05000410/LER-2003-001

LER-2003-001,

Event date:	07-24-2003
Report date:	09-22-2003
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
4102003001R00 - NRC Website

I. Description of Event

On July 24, 2003, at approximately 0550 hours0.00637 days <br />0.153 hours <br />9.093915e-4 weeks <br />2.09275e-4 months <br />, Nine Mile Point Unit 2 (NMP2) experienced a failure of instrument power supply C33-K611, which caused a partial loss of power to feedwater control, main steam flow Instrumentation, and reactor recirculation flow control. Prior to the failure of the power supply, two feedwater pumps and two recirculation pumps were operating. The loss of power to recirculation relays resulted in both recirculation pumps shifting from fast speed to stow speed. The reduction In flow caused reactor power to drop from 100 percent to approximately 45 percent. The reduction in core flow caused the reactor to enter the exit region of the power to flow map in which core Instability is possible.

The power supply failure also caused the loss of 3 out of 4 steam flow input signals to feedwater level control, which resulted in feedwater level control valve 2FWS-1V10A closing. Additionally, the power supply failure caused feedwater levet control valve 2FWS-LV10B to fail 'as is' at approximately 52 percent open. With 2FWS-LV10B at 52 percent open and reactor power reduced, reactor vessel level began to increase. To keep reactor water level from increasing further, operators secured reactor feedwater pump 2FWS-P1B. Upon securing feedwater pump 2FWS-P1B a reactor recirculation flow runback occurred, reducing power to approximately 35 percent. A false low reactor vessel water level, resulting from the failed power supply, coincident with only one running feedwater pump (after securing the B reactor feedwater pump) satisfied the recirculation flow runback initiation logic. Approximately three minutes after the recirculation flow runback, Oscillation Power Range Monitor (OPRM) channels 1 and 4 detected power oscillations and Initiated an automatic reactor scram (a period based algorithm scram) at approximately 0557 hours0.00645 days <br />0.155 hours <br />9.209656e-4 weeks <br />2.119385e-4 months <br />. At the time of the scram, reactor power was approximately 45 percent and core flow was approximately 28 percent.

All control rods fully inserted. After the scram, the turbine bypass valves were used to control reactor pressure. Post scram, operators took manual-electric control of 2FWS-LV10A (from the control room) and used feedwater pump 2FWS-PIA to control reactor water level.

Troubleshooting determined that power supply C33-K611, a Lambda model LRS-54-24, had failed. A temporary power supply was installed in parallel with the failed power supply C33-K611 and power to control 2FWS-LV1013 was re-established. Power supply C33-K611 is located in panel 2CEC-PNL612, along with power supplies C33-K612 and C33-K613. All three power supplies were Lambda model LRS-54-24 and all three were replaced with auctioneering type Lambda model 12S-250-3. The auctioneering type power supply reduces the single point vulnerability.

II. Cause of Event

The cause of the power and flow perturbations leading to the reactor scram was failed power supply C33-K611. The power supply failed because of the age related failure of internal components and no specific preventive maintenance program activity to refurbish or replace the power supply. Power supply C33-K611 was manufactured circa 1985.

There was no specific preventive maintenance program activity to refurbish or replace the power supply. Contributing causes were the lack of power supply redundancy for important operational loads and inadequate distribution of loads on the power supply, i.e. the powering of 3 out of 4 main steam line flow instruments from one power supply. The significance of the load distribution is reduced by using the auctioneering type power supply.

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III. Analysis of Event

This event is reportable in accordance with 10 CFR 60.73(a)(2)(iv)(A) as a critical reactor scram.

Given the power supply failure, the feedwater system and recirculation system responded as designed.

All control rods fully inserted.

Reactor Core Isolation Cooling was available post scram but was not needed.

No Emergency Core Cooling Systems actuated or should have actuated.

A General Electric evaluation confirmed that Minimum Critical Power Ratio (MCPR) safety limit protection was provided by the OPRM reactor trip.

A probabilistic risk assessment of the event concluded that the estimated Conditional Core Damage Probability for the event was less than 1E-6 and therefore, the event was not considered risk significant.

Based on the above, the event did not pose a threat to the health and safety of the public.

IV. Corrective Actions

1. Power supply C33-K611 and two additional power supplies (C33-K612 and C33-K613) were each replaced with an auctioneer style power supply, Lambda Model L2S-250-3.

2. Activities were initiated to replace non-safety related power supplies whose failure could result in a reactor scram or impact mission critical equipment, or cause a significant plant transient.

3. An engineering evaluation of load redistribution on power supply C33-K611 has been initiated.

4. Power supply C33-K611 was added to the preventive maintenance program.

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V. Additional Information

1. Failed Components:

Lambda power supply, Model Number: LRS-54-24, Manufacturer: Lambda Electronics 2. Previous similar events:

Licensee Event Report 89-024. "Manual Scram Due to Equipment Failure and Entry into Restricted Zone,' documents the failure of a Lambda LRS-54-24 power supply on September 8, 1989. The cause of the reactor scram was identified as failure of the power supply. The cause of the power supply failure was not identified.

3. Identification of components referred to in this Licensee Event Report:

Components IEEE 805 System ID � JEEE 803A Function Feedwater System � SJ � NIA Mainsteam System � SB � WA Turbine Bypass � TG � N/A Oscillation Power Range Monitors � IG � N/A Reactor Protection System � JC � N/A Recirculation System � AD � WA Reactor Core � AC � N/A Reactor Core Isolation Cooling System � BN � N/A Instrument Power System � EE � NIA Control rod � AC � ROD Pump � AD � P Valve � SB, TG , AD � LCV, PCV, FCV Power Supply � SJ, SB, AD, EE � RJX Reactor Vessel � AD � VSL Panel � EE � PL Flow Indicator � SB � Fl Relay � AD � RLY