05000445/LER-2002-003
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 4452002003R01 - NRC Website | |
I. DESCRIPTION OF REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
An event or condition that resulted in valid actuation of any system listed in 10CFR50.73 (a)(2)(iv)(B). Specifically, an emergency diesel generator automatically started.
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
On October 7, 2002, Comanche Peak Steam Electric Station (CPSES) Unit 1 was in Mode 6 with the reactor cavity water level > 23 feet above the vessel flange. Core off-load was in progress with approximately one third of the core off-loaded. Core decay heat was being removed by train B of the Residual Heat Removal system. Unit 2 remained at power throughout the event.
C. STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE
INOPERABLE AT THE START OF THE EVENT AND THAT CONTRIBUTED TO
THE EVENT
There were no inoperable structures, systems, or components that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND
APPROXIMATE TIMES
On October 07, 2002, at 0936, during 345 KV switchyard east bus relay testing, an unexpected pickup of a relay contact occurred which generated a signal that caused the Unit 1 6.9 KV safeguards busses to transfer to the alternate power source. The Unit 1 train B emergency diesel generator (EIIS:(DG)) 1-02 unexpectedly auto started while, as designed, the bus re- energized to station transformer XSTI. Bus load shedding initiated by bus undervoltage caused both Spent Fuel Pool pumps and the operating Residual Heat Removal pump to trip.
The blackout sequencer (BOS) actuated and functioned as designed. The fuel assemblies in transit were secured in safe locations and core alterations were stopped. The plant operators responded to the loss of normal power to the train B bus and restored equipment shed from the busses by the momentary undervoltage condition.
Residual Heat Removal (RHR) shutdown cooling was restored on the train B RHR pump at 0944 and Spent Fuel Pool cooling restored at 0946. The batteries normally providing backup DC power to the normal (non-safeguards) breaker controls were undergoing replacement at the time, complicating recovery of the non-safeguards busses. Normal plant power was restored at 1112 and core off-load resumed at 1319.
E. THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM
FAILURE, OR PROCEDURAL OR PERSONNEL ERROR
Control board indicators and alarms alerted the Reactor Operator (Utility, Licensed) that the train B emergency diesel generator 1EA2 (EIIS:(DG)) auto started.
II. COMPONENT OR SYSTEM FAILURES
A. FAILURE MODE, MECHANISM, AND EFFECTS OF EACH FAILED COMPONENT
The emergency diesel generator auto start in response to the bus transfer was unexpected.
Troubleshooting determined that the time delay relay 27-2X/1EA2 (EIIS:(2)) was operating erratically.
B. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE
Performance of multiple tests on the time delay relay 27-2X/1EA2 (EllS:(2)) determined that the time delay relay degraded to a point where repeatability was lost.
C. SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE
OF COMPONENTS WITH MULTIPLE FUNCTIONS
Not applicable -- No failures of components with multiple functions have been identified.
D. FAILED COMPONENT INFORMATION
Manufacturer: Tyco Electronics Model No. E7012PA004 Agastat relay
III. ANALYSIS OF THE EVENT
A. SAFETY SYSTEM RESPONSES THAT OCCURRED
Train B Control Room HVAC shifted to emergency recirculation and the BOS actuated during this event. The BOS restarted the B train Station Service Water and Component Cooling Water pumps. Both Spent Fuel Pool pumps and the Residual Heat Removal pump were load shed and were restarted manually.
B. DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY
Not applicable -- No safety system was rendered inoperable
C. SAFETY CONSEQUENCES AND IMPLICATIONS
The blackout sequencer functions to reload the associated 6.9 KV safeguards bus in a pre- established sequence following an undervoltage condition on the bus after the bus has been re- energized from the alternate power source or the emergency diesel generator. When the time delay relay 27-2X/1EA2 was slow to pick up, relay 27-2X1/1EA2 picked up and the train B emergency diesel generator started. Bus lEA2 re-energized, on closing of the alternate source breaker, and the BOS began the loading sequence. Even though the emergency diesel generator auto start was not anticipated, evaluation of the event concluded that all other systems operated as designed. Continued cooling of the fuel assemblies in the reactor vessel and in the spent fuel pools was maintained by the large mass of water in the reactor cavity and in the spent fuel pools, respectively. No observable temperature changes occurred in either the refueling cavity or the spent fuel pools. Based on this analysis it was concluded that this event did not adversely affect the safe operation of CPSES Unit 1 or the health and safety of the public.
Unit 2 was not affected in that the Unit 2 preferred source of power remained available and Unit 2 remained at power throughout the event.
IV. CAUSE OF THE EVENT
TXU Energy believes the 27-2X/1EA2 relay was operating erratically and was too slow to pick up allowing the 27-2X1/1EA2 relay to time out and start the train B emergency diesel generator.
Subsequent testing of relay 27-2X/1EA2 found that some of the pick up times would have been long enough to allow the 27-2X1/1EA2 relay to time out and start the emergency diesel generator.
Although subsequent evaluation of relay 27-2X/lEA2 was not possible because the relay was destroyed and scrapped, it is possible that the relay erratic operation could be age related.
V. CORRECTIVE ACTIONS
The time delay relay 27-2X/1EA2 was replaced with a new relay. The new relay was recalibrated and is within design specifications. Additional relays in associated loss of power emergency diesel generator start circuits were checked and found to be functioning within design requirements.
In addition to the above, as a part of the CPSES Corrective Action Program, Engineering is evaluating possible causes for similar relays to exceed their expected response time during surveillance testing. The scope of the ongoing evaluations also include reassessing the manufacturer's recommendations for these relays, and determining any preventive maintenance, or planned replacement of aged relays as may be appropriate.
VI. PREVIOUS SIMILAR EVENTS
There have been other events involving the start of an emergency diesel generator. However, details/causes are sufficiently different from the event described in this LER such that the previous corrective actions could not have prevented this event. There have also been previous failures of Agastat relays, but none involving an auto start of an emergency diesel generator.