ENS 41791
ENS Event | |
|---|---|
20:49 Jun 23, 2005 | |
| Title | Offsite Power Source Declared Inoperable Due to Low Voltage |
| Event Description | At 15:49 hours Central Daylight Savings Time, June 23, 2005, entered DOA 6500-12, Low Switchyard Voltage. Bulk Power Operations reported predicted post Unit trip with LOCA switchyard voltage for Unit 2 is 342.5 KV and for Unit 3 is 342.3 KV. This rendered both Unit 2 and Unit 3 offsite electrical power sources inoperable. This predicted voltage is the value required to support Loss Of Coolant Accident (LOCA) loading. Entered applicable Technical Specification required actions for both Units due to both offsite power sources being declared inoperable for Unit 2 and Unit 3.
As of 1744 hours0.0202 days <br />0.484 hours <br />0.00288 weeks <br />6.63592e-4 months <br />, restored Unit 2 offsite electrical power source to operable status by performing Transformer 86 Tap Changer adjustment per applicable procedure. TR 86 [transformer 86] supplies Unit 2 offsite electrical power. As of 1948 hours0.0225 days <br />0.541 hours <br />0.00322 weeks <br />7.41214e-4 months <br />, switchyard voltage was restored to operable limits. This restored Unit 3 offsite electrical power source to operable status. This notification is provided in accordance with 10CFR50.72(b)(3)(v)(D) to report a condition that at the time of discovery could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident. The licensee will notify the NRC Resident Inspector and the state.
The purpose of this report is to retract ENS 41791 (June 23, 2005) for Dresden Nuclear Power Station (DNPS). The report was made following notification from Bulk Power Operations that the predicted post Unit trip with LOCA switchyard voltage rendered both Unit 2 and Unit 3 offsite electrical power sources inoperable. Both offsite electrical power sources were declared inoperable, the appropriate Technical Specification required actions were taken and an ENS notification was made in accordance with 10CFR 50.72(b) (3)(v)(D) for an event that could have prevented fulfillment of a safety function. In July 2006, DNPS performed a reanalysis of the June 23, 2005 event. The reanalysis determined that the computer model used to determine the required switchyard voltage for the June 23, 2005 event conservatively assumed that the Reserve Auxiliary Transformers (RATs) were supplying all plant 4 kilovolt (Kv) electrical loads and the second source of offsite power for each unit was based on a post unit trip. During normal plant operation the 4kV electrical loads are split between the RATs and the Unit Auxiliary Transformers (UATs). The reanalysis used the split bus configuration and the actual switchyard voltages at the time of the June 23, 2005 event. The reanalysis concluded that the voltages were greater than required, ensuring that the second source of offsite power for each unit would remain operable following a postulated design basis accident. Therefore, there was no condition present on June 23, 2005, that could have prevented fulfillment of a safety function and this event is not reportable under 10CFR 50.72(b)(3)(v)(D). The licensee notified the NRC Resident Inspector. Notified R3 DO (R. Lanksbury) |
| Where | |
|---|---|
Dresden  Illinois (NRC Region 3) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | |
| Time - Person (Reporting Time:+1.32 h0.055 days <br />0.00786 weeks <br />0.00181 months <br />) | |
| Opened: | Dan Daly 22:08 Jun 23, 2005 |
| NRC Officer: | Mark Abramovitz |
| Last Updated: | Jul 28, 2006 |
| 41791 - NRC Website
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Unit 2 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (99 %) |
| After | Power Operation (99 %) |
Unit 3 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (99 %) |
| After | Power Operation (99 %) |