05000316/LER-2003-002

LER-2003-002, Supplemental LER for Unit 2 Reactor Trip due to Instrument Rack 24 Volt Power Supply Failure

Docket Number
Event date:	02-05-2003
Report date:	02-23-2005
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
3162003002R01 - NRC Website

Conditions Prior to Event Unit 1 - Mode 5, 0 percent power Unit 2 - Mode 1, 100 percent power

Description of Event

At 2058 hours0.0238 days <br />0.572 hours <br />0.0034 weeks <br />7.83069e-4 months <br /> on February 5, 2003, the Unit 2 reactor automatically tripped from 100% power. The direct trip signal was low steam generator water coincident with steam flow/feed flow mismatch in #23 steam generator (JB]. This condition was caused by the failure of the 24 VDC dual power supplies in rack 21 of control group III, which caused the steam generator feedwater regulating valve (2-FRV-230)(SJ) to #23 steam generator to fail closed. After recognizing the failed feedwater regulating valve and taking actions to restore feed flow manually, it was determined that steam generator level was not recoverable and a manual reactor trip was directed. Just before the manual trip could be initiated, an automatic trip occurred.

The Reactor Coolant System (RCS)W1 cooldown experienced after the trip was greater than expected. As temperature reached 539 degrees F and continued lowering, the steam generator stop valves were tripped closed, as directed by the Response Not Obtained guidance for Step 1 of procedure ES-0.1, Reactor Trip Response. This action terminated the cooldown. RCS temperature remained above 538 degrees F, and RCS pressure reached a low of approximately 1920 psig. The RCS cooldown is primarily attributed to sustained high auxiliary feedwater [M]' flow following the reactor trip. The turbine driven auxiliary feedwater pump was secured 14 minutes after the reactor trip.

The level in #23 steam generator dropped to less than normal for a reactor trip.

The nominal level is 50% wide range, and level fell to approximately 33%. This is attributed to the feedwater regulating valve failing closed for this steam generator, as well as the steam load of the turbine driven auxiliary feedwater pump. The #22 steam generator was also affected to a lesser degree. The loss of the 24 VDC power supplies also resulted in the loss of the refueling water storage tank (RWST) low-level alarm function. There were no complications from loss of the low- level alarm function.

In accordance with 10 CFR 50.72(b)(2)(iv)(B), the reactor shutdown is reportable as a valid actuation of the reactor protection system. This LER Supplement is being submitted in accordance with 10 CFR 50.73(a)(2)(iv)Wand NUREG-1022, Section 5.1.5.

Cause of Event

The root cause of event was specific component age degradation of the Control Group III power supplies. Internal components failed on the power supply's input and output circuits. The primary failure mode for this component is attributed to age.

A contributing programmatic cause to this event was untimely corrective action.

Following a forced outage caused by power supply failure in May 2002, a decision was made to delay replacement of all installed 24 VDC power supplies until the next refueling. This was due, in part, to the organization being unaware of an EPRI recommendation to replace critical power supplies at an age of 7.5 years, and an interim measure had been written to inspect the power supplies on a weekly basis.

Additionally, the stock of replacement power supplies was depleted during the forced outage in May 2002.

Analysis of Event

The safety significance of this event was determined to be low. Although several automatic control functions were lost and multiple alarms were generated, review of operations procedures and actions determined that adequate alternate indications and controls were available to effectively mitigate the complications encountered during trip recovery. The RCS cooldown experienced after the trip was greater than desired, prompting closure of the main steam isolation valves. Although this was considered a loss of normal cooling pathway, it was as directed by emergency operating procedure, response not obtained column. Cooldown rate was controlled by auxiliary feedwater flow.

There was no impact on the health and safety of the public because of this event.

Corrective Actions

The Unit 1 and Unit 2 Control Group and WSI Cabinet 24 VDC Power Supplies were replaced with new or remanufactured power supplies. (CRA 03036056-04,-05) An optimized preventive maintenance program for critical power supplies was developed and implemented. (CRA 03036056-03,-20)

Previous Similar Events

Failure. The root cause for LER 50-316/2002-005-00 was age-related failure of components within critical power supplies. A contributing factor to the event was not having a provision for periodic monitoring of the 24 VDC power supplies.

A corrective action from LER 50-316/2002-005-00 was routine monitoring of the 24 VDC power supplies. However the prescribed monitoring task was not rigorously performed. This factor, in addition to the decision to delay replacement of the power supplies until the next refueling, contributed to LER 50-316/2003-002-01 event.