05000323/LER-2013-004
| Diablo Canyon Power Plant, Unit 2 | |
| Event date: | 06-08-2013 |
|---|---|
| Report date: | 03-30-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3232013004R01 - NRC Website | |
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I. Plant Conditions
At the time of the event, Unit 2 was in Mode 1 (Power Operation) at approximately 100 percent reactor [RCT] power with normal operating reactor coolant temperature and pressure.
II. Problem Description
A. Background
Diablo Canyon Power Plant (DCPP) Units 1 and 2 each have three emergency diesel generators (EDGs) [DG] that provide vital backup power to each unit's three electrical buses [BU] to mitigate the consequences of a design basis accident (DBA) if off-site power sources are unavailable. DCPP EDGs are designed to function so that a single failure of any EDG will not jeopardize the capability of the remaining EDGs to start and provide power to operate the systems required to mitigate any DBA condition. In order to ensure that an EDG can perform its design function, it is tested by continuously operating at full-load for an interval of not less than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
Each EDG has differential relays [87] that monitor phase currents and compare them with the corresponding phase currents on the 4 kV Bus side of the generator output breaker [BKR]. If a phase current is different between generator [GEN] and bus sides of the breaker, that differential relay actuates. This condition actuates an alarm [ALM] in the Control Room [NA], energizes blue lights in the Control Room and on the 4 kV relay board [PL], and energizes the diesel shutdown lockout relay [86], which shuts down the EDG.
The wire terminations for the current transformer inputs to the differential relays are located in junction boxes [JBX]. The configuration of the junction boxes in the EDG rooms is not the same for all six EDGs. EDGs 1-1, 1- 2, 1-3, 2-1, and 2-2 have their junction box mounted directly onto the generator housing, roughly 4-1/2 feet above ground on the generator centerline. They are lower to the ground and extremely secure, whereas the junction box on EDG 2-3 is mounted to two supports [SPT] that are connected to the top of the high voltage termination cabinet [CAB], roughly 7 feet above ground. When EDG 2-3 is running, its junction box experiences a higher level of vibration than the other EDG junction boxes. Additionally, the equipment inside the junction boxes is not the same for all six EDGs. EDGs 1-1, 1-2, 1-3, 2-1, and 2-2 have a terminal block [BLK] inside of their junction boxes and no terminal block cover. EDG 2-3 has a terminal block and state switches [89] beneath a terminal block cover inside of its junction box.
B. Event Description
During a performance of DCPP Procedure STP M-9G, "Diesel Generator 24-Hour Load Test and Hot Restart Test," on June 8, 2013, the control room received two unexpected alarms for EDG 2-3. Operators recognized that 4 kV, Bus F, Differential Relay 87HFG had actuated and shut down EDG 2-3 21 hours2.430556e-4 days <br />0.00583 hours <br />3.472222e-5 weeks <br />7.9905e-6 months <br /> and 42 minutes into the 24-hour load run. DCPP determined that a broken wire lug [CON] in the Differential Relay 87HFG circuit removed an input signal to the relay. Subsequent evaluation concluded that terminal block cover vibration against the wire [WEL] induced a cyclical fatigue in the wire lug. Inspection of the junction boxes for the other EDGs showed no signs of wear.
Due to the nature of the failure, DCPP determined that the last time that EDG 2-3 would have been able to meet its surveillance requirement to complete a 24-hour load run was April 11, 2013.
C. Status of Inoperable Structure, Systems, or Components That Contributed to the Event EDG 2-3 was unable to perform its safety function from April 11, 2013, until June 8, 2013. During this window, EDGs 2-1 and 2-2 were declared inoperable on multiple occasions, but remained capable of performing their safety function via manual operator action.
EDG 2-1 was declared inoperable on the following four occasions (all times PDT):
- May 1, 2013, from 01:32 — 04:21, for scheduled surveillance
- May 28-29, 2013, from 21:43 — 00:13, for scheduled surveillance
- May 27, 2013, from 20:19 — 22:57, for corrective maintenance
- May 29, 2013, from 03:05 — 14:21, for corrective maintenance EDG 2-2 was declared inoperable on the following two occasions (all times PDT):
- April 25-26, 2013, from 21:59 — 00:32, for scheduled surveillance
- May 24-25, 2013, from 10:01 — 16:38, for scheduled surveillance For those periods that EDG 2-1 or 2-2 were declared inoperable, DCPP Unit 2 only had one operable EDG, when two are required to be operable to perform the designed safety function of providing vital emergency power. This condition could have prevented the fulfillment of the EDG's designed safety function to mitigate the consequences of a DBA.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
Annunciators PK18-14, "DSL GEN 2-3 BKR DIFFERENTIAL TRIP" and PK18-15, "DSL 2-3 SHUTDOWN RELAY TRIP" alarmed in the Control Room.
F. Operator Actions
Operators observed EDG 2-3 coasting down normally, DG 2-3 blue light lit on Panel VB4, and 4 kV, Bus F, Differential Relay 87HFG actuated. Operators declared EDG 2-3 inoperable.
G. Safety System Responses
None.
III. Cause of the Problem
The apparent cause of the EDG 2-3 trip was a broken wire lug in the Differential Relay 87HFG current transformer circuit. This broken lug removed one phase of the generator-side current input signal to the differential relay.
Subsequent evaluation concluded that terminal block cover vibration against the wire induced a cyclical fatigue in the wire lug.
IV. Assessment of Safety Consequences
DCPP performed a quantitative risk analysis for a 58-day exposure period. This timeframe is based on the last time that EDG 2-3 would have been able to successfully complete a 24-hour load run. EDGs 2-1 and 2-2 were capable of performing their safety function (via manual operator action) and EDG 2-3 would start, load the associated bus, and run for at least 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br /> prior to failure. In that time, the offsite power supply would likely have been recovered. Based on these considerations, DCPP concluded that the incremental conditional core damage probability was less than 1E-06.
V. Corrective Actions
A Immediate Corrective Actions DCPP replaced and reconnected the broken wire lug.
B. Other Corrective Actions
DCPP permanently removed the terminal block cover, and will revise DCPP Procedure STP M-81G, "Diesel Engine Generator Inspection," to include a step to inspect terminations at the generator junction box for tightness and cyclic fatigue.
VI. Additional Information
A. Failed Components
DCPP discovered a broken wire lug at Junction Box BTA503 in the current transformer input circuit for Differential Relay 87HFG.
B. Previous Similar Events
Diablo Canyon — On December 11, 1989, EDG 1-2 tripped due to a differential relay actuation. Troubleshooting found a broken lug in the current transformer output circuit for a differential relay.