United States Nuclear Regulatory Commission - Protecting People and the Environment


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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