05000336/LER-2013-004

LER-2013-004, Reactor Trip While Backwashing D Waterbox
	Millstone Power Station - Unit 2
Event date:	11-09-2013
Report date:	12-19-2013
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation; 10 CFR 50.73(a)(2)(IV)(A); 10 CFR 50.73(a)(2)(iv)(A), System Actuation
	3362013004R00 - NRC Website
	v • d • e

1. EVENT DESCRIPTION

On November 9, 2013, at 1514, Millstone Power Station Unit 2 (MPS2) experienced a turbine trip and an automatic reactor trip from 95% power MODE 1 due to loss of condenser vacuum.

The Unit was in the process of condenser backwashing operations in accordance with plant procedures. The loss of condenser vacuum was due to unexpected pump ramp-down of the 'C' circulating water pump (CWP) when the 'D' CWP was secured as required by procedure. This resulted in both CWPs being secured in the B' condenser. This caused the condenser vacuum to drop resulting in a turbine trip which immediately caused the reactor trip breakers to open. All the control rods inserted into the reactor core. Reactor coolant (RCS) and main steam systems responded as expected. There were no indications of safety valve actuation. Steam generator (SG) pressures were maintained below 920 psia. RCS temperature response was as expected post trip and returned to its nominal no-load Tcold of 532 degrees F. An auxiliary feedwater (AFW) automatic start occurred post trip, as expected and SG water level was recovered. All safety systems functioned as required.

This event is being reported 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 AFW system are reportable under this paragraph.

2. CAUSE

Post trip troubleshooting revealed that the direct cause of the event was the MPS2 'C' CWP ramped off due to failure of contacts on a time-delay relay to deenergize as designed. This relay is a pump permissive relay designed to de-energize when the 'D' CWP is fully secured.

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 AFW system started in response to low steam generator level as designed.

All control rods inserted on the reactor trip. With the 'A' and 'B' CWPs 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 SGs.

Based on the above discussion, there were no safety consequences for the event.