05000336/LER-2010-003
| Docket Numbersequential Revmonth Day Year Year No Month Day Yearnumber 05000 | |
| Event date: | 11-28-2010 |
|---|---|
| Report date: | 01-27-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3362010003R00 - NRC Website | |
1. Event Description On November 28, 2010, at 1515, Millstone Power Station Unit 2's (MPS2) reactor automatically tripped from 100% power MODE 1. Prior to the event, MPS2 was establishing conditions to perform a backwash of the "B" condenser [COND] waterbox. Operators pressed the stop pushbutton for the "B" circulating water (CW) pump [P] and were closing the "A" CW pump's water box outlet valve (2-CW-11A) [V] per established operating procedure OP-2325D when the "A" CW pump automatically ramped off. Upon loss of the second pump in the condenser, condenser pressure increased to the low condenser vacuum trip set point, causing an automatic turbine generator trip and a reactor trip.
The CW pumps' operation had changed due to plant modifications which installed Variable Frequency Drives (VFDs) for these pumps at Millstone Station. In accordance with the original systems' design, when the control logic sensed that the "A" CW pump's outlet valve was closed with the "B" CW pump still running, it caused the "A" CW pump to automatically shut down because the system detected that no flow path was available. The VFD modification changed the CW pump operation in that the pumps ramped off instead of tripping off when the operator stopped the pump in VFD Mode. This new configuration created a delay in completing the logic for pump shutdown.
2. Cause The cause of the event was an organizational failure to properly develop and train on the procedure needed to conduct backwashing operations using Variable Frequency Driven (VFD) CW pumps.
3. Assessment of Safety Consequences The operating crew responded to the reactor trip by completing EOP 2525, Standard Post Trip Actions, and entering EOP 2526 Reactor Trip Recovery. The auxiliary feedwater system started in response to low steam generator level as designed.
All control rods inserted on the reactor trip. With the "C" and "D" CW pumps still running, condenser vacuum remained adequate for operation of the condenser dump valves following the reactor trip. Both main and auxiliary feedwater provided makeup to the steam generators.
Based on the above discussion, there were no safety consequences for the event.
This event is reportable per 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in a manual or automatic actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B). Actuations of the reactor protection system and the auxiliary feedwater system are reportable under this paragraph.
4. Corrective Action OP-2325D was revised prior to MPS2 restart to provide additional directions to operators about circulating water pump operations during backwashing. All other VFD procedures on MPS2 and MPS3 were reviewed and found to contain adequate procedure guidance or corrective actions were established to correct the procedures.
Corrective actions to prevent recurrence being implemented will ensure proper development and training on procedures are made during plant modifications.
5. Previous Occurrences No similar occurrences were found at Millstone Station where plant trips occurred caused by failure to properly implement a procedure due to design changes within the last 3 years.
Energy Industry Identification System (EDS) codes are identified in the text as [XX].