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
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
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:
SUPPLEMENTAL REPORT EXPECTED: NO
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) 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.
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
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
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,
TEXT PAGE 6 OF 7
10CFR PART 21, REPORT NO. 21-98-022/935
On February 13, 1998, a Nuclear Systems Power System 12 volt direct
current power supply failed causing the Division II NSPS inverter to
reverse transfer to the bypass transformer which was out of service for
maintenance. This resulted in a loss of shutdown cooling as described in
the Description of Event section of this report. The cause of the power
supply failure was an inadequately designed output transistor pair
combined with degraded capacitors.
(i) Walter G. MacFarland, IV, Senior Vice President and Chief
Nuclear Officer of Illinois Power Clinton Power Station, Post
Office Box 678, Highway 54, six Miles East, Clinton, Illinois,
61727, is informing the Nuclear Regulatory commission (NRC) of
a condition reportable under the provisions of 10CFR, Part 21.
(ii) The component involved in this report is a 12 VDC power supply,
model number RMX 12-D-20993, supplied to Clinton Power Station.
This condition was also identified to effect model numbers RMX
24-D-20804 and RMT001-AA-20995.
(iii) The power supply wan manufactured by KEPCO, Incorporated, and
was purchased by General Electric (GE) as a non-safety related
(iv) The supplier provided marginally rated output transistors in
the power supply. These transistors, aggravated by the
degradation of their electrolytic filter capacitors, failed,
producing a phase to phase short on the NSPS bus. The large
current draw caused the inverter to reverse transfer as
designed to clear this fault. Since the alternate source was
unavailable, the condition resulted in a loss of power to NSPS
bus loads which Included the primary valve for the shutdown
cooling mode at the time of the event. The loss of power to
the valve caused the valve to close and shut off shutdown
cooling. In addition, GE parts list drawing 169C8805
identifies the power supplies as non-safety-related and CPS
purchased the power supplies as non-safety related. However,
based on the application of the power supplies and GE
specification 22A7822 Revision 1, the power supplies should be
classified as safety-related. Subsequently, CPS has revised
the master equipment list to change the classification of the
(v) IP Identified on March 12, 1998, that the power supply failure
during startup of the division 2 NSPS bypass transformer on
February 13, 1998, may have caused the inverter to reverse
transfer. Subsequently, on March 24, 1998, the results of the
failure analysis performed by Integrated Resources Inc.
confirmed that the power supply failure was the cause of the
reverse transfer. The output transistors were under rated and
are therefore reportable under 10CFR21.
(vi) RMX type KEPCO, Inc., power supplies are in use at CPS for all
four divisions of NSPS (16 total), Neutron Monitoring System
(24 total), and the Display Control System (2 total).
Currently, CPS is evaluating the impact on all KEPCO supplied
RMX type power supplies.
TEXT PAGE 7 OF 7
(vii) IP will replace the output transistors and degraded capacitors,
due to premature aging, in KEPCO RMX type power supplies.
(viii) IP recommends evaluation of RMX type power supplies
manufactured by KEPCO, Inc., for conditions similar to these
described in this report.
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