05000336/LER-2003-005
| Event date: | 10-14-2003 |
|---|---|
| Report date: | 11-21-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| Initial Reporting | |
| ENS 40245 | 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat, 10 CFR 50.72(a)(1)(i), Emergency Class Declaration |
| 3362003005R00 - NRC Website | |
1. Event Description On October 14, 2003, at 0407 with the plant in Mode 5, a loss of shutdown cooling (SDC) occurred when the SDC [BP) heat exchanger flow control valve (FCV) [FCV] failed closed due to a loss of power at vita! 120 vac panel VA10. When the FCV closed, the SDC heat exchangers were bypassed and resulted in a reactor coolant temperature rise of approximately 14 degrees F. The loss of SDC Is reportable as a loss of safety function in accordance with 10CFR50.73(a)(2)(v)(B). In addition, as expected upon a loss of VA10, one of the containment radiation monitors sent a signal to close the containment purge valves. Contrary to the normal response, the purge valves did not close and the operators manually closed them from the control room as directed by procedure. This unanticipated plant response did not complicate the event and Is not reportable as the containment purge isolation function is not required in Mode 5. Operator action restored power to VAIO within 13 minutes causing the FCV to automatically open and reestablish cooling. The plant declared an Unusual Event (UE) at 0423 (Event Number 40245) when reactor coolant temperature rise exceeded 10 degrees F and subsequently terminated the UE at 0532 after stabilizing plant conditions.
At the time of the event, the unit was in day 4 of a scheduled refueling outage and the following conditions existed:
- reactor coolant system (RCS) was vented through the pressurizer manway and filled to approximately 27%
- RCS temperature was 101 degrees F
- containment personnel and equipment hatches were open and containment dosure teams were In place
- both emergency diesel generators were available, 3 out of 4 vital 120 vac panels were available
- two high pressure safety injection pumps and two charging pumps were available A recent change in the method of performing inverter [INVT] maintenance required the use of an existing design configuration that was subsequently determined, as a result of this first-time evolution, to have a limitation.
Specifically, the circuits for the vita! inverter contain manual bypass and static switches, which when used as directed in the approved operations procedure allow the momentary paralleling of two asynchronous power sources. The procedure instructions reflect equally inaccurate vendor instructions provided with an earlier version of the vendor technical manual. The vendor manual had been subsequently corrected to compensate for the design limitation and was available at the time of equipment installation (1992). However, the testing and procedure reviews performed at the time of equipment installation did not uncover the procedure deficiency and it became a latent procedure error.
2. Cause The root cause of the event was determined to be a procedure deficiency resulting from using an incorrect version of the vendor technical manual. Specifically, the operations procedure allows switch manipulations that momentarily parallel two energized inverters without synchronous protection.
3. Assessment of Safety Consequences There were no significant safety consequences as a result of the loss of SDC. Operators are routinely trained on the emergency and abnormal operating procedures related to a loss of vital ac power and manual restoration of SDC. Operations personnel had been successfully dispatched to await Instruction to manually open the FCV to reestablish cooling and to close containment. These actions were not required since the FCV automatically reopened when vital power was restored to VA10 through its alternate supply. It is important to note that although the SDC heat exchanger was bypassed during this event, the SDC pump continued to operate and supply flow to the reactor coolant system (RCS). In addition, the estimated time to boll was approximately 1.3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> based on the event heat-up rate and core uncovery in this instance was estimated to occur 8.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> after boiling. This is considered to be sufficient time to both diagnose and mitigate the event. Also, the charging and high pressure safety injection systems were available to provide makeup to the RCS in the event of boiling. The overall safety significance of this event is considered to be low.
4. Corrective Action The compensatory corrective action to ensure inverters are returned to Operations in a deenergized state has been implemented. This action allows the existing procedure to be performed as written with the intended result of successfully swapping power supplies for VA10.
The corrective actions to prevent recurrence are to modify the operations procedure, vendor manual, and associated training documents to reflect safe operation of the inverter and static switch.
A root cause investigation was performed and additional corrective actions are being addressed in accordance with the Millstone Corrective Action Program.
5. Previous Occurrences No previous similar events were identified.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].