Part 21 Report - 1998-243

ACCESSION #: 9806040046 LICENSEE EVENT REPORT (LER) FACILITY NAME: Clinton Power Station PAGE: 1 OF 9 DOCKET NUMBER: 05000461 TITLE: Division 2 Nuclear Systems Protection System Inverter Not in Accordance With the Plant's Design Basis Due to Various Deficiencies EVENT DATE: 01/27/98 LER #: 98-004-01 REPORT DATE: 05/28/98 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)(ii) OTHER LICENSEE CONTACT FOR THIS LER: NAME: M. D. Wagner, System Engineer TELEPHONE: (217) 935-8881, Extension 4071 COMPONENT FAILURE DESCRIPTION: CAUSE: X SYSTEM: EF COMPONENT: INVT MANUFACTURER: E209 REPORTABLE NPRDS: Y SUPPLEMENTAL REPORT EXPECTED: NO ABSTRACT: With the plant in COLD SHUTDOWN technicians identified various deficiencies in the installed Silicon Controlled Rectifiers (SCR) and power diodes of the Division 2 Nuclear Systems Protection System (NSPS) inverter. The compressive force applied to the heatsink clamp hardware for SCRB and power diodes had been applied,and verified incorrectly causing loose connections. Therefore, the inverter likely would not have been capable of performing its safety function during a seismic event. Further, improper calibration of the inverter lock-up detector circuit resulted in a loss of the simulated inverter loads during testing. The improper calibration caused the inverter to reverse transfer to a de-energized bypass power test source. These conditions are not in accordance with the plant's design basis. The cause for this event is ineffective and inadequate preventive maintenance. The Division 2 NSPS inverter deficiencies have been corrected and the inverter has been calibrated. Other inverters will be inspected for similar deficiencies and corrected. A preventive maintenance program is being developed for inverters. This event is also reportable under 10CFR21. END OF ABSTRACT TEXT PAGE 2 OF 9 DESCRIPTION OF EVENT On December 25, 1997, the plant was in Mode 4 (COLD SHUTDOWN) for the sixth refueling outage (RF-6), and reactor [RCT] coolant temperature was being maintained within a band of 95 to 115 degrees Fahrenheit (F) and pressure was zero pounds per square inch. At about 1245 hours, operators in the Main Control Room (MCR) received an unexpected alarm indicating that the Nuclear Systems Protection System (NSPS) [EF] inverter (INVT], 1C71-S001B, automatically transferred from its normal power source to its alternate source (reverse transfer). At 1246 hours, an area operator verified that the Division 2 NSPS inverter was on the alternate power source. Electrical Maintenance technicians were dispatched to the inverter, and Maintenance Work Request (MWR) D79429 was initiated to investigate the automatic transfer. Four uninterruptible NSPS buses [BU] supply Class 1E 120 Volts Alternating Current (AC) power to the four logic divisions of the Reactor Protection System (RPS) [JC]. An NSPS bus is normally fed via a Direct Current (DC) to AC inverter supply. The inverter is fed by a 125 Volts DC divisional battery charger [BYC] with a floating battery [BTRY). In the event of an inverter failure/power loss, the NSPS bus automatically transfers via a solid state transfer switch, to an alternate 120 Volts AC source derived from a 480 Volts AC to 120 Volts AC transformer [XFMR] supply. Also, 120 Volts AC from the transformer can be supplied to the Division A and B NSPS buses by manual transfer through an inverter maintenance bypass feed. On January 5, 1998, during troubleshooting of the Division 2 NSPS inverter in accordance with MWR D79429, technicians initiated Condition Report 1-98-01-038 to identify several discrepancies which could have adversely affected operation of the inverter. Two lead wires on the Fuse Sense and Interconnect Board (FSIB) were found loose. One of the wires is connected to a voltmeter [EI] indication of NSPS bus voltage, and the other wire is connected to the inverter lock-up circuit. A resistor on current transformer [XCT] CT-1 was found to have signs of excessive heat and to be burnt open. A wire on current transformer CT-1 was found disconnected. The impact of the current transformer and inverter lock-up issues is a potential malfunction of the inverter lock-up circuit which provides inverter protection in the event of inverter failure. The CT-1 circuit provides a means to reverse transfer the inverter when an internal fault within the inverter provides sufficient current through the primary side of the current transformer. However, inverter protection was still available through a fuse [FU] in the circuit. Several Silicon Controlled Rectifier [SCR] gate leads were not twisted properly. Failure to properly twist SCR gate leads could result in noise injection into the inverter, potentially causing the SCRs to misfire, fuses to blow, and shutdown of the inverter. Heatsink clamp hardware (General Electric Press Pac Clamp Assembly) spring bare on SCRs did not appear to be evenly compressed. Improperly compressed Press Pac spring bars on semiconductors may cause premature degradation of the semiconductors, thereby creating the potential for the inverter to not operate as designed. On January 7, 1998, during performance of MWR D79429, technicians identified several additional deficiencies in the installed SCR and power diodes of the Division 2 NSPS inverter. The deficiencies included: SCR gate and cathode leads were not tightly twisted, making the inverter more susceptible to electrical noise from outside sources; SCRs and power diodes had evidence of arcing on the conducting surfaces of the devices, indicating TEXT PAGE 3 OF 9 that these devices had degraded electrical continuity; and excessive Wakefield heat transfer compound was applied to the conductive surfaces of the SCRs and power diodes, creating the potential for hot spots and long term degradation of the SCRs, power diodes, and the inverter. Technicians also identified that SCRs had dimples on their conductive surfaces from the SCR heatsink clamp alignment pin impinging on the SCR surface due to the SCRs not being properly centered during previous installations of the SCRs, and additional mechanical defects were noted on SCRs and power diodes which did not appear to be directly related to the alignment pin dimples. These conditions affect the conductive surfaces of the semiconductors, thereby degrading electrical continuity. In addition, the compressive force applied to the heatsink clamp hardware (General Electric Press Pac Clamp Assembly) during previous installations of SCRs and power diodes had been applied and verified incorrectly using a torque value. The application of excessive force caused the Press Pac spring bars to flex beyond their yield point, resulting in inadequate compressive force applied to the installed SCRs and power diodes. One semiconductor assembly was installed in an incorrect order, resulting in the device being out Of position with regards to the compression centering plates. This condition degraded electrical continuity because the centering pins were not in the proper position. Electrical testing of the improperly installed SCRs and power diodes in the Division 2 NSPS inverter during performance of MWR D79429 indicated that immediate failure of the these components was not likely and immediate operability of the inverter was not affected by the deficiencies identified on January 7, 1998. The improper installation of devices described above can result in increased heating and arcing of the affected devices, and long term degradation of the semiconductors. Failure to properly twist SCR gate and cathode leads could result in noise injection into the inverter, potentially causing the SCRs to misfire, fuses to blow, and shutdown of the inverter. The improper compressive force on SCRB and power diodes may cause premature degradation of the SCRs, thereby creating the potential for the inverter to not operate as designed. Condition Report 1-98-01-066 was initiated to track an investigation and resolution for these issues. MWRs were initiated to inspect other potentially affected equipment including the Division 1 NSPS inverter, 1C71-S001A, the Division 3 NSPS inverter, 1C71-S001C1 the Division 4 NSPS inverter, 1C71-S001D, the A NSPS Solenoid(RPS) Uninterruptible Power Supply (UPS) Inverter, 1C71-S004A, and the B NSPS Solenoid (RPS) UPS Inverter, 1C71-S004B. On January 12, 1998, during performance of MWR D79429 on the Division 2 NSPS inverter, technicians noted that the bolts in the Press Pac spring bar (General Electric Press Pac Clamp Assembly), used to determine the amount of force being applied to the SCRs and power diodes, felt loose although the Press Pac spring bar was indicating the proper compressive force. Investigation of this issue identified that the Press Pac spring bar was bent, resulting in less than the required compressive force being applied to the SCRS and power diodes. CR 1-98-01-137 was initiated to track an investigation and resolution for these issues. This issue also potentially affects the Divisions 1, 3, and 4 NSPS inverters, and the A and B NSPS Solenoid (RPS) UPS inverters. On January 27, 1998, during evaluation of the inadequate compressive force issue as documented in CR 1-98-01-137, the system engineer concluded that torquing the General Electric Press Pac Clamp Assembly to 90 to 100 inch-pounds caused the Press Pac spring bar TEXT PAGE 4 OF 9 to bend beyond its yield point, resulting in inadequate compressive force on the SCRB and power diodes, and loose connections and arcing on the semiconductor surfaces. As a result of the bent Press Pac spring bar and loose connections, the inverter likely would not have been capable of performing its intended safety function during a seismic event. Therefore, the Division 2 NSPS inverter was not in accordance with the plant's design basis. On about January 28, 1998, during performance testing of the Division 2 NSPS inverter in accordance with MWR D79429 and in response to an inverter reverse transfer on January 18, 1998, technicians reported that a loss of the simulated bypass power test source under inverter full load conditions (while on a resistive load bank) resulted in a reverse transfer of the inverter to the bypass power test source (which was intentionally deenergized as part of the test procedure). The reverse transfer resulted in a lose of power to the simulated inverter load bank. This condition is not in accordance with the plant's design basis. However, this condition did not exist at lower test loads which are more representative of actual inverter load conditions. Investigation of the reverse transfer issue with the inverter supplier identified that the deficiency is the result of improper calibration of the lock-up detector adjustment. The improper calibration method provided a "rough set calibration" of the adjustable resistor (R134) in the calibration of the lock-up detector circuit based on a number of turns of the resistor adjustment rather than using a precise calibration based on a voltage measurement of the lock-up detector circuit. The improper calibration was performed in accordance with maintenance procedure CPS 8503.03, "NSPS Inverter Calibration," as specified in vendor manual K2801-0144. CR 1-98-01-455 was initiated on February 1, 1998, to track an investigation and resolution for these issues. This issue also potentially affects the Divisions 1, 3, and 4 NSPS inverters, and the A and B NSPS Solenoid (RPS) UPS inverters. On March 28, 1998, while investigating the cause of the Division 2 NSPS bus de-energizing on February 13, 1998, in accordance with MWR D82801 and CR 1-98-02-243, technicians discovered a cold solder connection on Metal Oxide Varistor (MOV) R3. (The February 13, 1998, event is reported in Licensee Event Report 98-003.) Illinois Power (IP) Electrical Maintenance technicians improperly soldered the connection during replacement of MOV R3 in March 1990. MOV R3 provides a protective feature to prevent inverter damage from voltage surge. Thus, the deficient solder connection could have resulted in damage to voltage sensitive components. However, during the investigation and repair of the Division 2 NSPS inverter no voltage sensitive components related to MOV R3 were found damaged. Several other condition reports were initiated during the investigation and repair of the Division 2 inverter for various discrepancies. These discrepancies did not adversely affect the operability of the inverter. No automatic or manually initiated safety system responses were necessary to place the plant in a safe and stable condition. This event was not directly affected by other inoperable equipment or components. CAUSE OF EVENT The root cause of the spurious inverter reverse transfer events on December 25, 1997, and January 18, 1998, has been determined to be ineffective and inadequate preventive TEXT PAGE 5 OF 9 maintenance. Numerous minor hardware deficiencies were discovered during troubleshooting. Contributing factors were incomplete and incorrect calibration procedures and inadequate training of maintenance personnel on specific techniques of various internal component maintenance. CORRECTIVE ACTION The loose lead wires on the Division 2 NSPS inverter FSIB were tightened to an acceptable condition. The resistor with signs of heat damage was replaced. SCR gate and cathode leads were properly twisted. The disconnected wire on current transformer CT-1 was reconnected and soldered. The guidance for applying compressive force to heatsink clamp hardware using a torque value was incorrect. The method for applying and verifying compressive force was revised in vendor manual K2801-0144 in accordance with supplier recommendations to use a gauge to provide visual indication of the flex of a Press Pac spring bar. Heatsink clamp hardware (General Electric Press Pac Clamp Assembly) was corrected by replacing the clamp hardware and applying appropriate compressive force to the clamp. SCRs and power diodes with evidence of arcing on the conducting surfaces were replaced, and the conducting surfaces were cleaned as needed. Excessive Wakefield heat transfer compound was removed from the conductive surfaces of SCRs and power diodes, and was properly reapplied. SCRs with dimples on their conducting surfaces due to non-centering of the semiconductor and other mechanical defects were replaced. New heatsink clamp hardware was properly installed on SCRB and power diodes using the revised installation method provided in vendor manual K2801-0144. Bent Press Pac spring bars were replaced with new clamp assemblies installed with proper compressive force per the revised instructions. The semiconductor installed in an incorrect order was replaced with a correctly installed semiconductor. The cold solder connection on MOV R3 was reworked to an acceptable condition. After various calibrations and supplier recommendations, an acceptable calibration of the lock-up detector was completed. Maintenance procedure CPS 8503.03 Was revised to incorporate the corrected calibration method which includes a precise calibration based on a voltage measurement of the lock-up detector circuit. Vendor manual K2801-0144 will be revised to include the corrected calibration method for the inverter lock-up detector and various vendor recommendations for the Divisions 1, 2, 3 and 4 NSPS inverters. Vendor manual K2801-0223 will be revised to include the vendor calibration recommendations for the A and B NSPS Solenoid (RPS) UPS inverters. Training will be provided to appropriate Maintenance personnel on the following procedures and techniques: proper twisting of SCR leads; proper application of heat transfer compound; proper installation of power semiconductors (SCRs and diodes); and proper installation of compression clamp assemblies used in battery chargers and inverters. TEXT PAGE 6 OF 9 A soldering techniques seminar was provided for Electrical Maintenance personnel in June 1997. IP will evaluate the need to provide advanced soldering training to Electrical Maintenance personnel. During investigation and repair of the Division 2 NSPS inverter, a factory certified service organization representative was extensively present, providing guidance to Maintenance technicians in identifying and correcting the inverter deficiencies. The Divisions 1, 3, and 4 NSPS inverters, and the A and B NSPS Solenoid (RPS) UPS inverters will be inspected in accordance with MWRs D77402, D77403, D77404, D77405, and D77406 for deficiencies similar to those found on the Division 2 NSPS inverter. The MWRs will contain sufficient detail in jobsteps to enable Maintenance technicians to identify and properly correct deficiencies similar to those found on the Division 2 NSPS inverter. A preventive maintenance program will be developed for the inverters based on the maintenance improvements recommended in the root cause investigation for this event. ANALYSIS OF EVENT This event is reportable under the provisions of 10CFR50.73(a)(2)(ii)(B) because the Division 2 inverter was not in accordance with the design basis of the plant. An assessment of the safety consequences and implications associated with this event identified that this event had nuclear safety significance. The divisional inverters are designed to provide the required capacity, capability, redundancy, and reliability to ensure the availability of necessary power to the RPS and Emergency Core Cooling System (ECCS) instrumentation and controls so that the fuel, Reactor Coolant System, and containment design limits are not exceeded. The RPS solenoid bus inverters are designed to provide the required capacity, capability, redundancy, and reliability to ensure the RPS and Main Steam Isolation Valve (MSIV) solenoids function and are not damaged. As a result of the bent Press Pac spring bar and loose connections, the inverter likely would not have been capable of performing its intended safety function during a seismic event. The reverse transfer during testing was a result of an inadequate calibration procedure and caused a loss of power to the simulated inverter load bank. Investigation of this issue is not complete, however, if a similar condition occurred while the Division 2 NSPS inverter was performing its safety function, power could be lost to safety-related loads supplied by the inverter. The disconnected wire and open resistor on current transformer CT-1 could cause a malfunction of the inverter lock-up circuit which provides inverter protection in the event of an internal fault and a means to reverse transfer the inverter to the alternate power source. If a malfunction occurred while the Division 2 NSPS inverter Was performing its safety function, power could be lost to safety-related loads supplied by the inverter. TEXT PAGE 7 OF 9 In addition, the Press Pac and inadequate calibration procedure issues are postulated to also impact the Divisions 1, 3, and 4 NSPS inverters and the A and B NSPS Solenoid (RPS) UPS inverters. ADDITIONAL INFORMATION The Divisions 1, 2, 3, and 4 NSPS inverters are model number INV-752-1-101, manufactured by Elgar Corporation. The A and B NSPS Solenoid (RPS) inverters are model number UPS-103-1-189, manufactured by Elgar Corporation. Clinton Power Station has not issued Licensee Event Reports for similar events; however, the inverters have been a long standing problem requiring significant maintenance attention. From 1993 to February 1998, 76 condition reports were initiated for inverter problems. Since January 1988, 102 maintenance work requests were initiated for inverter problems. The Main Control Room Operator Logs from January 1995, to December 1997, recorded nine instances of inverter reverse transfers without a definitive, known cause, four of the reverse transfers occurred on the Division 2 NSPS inverter. For further information regarding this event, contact M. D. Wagner, System Engineer, at (217) 935-8881, extension 4071. 10CFR, PART 21 REPORT 21-98-004/015 IP is providing the following information in accordance with 10CFR21.21(d)(4). Initial notification of this matter will be provided by facsimile of this letter to the NRC operations Center in accordance with 10CFR21.21(d)(3) within two days of the date the responsible officer signs this letter. (i) Walter G. MacFarland, IV, Senior Vice President and Chief Nuclear Officer of IP, Clinton Power Station, Highway 54, 6 Miles East, Clinton, Illinois, 61727, is informing the Nuclear Regulatory Commission of a condition reportable under the provisions of 10CFR, Part 21. (ii) The basic component involved in this condition is the safety-related Division 2 NSPS inverter, manufacturer model number INV-752-1-101. (iii) The Division 2 NSPS inverter was supplied to Clinton Power Station by Elgar Corporation. (iv) During troubleshooting of the Division 2 NSPS inverter, IP noted that the bolts in the Press Pac spring bar (General Electric Press Pac Clamp Assembly), used to determine the amount of force being applied to the SCRs and power diodes, felt loose although the Press Pac spring bar was indicating the proper compressive force. Investigation of this issue identified that the Press Pac spring bar was bent, resulting in less than the required compressive force being applied to the SCRs and power diodes. TEXT PAGE 8 OF 9 During evaluation of the inadequate compressive force issue, IP concluded that torquing the General Electric Press Pac Clamp Assembly to go to 100 inch pounds caused the Press Pac spring bar to flex beyond its yield point, resulting in inadequate compressive force on the SCRs and power diodes, and loose connections and arcing on the semiconductor surfaces. As a result of the bent Press Pac spring bar and loose connections, the inverter likely would not have been capable of performing its intended safety function during a seismic event. The safety function of the Division 2 NSPS inverter is to supply uninterruptible NSPS buses with Class 1E 120 Volts AC power for the Division 2 logic of the Reactor Protection System. Therefore, the Division 2 NSPS inverter was not in accordance with the plant's design basis. Illinois Power has determined that the inverter supplier provided incorrect guidance for installing the Press Pac spring bar. A record of coordination between IP and the inverter supplier, dated August 4, 1988, and contained in vendor manual K2801-0144, provided the incorrect method of applying compressive force using a torque value. Recent guidance from the inverter supplier recommends not using torque as the means for verifying the compressive force applied to the semiconductor devices installed in the heatsinks. The current supplier-recommended method of applying and verifying compressive force applied to the semiconductor surfaces uses visual indication of a Press PaC spring bar. During performance testing of the Division 2 NSPS inverter, technicians reported that a loss of the simulated bypass power test source under inverter full load conditions (while on a resistive load bank) resulted in a reverse transfer of the inverter to the missing bypass power test source. The reverse transfer resulted in a loss of power to the simulated inverter load bank. This condition is not in accordance with the plant's design basis. However, this condition did not exist at lower test loads which are more representative of actual inverter load conditions. Investigation of the reverse transfer issue with the inverter supplier identified that the deficiency is the result of improper calibration of the lock-up detector adjustment. The improper calibration was performed in accordance with maintenance procedure CPS 8503.03, "NSPS Inverter Calibration," as specified in vendor manual K2801-0144. Illinois Power has determined that the inverter supplier did not provide proper guidance for calibrating the inverter lock-up detector adjustment. The improper calibration method provided a "rough set calibration" of the adjustable resistor (R134) in the calibration of the lock-up detector circuit based on a number of turns of the resistor adjustment rather than using a precise calibration based on a voltage measurement of the lock-up detector circuit. (v) The incorrect application and verification methods for applying compressive force to SCRs and power diodes was identified on January 7, 1997, and determined to be potentially reportable under 10CFR21. On January 12, 1998, a bent Press Pac spring bar was identified on the Division 2 NSPS inverter. The incorrect calibration of the Division 2 NSPS inverter was identified and determined to be potentially reportable under 10CFR21 on February 1, 1998. TEXT PAGE 9 OF 9 (vi) CPS has six safety-related inverters that may be affected by these deficiencies, including the Division 1 NSPS inverter, 1C71-S001A, Division 2 NSPS inverter, 1C71-S001B, the Division 3 NSPS inverter, 1C71-S001C, the Division 4 NSPS inverter, 1C71-S001D, the A NSPS Solenoid (RPS) UPS Inverter, 1C71-S004A, and the B NSPS Solenoid (RPS) UPS Inverter, 1C71-S004B. IP is not aware of other facilities that could be affected by this deficiency. (vii) Corrective actions that IP is taking are discussed in the LER CORRECTIVE ACTION section of this report. (viii) IP has no advice for other purchasers or licensees regarding this issue. *** END OF DOCUMENT ***

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