05000336/LER-2010-001

LER-2010-001, Millstone Power Station Unit 2 Reactor Trip

Docket Number
Event date:	02-26-2010
Report date:	04-26-2010
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
3362010001R00 - NRC Website

1. EVENT. DESCRIPTION:

On February 26, 2010, with the unit in Mode 1, at 100 % power, the Millstone Power Station Unit 2 (MPS2) reactor was manually tripped as required by procedure due to a loss of circulating water (CW) [SG] flow to one of the two sections of a main condenser [COND]. All control rods fully inserted into the reactor and all emergency systems functioned as designed. At the time of the trip, the 'D' CW pump [P] was out of service for planned maintenance and divers were working in the 'D' CW pump bay to remove thermal barriers [NN] and install stop logs in support of the planned work. The work required close coordination with the control room staff as the 'C' CW pump bay screen wash system had also been removed from service in support of the planned work. A contingency plan was in place to support restoration of 'C' CW pump bay screen wash if needed. The contingency plan was not effective since during the removal of the thermal barriers a larger than expected influx of debris from the 'D' bay resulted in differential level across the 'C' CW screen reaching the automatic trip point of the 'C' CW pump. Due to the loss of the two CW pumps in one CW bay the condenser was unavailable.

Following the trip, the operators closed the main steam isolation valves to protect the condenser from overpressure. The auxiliary feedwater system started in response to low steam generator water levels and restored the steam generator levels to their normal operating band.

There were no radiological challenges as a result of the event.

This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B).

2. CAUSE:

The cause was determined to be the failure of the organization's individual departments to work together as a team to clearly identify the risks associated with the work evolution. This led to failure to mitigate risk during the 'D' CW pump work activity.

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 water levels and restored the steam generator levels to their normal operating band. The reactor coolant system (RCS) heat removal safety function was maintained. In addition all other safety functions, including reactivity control, RCS inventory and pressure control, and containment safety functions continued to be satisfied.

With the loss of the 'C' CW pump and the 'D' CW pump out of service for maintenance the operators closed the main steam isolation valves and broke condenser vacuum in accordance with EOP 2525. With the condenser unavailable, the main feedwater system and the steam dump to condenser valves were not available. RCS heat removal was satisfied utilizing auxiliary feedwater and the main steam atmospheric dump valves.

The operator actions and plant mitigating equipment responded as expected with no safety system failures.

There were no challenges to any fission product barrier. Therefore, there were no safety consequences to the reactor trip.

4. CORRECTIVE ACTION:

The risk assessment and mitigation control process has been reviewed and the appropriate actions are being taken in accordance with the station's corrective action program.

5. PREVIOUS OCCURRENCES:

No previous similar events/conditions were identified.

Energy Industry Identification System (EllS) codes are identified in the text as [XX].