05000423/LER-2005-003

LER-2005-003,

Docket Number
Event date:	9-29-2005
Report date:	11-10-2005
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
4232005003R00 - NRC Website

1. Event Description On September 29, 2005, at approximately 1313, with the unit in Mode 1 at 100% power, Millstone Unit 3 reactor was manually tripped in accordance with established procedures, due to the loss of two circulating water [CW] pumps in the same condenser section.

Weather conditions began to deteriorate at approximately 1216 as a result of a passing storm. At approximately 1311 the 'A' [CW] pump 3CWS-P1A tripped due to high traveling screen differential pressure. At approximately 1313 the 'B' CW pump 3CWS-P1B tripped due to high traveling screen differential pressure. Once the control operator announced the second CW pump trip, the unit supervisor ordered the manual reactor trip and entry into emergency operating procedure (EOP) E-0, "Reactor Trip or Safety Injection.

Plant systems responded as designed and no adverse off-site radiological consequences resulted from this event.

Personnel in the intake structure during the event identified some debris loading, heavy wave action and strong winds. According to personnel dispatched, the conditions at the intake structure had an estimated wave height of approximately 8 ft. with water in the intake bay changing levels by approximately 4 ft. The plant computer records show sustained wind speeds of > 30 mph (gusting between 35-50 mph). The wind direction/speed, wave height, and debris loading created conditions that challenged the intake equipment.

2. Cause Unusually severe environmental conditions challenged the design of the MP3 Intake structure causing the loss of two circulating water pumps and thus required the Manual Trip of the Reactor.

The 'A' and 'B' CW Pumps tripped due to a high differential pressure across the traveling screens. The differential pressure (dp) setpoints of 'A' and 'B' traveling screens were exceeded, due to the combination of wind direction/speed, wave height, and debris loading.

Large swell oscillations in the intake bay prevented accumulating, suspended debris from collecting on the traveling screens until debris density became significant. Subsiding oscillations allowed the debris to collect on the screens causing the 'A' CW pump to automatically trip. The debris then migrated to the 'B' bay, adding to the loading on the 'B' traveling screen and the subsequent trip of the 'B' CW pump.

3. Assessment of Safety Consequences The reactor trip resulted in a loss of normal heat removal (condenser not available). The condenser was not available due to the loss of two circulating water pumps and loss of the C-9 interlock, prohibiting the use of the condenser steam dump valves. When the 'A' CW pump was restored to service at 1343, the condenser became available and the C-9 interlock was actuated.

The auxiliary feedwater system started automatically on the trip as expected and restored the steam generator levels to their normal operating band. Heat removal capability was maintained.

The operator actions and plant mitigating equipment responded as expected with no failures. There were no challenges to any fission product barrier. Therefore, there were no safety consequences to the reactor trip on September 29, 2005.

4. Corrective Action Actions to Prevent Recurrence:

Actions were initiated in the station corrective action program to review possible design changes to the MP3 intake structure that would reduce the susceptibility to environmentally induced trips.

5. Previous Occurrences None Energy Industry Identification System (El IS) codes are identified in the text as [XX].