United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #:  9806050378

                       LICENSEE EVENT REPORT (LER)

FACILITY NAME: Clinton Power Station                      PAGE: 1 OF 7

DOCKET NUMBER:  05000461

TITLE:  Loss of Nuclear Systems Protection System Bus Due to a

        Power Supply Failure Results in Inadvertent Isolation of

        Shutdown Cooling

EVENT DATE:  02/13/98   LER #:  98-003-01   REPORT DATE:  06/02/98

OTHER FACILITIES INVOLVED:                          DOCKET NO:  05000







NAME:  Shawn A. Wadhams, Nuclear Station    TELEPHONE:  (217) 935-8881,

       Engineering Department                           Extension 4389






On February 13, 1998, with the plant in COLD SHUTDOWN, an inadvertent

isolation of shutdown cooling occurred when a common line auction valve

for the Residual Heat Removal system went shut.  The valve closed due to

a de-energization of the Division II Nuclear Systems Protection System

(NSPS) bus.  The NSPS bus became de-energized because the inverter

reverse transferred to the bypass transformer which was out of service

for maintenance.  The inverter reverse transferred due to the failure of

a 12 volt power supply that was being supplied by the Division II NSPS

bus.  The root cause of this event was the inadequate guidance of a work

management procedure to perform a high risk evolution without a

contingency plan.  Clinton Power Station (CPS) has revised & work

management procedure to strengthen the requirements for a contingency

plan and will revise other work management procedure to provide clearer

guidance for risk assessments for high risk evolutions.  CPS will

upgrade/refurbish the associated power supplies in the NSPS system.  CPS

will also monitor similar power supplies in the Neutron Monitoring System

and the Display control System and refurbish these power supplies.  This

issue is also reportable under 10CFR21.


TEXT                                                          PAGE 2 OF 7


On February 13, 1998, the plant was in Made 4 (COLD SHUTDOWN) for the

sixth refueling Outage (RF-6), and reactor [RCT] coolant temperature was

being maintained between 95 to 115 degrees Fahrenheit (F) and pressure

was zero pounds per square inch.  Residual Heat Removal (RHR)[30] train

"A" was in service providing shutdown cooling.  Maintenance was being

performed on the Bypass transformer [XFMR] associated with Division II

Nuclear Systems Protection System (NSPS)[EF] bus (BU).

At approximately 0340 hours, operators received multiple annunciators

[ALK] that were indicative of a loss of the Division II NSPS bus.  The

Reactor Water Cleanup (RT)[CE] system isolated, and operators observed

indications that the RHR auction valve (V), 1E12F009, was going in the

closed direction.  Operators immediately shut down RHR "A" train to

preclude damage to the RHR "A" pump [P) in accordance with appropriate

procedures.  Due to this automatic isolation, operations entered the

Clinton Power Station (CPS) off-normal procedure CPS 4001.02, "Automatic

Isolation." Reactor pressure vessel water (RPV) level was 104 inches, 4

inches out of band, as indicated by the shutdown range instrumentation.

The operators initiated action to shut down the control rod drive (RD)

[AA] hydraulics to secure a source of inserting water to the vessel so

that the increase in level would be minimized.  Operators entered CPS

off-normal procedure CPS 4006.01, "Loss of Shutdown Cooling," performed

steps to evacuate the drywell, turned on the Automatic Depressurization

System (ADS) safety valve temperature recorder [TR], and commenced

surveillance procedure CPS 9000-06D001, "RPV Heatup/Cooldown Log," for

tracking reactor coolant temperature.

At Approximately 0345 hours, the main control room sent the area operator

to confirm the status of the Division II NSPS inverter [INVT],

1C71-S001B.  The area operator found that the inverter had reverse

transferred from its normal power source to the alternate power source,

the bypass transformer, which was out of service for scheduled

maintenance.  The area operator also confirmed that the inverter was

lined up for normal operation being fed 125 volt Direct Current (VDC)

from the 1B Motor Control Center [MCC].  The NSPS bus gets its normal

power feed from a 125 volt direct current (VDC) divisional battery

through an inverter that supplies 120 volts alternating current (VAC) to

Class 1E loads.  There is an alternate feed through a bypass transformer

which steps down a 480 VAC source to the required 120 VAC.

At approximately 0420 hours, the on-duty Shift Supervisor (SS) declared

an Alert to obtain additional assistance in restoring shutdown cooling

capabilities.  The shift supervisor made this declaration based on his

judgment and the guidelines of section 13.6 of procedure EC-02,

"Emergency Classification," which states that other plant conditions

could exist which warrant activation of the Technical Support Center.

At approximately 0415 hours, Operations began steps to restore the

Division II NSPS bus.  Electrical maintenance was contacted to facilitate

in the restoration of the bypass transformer.

TEXT                                                          PAGE 3 OF 7

At 0445 hours, Operators determined that Technical Specification (TS)

Limiting Condition for Operation (LCO) 3.4.10, Action B.2, "verify

reactor coolant circulation by alternate means," could not be completed

within the one hour completion time.  Operations personnel in the Main

Control Room (MCR) were briefed on contingencies in the event plant

conditions continued to degrade.

At 0537 hours, Operations completed restoration of the Division II NSPS

bus.  Operators energized the Division II NSPS bus by restoring the

bypass transformer from maintenance.

At 0545 hours, ERO personnel manned the Technical Support Center (TSC)

and Operations Support Center (OSC).  This evolution helped resolve the

burden of plant recovery from the Shift Supervisor and provided

additional manpower for restoring plant systems.  The Station

Emergency Director assumed command authority from the SS to coordinate

efforts in restoring from the event.

At 0625 hours, operators started both "A" and "B" Reactor Water Cleanup

(RT) pumps to establish reactor coolant circulation.  Operators commenced

rejecting RPV water to the Radwaste system to maintain level within the

normal control band of 90 to 100 inches.  Operators verified that the RT

beat exchanger [HX] lineup was correct and were able to comply with TS

LCO 3.4.10, Action B.2.

At 0735 hours, operators exited from the Off Normal procedure 4006.01,

"Loss of Shutdown Cooling," after verifying that RPV temperature control

had been established.  There was a one degree Fahrenheit rise in reactor

coolant temperature during this event.

At approximately 0945 hours, RHR "A" was started in the shutdown cooling

made.  Operators verified that the 1E12F009 valve was open and opened its

associated breaker to prevent any further inadvertent isolations of

shutdown cooling from a loss of the NSPS bus.

At 1000 hours, the Alert was terminated.

There have been recent problems with the operation of the Division II

NSPS Inverter.  On December 23, 1997, the inverter experienced a reverse

transfer from it's normal feed to the alternate (Bypass transformer)

source.  A Maintenance Work Request (MWR) D79429 was initiated to

troubleshoot and repair the inverter. On January 18, 1998, the inverter

reverse transferred again during the post maintenance testing of work

performed under MWR D79429.  Details of this work were provided in

License Event Report (LER) 98-004-00 (IP letter U-602945, dated February

26, 1998).  The root causes of the reverse transfer on December 25, 1997,

and January 18, 1998, had not yet been determined when the event on

February 13, 1996 occurred.  Electrical maintenance was performing

extensive troubleshooting of the inverter and had just completed special

testing with a follow-up post maintenance test to complete their repairs

of the inverter.  The inverter was placed back in service an February 12,

1998, but was not yet declared operable.  Maintenance on the bypass

transformer began on February 12, 1998.

This event was not affected by other inoperable equipment or components.

TEXT                                                          PAGE 4 OF 7


The initiating action for this event was the failure of a 12 vdc power

supply (IH13-P662-H-A195, model number RMX 12-D-20993) that was a load to

the Division II NSPS bus due to marginal output transistors.  The NSPS

inverter sensed the failure of the power supply and reverse transferred

to the bypass transformer.  Since the bypass transformer was out of

service for maintenance, the Division II NSPS bus lost power and

subsequently caused the RHR suction valve, 1E12F009, to shut which

secured the mode of shutdown cooling.  The root cause of this event is

the lack of a contingency plan for placing the bypass transformer out of

service for maintenance.  The CPS work management procedure 1151.04,

"Planned Outage Scheduling," did not provide adequate guidance to

schedule the work on the bypass transformer.


CPS has revised a work management procedure 1151-09, Methodology for

Outage Safety Reviews and Maintenance of Acceptable Shutdown Risk," to

strengthen the requirements for a contingency plan for high risk

evolutions that could impact the availability of a safety function

CPS will revise work management procedures, like 1151.04, to incorporate

expectations for risk reviewed and work package preparation in accordance

with 1151.09 prior to equipment being removed from service.

CPS will refurbish/upgrade the associated NSPS power supplies with output

transistors rated for a voltage greater than 428 V and the electrolytic

capacitors will be replaced.  CPS will also monitor and refurbish, the

other RMX type power supplies located In the Neutron Monitoring System

and Display Control System.


This event is reportable under the provisions of 10CFR50.73 (2) (iv) due

to the inadvertent lose of the Division II NSPS Bus which resulted in an

automatic isolation of shutdown cooling.

The safety consequences and implications associated with this event have

been determined as nuclear safety significant.  Inadvertent loss of

shutdown cooling requires immediate operator action, to restore heat

removal capability.  Any failure of an NSPS load could have caused the

inverter to reverse transfer and subsequently cause a similar condition

if the bypass transformer was unavailable.

Investigation of this event resulted in the discovery of deficiencies in

the areas of emergency preparedness, work management and equipment

maintenance.  Condition reports were generated to resolve these

deficiencies and corrective actions are underway.

TEXT                                                          PAGE 5 OF 7


The Division II NSPS Power Supply is model number RMX-12-D-20993,

manufactured by KEPCO, Inc.

Illinois Power (IP) reported a 10CFR Part 21 issue with the NSPS

inverters in Licensee Event Report (LER) 98-004 (U-602945, dated February

26, 1998).

For further information regarding this event, contact Shawn A.  Wadhams,

System Engineer, at (217) 935-8881, extension 4389, or Patrick J.  O'

Reilly, Senior Analyst - Experience Assessment at (217) 935-8861,

extension 3469.

TEXT                                                          PAGE 6 OF 7

10CFR PART 21, REPORT NO.  21-98-022/935

On February 13, 1998, a Nuclear Systems Power System 12 volt direct

current power supply failed causing the Division II NSPS inverter to

reverse transfer to the bypass transformer which was out of service for

maintenance.  This resulted in a loss of shutdown cooling as described in

the Description of Event section of this report.  The cause of the power

supply failure was an inadequately designed output transistor pair

combined with degraded capacitors.

(i)       Walter G.  MacFarland, IV, Senior Vice President and Chief

          Nuclear Officer of Illinois Power Clinton Power Station, Post

          Office Box 678, Highway 54, six Miles East, Clinton, Illinois,

          61727, is informing the Nuclear Regulatory commission (NRC) of

          a condition reportable under the provisions of 10CFR, Part 21.

(ii)      The component involved in this report is a 12 VDC power supply,

          model number RMX 12-D-20993, supplied to Clinton Power Station.

          This condition was also identified to effect model numbers RMX

          24-D-20804 and RMT001-AA-20995.

(iii)     The power supply wan manufactured by KEPCO, Incorporated, and

          was purchased by General Electric (GE) as a non-safety related


(iv)      The supplier provided marginally rated output transistors in

          the power supply.  These transistors, aggravated by the

          degradation of their electrolytic filter capacitors, failed,

          producing a phase to phase short on the NSPS bus. The large

          current draw caused the inverter to reverse transfer as

          designed to clear this fault.  Since the alternate source was

          unavailable, the condition resulted in a loss of power to NSPS

          bus loads which Included the primary valve for the shutdown

          cooling mode at the time of the event.  The loss of power to

          the valve caused the valve to close and shut off shutdown

          cooling.  In addition, GE parts list drawing 169C8805

          identifies the power supplies as non-safety-related and CPS

          purchased the power supplies as non-safety related. However,

          based on the application of the power supplies and GE

          specification 22A7822 Revision 1, the power supplies should be

          classified as safety-related.  Subsequently, CPS has revised

          the master equipment list to change the classification of the

          power supplies.

(v)       IP Identified on March 12, 1998, that the power supply failure

          during startup of the division 2 NSPS bypass transformer on

          February 13, 1998, may have caused the inverter to reverse

          transfer.  Subsequently, on March 24, 1998, the results of the

          failure analysis performed by Integrated Resources Inc.

          confirmed that the power supply failure was the cause of the

          reverse transfer.  The output transistors were under rated and

          are therefore reportable under 10CFR21.

(vi)      RMX type KEPCO, Inc., power supplies are in use at CPS for all

          four divisions of NSPS (16 total), Neutron Monitoring System

          (24 total), and the Display Control System (2 total).

          Currently, CPS is evaluating the impact on all KEPCO supplied

          RMX type power supplies.

TEXT                                                          PAGE 7 OF 7

(vii)     IP will replace the output transistors and degraded capacitors,

          due to premature aging, in KEPCO RMX type power supplies.

(viii)    IP recommends evaluation of RMX type power supplies

          manufactured by KEPCO, Inc., for conditions similar to these

          described in this report.


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