05000336/LER-2008-003

LER-2008-003, Failed Pilot Wire Causes Reactor Trip

Docket Number
Event date:	05-22-2008
Report date:	07-14-2008
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
LER closed by
IR 05000245/2008004 ()
3362008003R00 - NRC Website

1. Event Description On May 22, 2008 at 1359 with the Millstone Power Station Unit 2 (MPS2) at 100% power in Mode 1, the reactor automatically shutdown following a loss of load and subsequent turbine [TA] trip. Subsequent investigation determined that a lightning strike on a transmission line created an over current condition to which the unit responded.

During the event, the 345kV main generator output breakers [BKR] in the transmission switchyard [FK] opened when an over current situation was detected. This caused a trip of the turbine on loss of load and an automatic transfer of the in-house electrical buses from the Normal Station Service Transformer (NSST) [XFMR] to the Reserve Station Service Transformer (RSST) [EA, XFMR]. Letdown isolated automatically, the reactor trip circuit breakers opened, and all control element assemblies inserted. During the electrical transfer from the NSST to RSST, there was a momentary loss of the non-safety grade 120 volt power supplies (VR-11 and VR-21) [JX] to the annunciators and control boards when they transferred to their alternate power supplies. The steam generator #2 atmospheric dump valve [RV] and the steam generator #1 safety valve (2-MS-247) opened . Additionally, the "A" steam dump valve to the condenser modulated open, however the "B", "C" and "D" dump valves did not quick open due to a momentary loss of power from VR-11. Operators re-established letdown and all the steam dump valves were opened and steaming was re-established to the condenser. The unit was maintained in a stable condition, i.e., hot-standby (Mode 3).

This event is being reported pursuant to 10CFR50.73(a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of systems listed in 10CFR50.73(a)(2)(iv)(B).

2. Cause The direct cause of the switchyard breakers opening and remaining open was a mechanical failure of a connecting wire lug in the pilot wire circuitry causing an open circuit. Pilot wire relaying is used for primary protection of the 345 kV lines between the switchyard and the main step-up transformers. This open circuit created the conditions that caused a relay [RLY] to act as an over current protection device. The power surge from a lightning strike on the transmission line created the over current condition necessary for the system to react. The breakers and associated interlocks operated as expected protecting station equipment. Had the pilot wire lug been intact, the relay would have acted as a differential current relay and the switchyard breakers would have remained closed and the unit would have stayed on line.

The root cause of the failed lug in the pilot wire circuitry was ductile overload. Laboratory analysis indicated that there was excessive movement of the lug while it was tightly fastened to its contact point. Review of work history and wire configuration suggests the failure mechanism was a latent condition caused by a change in the wiring configuration for the lockout relay that occurred in 1996 as part of a larger cable separation project. Review of previous testing, plant conditions and work activities suggests that the lug most likely failed after the unit shutdown on April 6, 2008.

3. Assessment of Safety Consequences The event had very low risk significance. The reactor automatically shutdown following a loss of load and subsequent turbine trip. A lightning strike on a transmission line created an over current condition to which the unit responded. The unit responded as required for these conditions. The operators took actions as trained and in accordance with procedures. No equipment was damaged as a result of this event.

4. Corrective Action Short term corrective action was to repair the circuit by installing a new lug on the pilot wire, recalibrating the relay, and performing additional testing.

An investigation into this event was conducted and other appropriate corrective actions are being addressed in accordance with the Millstone Corrective Action Program.

5. Previous Occurrences No previous similar events/conditions were identified.

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