connection at the motor disconnect switch. The motor disconnect switch is in the breaker spring charging circuit. The springs in a breaker of this type are charged immediately after the breaker closes.
All evidence indicates that when the breaker was closed at approximately 2000 hours0.0231 days <br />0.556 hours <br />0.00331 weeks <br />7.61e-4 months <br /> on 6/24/02 the springs started recharging and almost reached full travel. It appears that at this point in time the degraded connection finally burned open and the charging motor stopped running. If the charging springs are not fully charged the breaker mechanism is not latched and cannot be closed. The breaker opening mechanism is separate and therefore there was no problem in opening the breaker at the end of the diesel generator IB run on the evening of 6/25/02. It is concluded that the diesel generator IB was inoperable between 6/24/02 at approximately 2000 hours0.0231 days <br />0.556 hours <br />0.00331 weeks <br />7.61e-4 months <br /> until correction of the problem on 6/29/02 at approximately 0125 hours0.00145 days <br />0.0347 hours <br />2.066799e-4 weeks <br />4.75625e-5 months <br />.
Laboratory examination of the damaged wiring connection concluded that the root cause of the failure was inadequate crimping of the wire within the ring lug barrel by the manufacturer. In cases where inadequate crimped connections exist, a condition known as static heating exists. Through time, the heating stress relieves the crimp increasing the temperature in the crimp area to a point where the heat becomes so hot the insulation burns or melts off and eventually melts the wire.
The breaker lETB-18 is a 4160-volt 5HK breaker manufactured by Asea- Brown-Boveri.
Operations personnel periodically inspect the breaker closing mechanism to verify the closing springs are charged. However, it is highly unlikely that this failure could have been detected by this visual MRQ FORM MA (1-2001) inspection because in this case, it would have appeared that the closing mechanism was properly positioned.
There are approximately 68 similar breakers in use at the site. Review of the operating experience data base for entries related to Asea- Brown-Boveri breakers charging motors and charging springs did not identify any cases with the same results as identified as the root cause of this event.
CORRECTIVE ACTIONS
Immediate:
1. A new ring terminal and motor disconnect switch were installed on breaker lETB-18 and the breaker was returned to service.
Interim:
1. This failure was discussed with breaker team technicians and they were requested to specifically inspect wire terminals on all metalciad circuit breakers when performing preventive maintenance work.
2. The diesel generator 1A output breaker was inspected on August 6, 2002 and no abnormal indications were identified.
3. Breaker maintenance procedures were revised to require close inspection of wire terminals during any breaker maintenance activity.
Any future corrective actions will be addressed via the Catawba Corrective Action Program. There are no NRC commitments contained in this LER.
SAFETY ANALYSIS
The Catawba Probabilistic Risk Analysis (PRA) was used to reach the conclusion that the increased risk was not significant.
The conditional core damage probability (CCDP) for the period of the breaker unavailability has been estimated to be approximately 6.4E-07.
This CCDP estimate excludes the effects of external events (i.e., fire,
- � tornado, and seismic). This estimate also considered two important changes to the current base case PRA (Revision 2b).
This estimate also considered 2 important changes to the current base case PRA (Revision 2b). Catawba has been replacing the reactor coolant pump seal packages on the reactor coolant pumps. The new seal packages include a high temperature o-ring material which significantly improves the seal performance when subjected to a loss of seal cooling. The PRA model has been modified for this analysis to reflect the expected improvement in RCP seal performance. Only the stage 1 seal of Reactor Coolant Pump 1D retains the original, non-high temperature o-ring material. The impact of this seal retaining the original material has been evaluated and has been determined to have an insignificant impact on the core damage frequency results. Additionally, the industry experience regarding turbine building flooding shows that maintenance on condenser cooling systems is a major contributor to the flooding events. Because the CNS condenser cooling systems were intact for the time period of interest, a reduction in the turbine building flood frequency by a factor of 0.5 has been assumed in the analysis.
The conditional early containment failure probability (ECFP) has been estimated to be 1.3E-07 for the time period of concern. For the condition that is the subject of this LER, the change in ECFP is an appropriate indicator of the change in the large early release probability. This estimate is arrived at by assuming that the entire change in core damage probability is due to station blackout sequences.
A conditional containment failure probability of 0.2 (consistent with NUREG/CR-6595) is applied.
The offsite consequence analyses performed by Duke as part of the PRA program have shown that such a small change in the early containment failure probability has a very small impact on the probability of early fatalities, the quantitative health objective (OHO) for which the LERF is used as a surrogate. The early fatality risk is dominated by sequences such as the interfacing-systems loss of coolant accident (ISLOCA).
The breaker unavailability had only a small impact on the core damage and large early release probabilities.
During the time period that diesel generator 1B was inoperable, there were no cases in which any 'A' train equipment was removed from
- service. Therefore, this equipment would have been available if needed during this period.
In conclusion, the overall safety significance of this event was determined to be low and there was no actual impact on the health and safety of the public.
ADDITIONAL INFORMATION
A review of LERs from the last two years found no LERs that involved a similar hardware failure. Catawba Nuclear Station Unit 2 LER 414/00-001 involved inoperability of a diesel generator caused by a failure of 2ETB-18 to close. This was attributed to the existence of loose parts in the control device mechanism that may have interfered with the control device mechanism and prevented the breaker close coil from being energized.
Energy Industry Identification System (EIIS) codes are identified in the text as [MIS: XX]. This event was reportable to Equipment Performance and Information Exchange (EPIX) Program as EPIX report 367.
This event does not represent a safety system functional failure.
There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.