05000388/LER-2005-003
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 41646 | 10 CFR 50.72(b)(2)(xi), Notification to Government Agency or News Release, 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3882005003R00 - NRC Website | |
PLANT CONDITIONS AT TIME OF EVENT
Unit 1, Model, 100% Unit 2, Mode 1, 75%
EVENT DESCRIPTION
Unit 2 has three single phase main transformers (EIIS Code: EL), A, B, & C that supply power to the 500 kV switchyard EIIS Code: FK) for grid distribution. The transformers are cooled via a circulating oil system that transfers the heat to the environment through forced air-cooling. At approximately 0700 hours0.0081 days <br />0.194 hours <br />0.00116 weeks <br />2.6635e-4 months <br /> on 4/28/2005, with Unit 2 in Mode 1 at 100% power, the Control Room received a B Phase Main Transformer alarm.
Operators were dispatched and found the cooling system for the transformer not operating. Attempts to restore the B Main Transformer cooling were unsuccessful. Without cooling, plant alarm procedures require removal of the transformer from service. During the recovery attempts, reactor power was reduced from 100% to 75% and at 0719 hours0.00832 days <br />0.2 hours <br />0.00119 weeks <br />2.735795e-4 months <br /> RPS (EIIS Code: JC) was manually actuated to scram the reactor. The Unit 2 electrical output breakers automatically opened as designed removing the electrical load from all three Main Transformers.
The plant response was per design. All control rods inserted and RPV water level decreased to —30", which is the setpoint for a RCIC (EIIS Code: BN) initiation. RCIC did initiate and with normal Feedwater (EIIS Code: SJ) flow, reactor water level was restored. A reactor water Level 3 (+13") containment isolation signal (EllS Code: JM) was received, but all affected isolation valves were already in their post-isolation alignment.
A reactor water Level 2 (-38") signal for Division 2 was also received. This resulted in a secondary containment ventilation isolation (Zones 2 and 3) and an auto-start of Division 2 of Standby Gas Treatment (EllS Code: BH) and Control Room Emergency Outside Air Supply (EllS Code: VI) Systems. A post scram evaluation determined that the instrumentation that caused the initiation signal was within its setpoint tolerance and the system response was per design.
The manual scram and subsequent injection of the RCIC system are considered unplanned actuations of systems that mitigate the consequences of significant events and are reportable per 10 CFR 50.73(a)(2)(iv)(A).
This event resulted in no actual adverse consequences to the health and safety of the public.
CAUSE OF THE EVENT
The cause of the event was attributed to two root causes:
1) The Main Transformer Cooling system contained a known design weakness that was not corrected in a timely manner. In 2003 the design weakness was discovered in the transformer cooling electrical logic. In the event of a ground fault in a transformer cooling circuit end device, the main power supply breaker could trip. In this scenario the backup power supply would also trip when called upon to energize the circuit. The evaluation of this vulnerability focussed on the component level impacts and did not adequately consider the overall potential impact to the plant.
2) Operators were unable to implement manual actions to restore cooling to one of the main transformers prior to initiating a reactor scram. The procedural guidance and training in response to the loss of transformer cooling were found to be weak and heavily reliant on operator experience.
ANALYSIS / SAFETY SIGNIFICANCE
Actual Consequences The health and safety of the public was not affected. The capability to electrically isolate the Unit 2 Main Transformers using existing plant equipment and procedures was demonstrated.
The total loss of all transformer cooling for the B Main Transformer was greater than 10 minutes. Dissolved gas analysis was performed on an oil sample taken from the transformer. The analysis reflected no increase in dissolved gas generation.
Potential Consequences The potential consequence from a total loss of all transformer cooling results in a negligible increase in the _ probabilistic risk to the health.and.safety Total loss of all transformer cooling does increase the probability of long term degradation of the transformer.
CORRECTIVE ACTIONS
Completed Actions
1) The fault that caused the loss of cooling to the B Main Transformer was found and repaired.
2) The A and C Main Transformer cooling circuits were inspected for similar conditions.
3) The B Main Transformer oil samples were tested with satisfactory results.
4) Procedures were reviewed and revised to increase the likelihood of restoring transformer cooling within the required administrative limit.
Planned Actions
1) Correct the design deficiency for Unit 2 main transformers.
2) Review plant procedures for enhancements that will prioritize the resolution of identified deficiencies based on the impact to the system/plant.
3) Enhance training material to improve operator proficiency when responding to time critical actions.
ADDITIONAL INFORMATION
None