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.
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