05000530/LER-2008-002

LER-2008-002,

Palo Verde Nuclear Generating Station (Pvngs) Unit 3
Event date:	09-27-2008
Report date:	11-26-2008
5302008002R00 - NRC Website

All times are Mountain Standard Time and approximate unless otherwise indicated.

1. REPORTING REQUIREMENT(S):

This LER is being submitted pursuant to 10 CFR 50.73 (a)(2)(iv)(A) to report a manual actuation of the reactor protection system (RPS). This event was reported to the NRC on the Emergency Notification System (ENS) on September 27, 2008, at 23:35. (ENS 44525)

2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):

The reactor protection system (RPS) (EIIS: JC) consists of sensors, calculators, logic, and other equipment necessary to monitor selected nuclear steam supply system (NSSS) conditions and to effect reliable and rapid reactor shutdown (reactor trip), if monitored conditions approach specified limiting safety system settings. The system's functions are to protect the core specified acceptable fuel design limits and reactor coolant system (RCS) (EIIS: AB) pressure boundary for incidents of moderate frequency, and also to provide assistance in limiting conditions for certain infrequent events and limiting faults. A manual reactor trip is also provided as part of the RPS to permit the operator to trip the reactor.

The General Electric main turbine (EIIS: TA) operates at 1800 revolutions per minute and includes one double-flow, high-pressure turbine; three double-flow, low-pressure turbines; and four moisture separator reheaters with two stages of reheating. The rotors are of monoblock construction. The main turbine is equipped with various protective trip features, which, when initiated, cause isolation of all turbine stop and control valves. Included in these protective trips is a protective trip for excessive vibrations (experienced in this event).

3. INITIAL PLANT CONDITIONS:

On September 27, 2008, Palo Verde Unit 3 control room operators were performing a unit downpower in response to a steam generator (EIIS: AB) chemistry sulfate excursion event.

Reactor rated thermal power level was approximately 38% when main turbine vibrations were noted to be increasing.

4. EVENT DESCRIPTION:

On September 27, 2008, at approximately 21:49, while performing a unit downpower in response to a chemistry excursion event (elevated sulfates in the steam generators), Palo Verde Unit 3 control room operators received main turbine high vibration alarms and noted that the vibrations appeared to be trending up. Licensed operators planned to perform a normal reactor shutdown by reducing reactor rated thermal power to approximately 20% and then initiating a manual reactor trip. Due to the increasing main turbine vibrations, a decision was made to manually trip the reactor sooner than planned, and then manually trip the main turbine. The manual reactor trip was initiated at 21:51 from approximately 34% rated thermal power. The plant was stabilized in Mode 3 with no observed abnormalities following the reactor trip.

A post-trip review of parameter records indicated that the main turbine vibrations had reached the automatic trip set point approximately 3.5 seconds before the manual reactor trip was initiated and as such, the main turbine had tripped automatically.

5. ASSESSMENT OF SAFETY CONSEQUENCES:

The event did not result in a transient more severe than those previously analyzed in the PVNGS UFSAR, Chapter 15. The event was considered an uncomplicated reactor trip.

The reactor trip was not automatically initiated as a result of any of the categories defined in UFSAR Section 15.0.1.2. The Specified Acceptable Fuel Design Limits and reactor coolant system pressure limit were not exceeded. Equipment and systems assumed in UFSAR Chapter 15 were functional and performed as required. No automatic engineered safety feature (ESF) actuations occurred during the event, and all safety related buses remained energized from normal offsite power during and following the reactor trip. The reactor trip was not necessary to mitigate the consequences of the event.

There are no actual safety consequences as a result of this condition; the condition would not have prevented the fulfillment of the safety function; and, the condition did not result in a safety system functional failure as defined by 10 CFR 50.73 (a)(2)(v).

6. CAUSE OF THE EVENT:

The direct cause of the reportable event (RPS actuation) was the decision reached by control room operators to initiate a manual reactor trip in response to the high vibrations on the main turbine.

The Unit 3 turbine was replaced in the most recent refueling outage which was completed in December 2007. Since the low pressure rotors and shaft packing are new, the vibrations are an indication that the clearances are fairly tight (and efficient). The main turbine vibrations may have also been exacerbated by erratic operation of a moisture separator reheater (MSR) (EIIS: SB) 2nd stage air operated control valve.

Vibration of GE monoblock rotors is not uncommon in response to temperature excursions.

7. CORRECTIVE ACTIONS:

There are no prescribed corrective actions for the operators' decision to trip the reactor at a power level above that originally planned based on chemistry considerations alone.

This decision was within the discretion of the operators in response to the changing conditions associated with the turbine vibrations.

8. PREVIOUS SIMILAR EVENTS:

There have been manual reactor trips in the past three years at Palo Verde Nuclear Generating Station but none with causes similar to this event. As such, the corrective actions for those events would not have prevented this event.