Part 21 Report - 1998-373

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 OPERATING MODE: 4 POWER LEVEL: 000 THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR SECTION: 50.73(a)(2)(iv) OTHER LICENSEE CONTACT FOR THIS LER: NAME: Shawn A. Wadhams, Nuclear Station TELEPHONE: (217) 935-8881, Engineering Department Extension 4389 COMPONENT FAILURE DESCRIPTION: CAUSE: SYSTEM: COMPONENT: MANUFACTURER: REPORTABLE NPRDS: SUPPLEMENTAL REPORT EXPECTED: NO ABSTRACT: 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. END OF ABSTRACT TEXT PAGE 2 OF 7 DESCRIPTION OF EVENT 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 CAUSE OF EVENT 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. CORRECTIVE ACTION 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. ANALYSIS OF EVENT 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 ADDITIONAL INFORMATION 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 component. (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. *** END OF DOCUMENT ***

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