Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 10, 1991
NRC INFORMATION NOTICE 85-18, SUPPLEMENT 1: FAILURES OF UNDERVOLTAGE OUTPUT
CIRCUIT BOARDS IN THE
WESTINGHOUSE-DESIGNED SOLID
STATE PROTECTION SYSTEM
Addressees
All holders of operating licenses or construction permits for Westinghouse
(W)-designed nuclear power reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice supplement to alert addressees to continuing problems associated with
the undervoltage (UV) output circuit boards (driver cards) in the solid
state protection system (SSPS) designed by the Westinghouse Electric
Corporation (Westinghouse). The failure of one driver card results in the
loss of automatic reactor trip redundancy and the failure of both output
driver cards would result in the loss of the automatic trip function of the
reactor protection system. It is expected that recipients will review the
information for applicability to their facilities and consider actions, as
appropriate, to avoid similar problems. However, suggestions contained in
this information notice are not NRC requirements; therefore, no specific
action or written response is required.
Description of Circumstances
On June 3, 1991, the Shearon Harris Nuclear Power Plant, Unit 1, (Harris)
experienced an automatic reactor trip from 100 percent power on a spurious
low reactor coolant system loop flow signal. The signal was generated as a
result of a surveillance test being performed on one of three loop flow
transmitters. The licensee attributed the spurious signal to both
procedural inadequacies and personnel error.
A control room operator verified that all control rods had fully inserted
following the trip signal and that reactor power was properly decreasing.
However, about 22 seconds after the automatic trip signal was generated,
operators discovered that the "A" reactor trip breaker (RTB) had not opened.
The RTB was manually opened using the reactor trip switch on the main
control board.
Discussion
After the trip, the licensee's review of this event found that the
undervoltage (UV) output driver card for the "A" train of the SSPS had
failed. During normal operation, this card maintains the required 48V
potential to the coil of
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IN 85-18, Supplement 1
September 10, 1991
Page 2 of 4
the undervoltage trip attachment (UVTA) and to the actuating relay
associated with the shunt trip attachment (STA). When a reactor trip signal
is generated, the output transistor in the driver card circuitry interrupts
the current flow thereby deenergizing the UVTA coil and the STA actuating
relay. These actions constitute a diverse means of opening the RTB. The
failure mode of the card prevented the power supply to the UVTA and the STA
relay from being interrupted on a valid automatic trip signal.
The staff discussed this failure mode in NRC Information Notice (IN) 85-18,
March 7, 1985. IN 85-18 described three operational events at the North
Anna Power Station and one event at the Sequoyah Nuclear Plant. These
events resulted from failures of undervoltage output driver cards. The
information notice delineated technical advice that Westinghouse had
provided to the North Anna licensee to reduce the potential for a repeat of
the same failure. Westinghouse later provided this advice to all
Westinghouse-designed plants in Westinghouse Technical Bulletin
NSID-TB-85-16. In the bulletin, Westinghouse stated that the failures of
the cards resulted from errors during maintenance and testing of the RTB
UVTA. These errors resulted in the UVTA coil being inadvertently short
circuited. This short circuit caused the output transistor of the UV output
driver card to fail to a shorted condition. In the technical bulletin,
Westinghouse recommended corrective procedural actions and described a
modified UV output driver card that could be used to replace the original
circuit card.
The recommended procedural actions included removing the card from the
system, whenever possible, when maintenance is performed on the switchgear.
If the card can not be removed, Westinghouse recommended that plant
personnel use extreme caution to prevent inadvertent short circuits.
Westinghouse recommended that, in either case, the semi-automatic test
feature of the SSPS be performed on each train after any maintenance is
conducted on the switchgear.
In the bulletin, Westinghouse also informed licensees of the availability of
a modified UV output driver card. The new card included a fusible link in
series with the output of the card. The fusible link is designed to open
under short circuit conditions to interrupt power to the UVTA coil and
deenergize the relay associated with the shunt trip attachment (STA). This
action opens the reactor trip breaker if it is closed and prevents closure
if the breaker is open. Westinghouse stated that the modified card design
significantly reduces the possibility that a failed card would remain
undetected.
The staff also discussed the failure mode of the card in NUREG-1341,
"Regulatory Analysis for the Resolution of Generic Issue 115, 'Enhancement
of the Reliability of Westinghouse Solid State Protection Systems,'" January
1989. The resolution produced no new regulatory requirements. However, the
staff set forth certain insights that it had gained in evaluating the issue
and that could assist in improving the SSPS reliability. The staff
concluded that implementation of the Westinghouse procedural recommendations
and hardware modification presented in Bulletin NSID-TB-85-16 would improve
the overall reliability of the SSPS.
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IN 85-18, Supplement 1
September 10, 1991
Page 3 of 4
The Harris licensee responded to the Westinghouse bulletin by adopting the
recommended corrective action to perform testing following maintenance using
the SSPS semi-automatic test feature. The licensee concluded that the
administrative controls were sufficient and, therefore, elected not to
install the modified circuit card.
After the June 3 event, the NRC performed an inspection at the Harris plant
and determined that the licensee had last functionally tested the failed UV
driver card on May 16, 1991, when the "A" train SSPS logic was functionally
tested. Subsequent to that test, during plant startup, the "A" RTB failed
to close on demand from the control room. The licensee performed
troubleshooting and found that both the "A" reactor trip breaker closing
coil and the control room reactor trip switch were defective. The licensee
replaced the breaker closing coil and the trip switch on May 18, 1991.
During interviews with the electricians who performed the troubleshooting on
the "A" reactor trip breaker, the inspectors learned that the licensee had
installed various electrical jumpers and test equipment to test the breaker
while it was racked out of the breaker cubicle. To facilitate testing, the
licensee installed one set of jumpers to provide 48 V to the breaker
undervoltage coil so that the breaker could be closed. This jumper was
installed in the breaker cubicle on the "A" contacts that were supplied from
the SSPS undervoltage output driver card. The electricians stated that
visibility and physical clearances for installing the jumpers were limited.
Following the June 3 event, the licensee's investigation concluded that
during the replacement of the reactor trip switch and breaker closing coil,
electrical leads were accidentally grounded or shorted through the use of
electrical jumpers. This caused the transistor on the undervoltage output
driver card to fail, which caused the card to maintain 48 V output to the
breaker undervoltage coil and the shunt trip relay in the presence of the
trip signal.
The NRC inspectors found that the post maintenance testing procedure was
inadequate to identify the failed UV driver card. The only testing required
was to cycle the reactor trip breakers from the main control board twice.
The UV output driver card was not functionally tested using the
semi-automatic tester as recommended in the Westinghouse bulletin and NRC IN
85-18. Furthermore, the inspectors determined that the licensee's testing
procedure did not verify all the contact functions of the reactor trip
switch. When informed of this finding, licensee personnel reviewed the
testing performed and likewise concluded that the testing would not check
both the shunt trip and the undervoltage trip functions of the reactor trip
switch. The licensee declared the switch inoperable and began to shut down
the plant on June 8, 1991, to test the switch.
The failure of the UV driver card appears to be associated with maintenance
activities that were not related to the SSPS. This is the same concern that
was identified in the original NRC information notice and the Westinghouse
Technical Bulletin. Maintenance conducted on components that are not
related to the SSPS can cause undetectable failures of the UV output driver
card. The SSPS semi-automatic tester is designed to uncover these potential
failures.
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IN 85-18, Supplement 1
September 10, 1991
Page 4 of 4
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: N. Fields, NRR
(301) 492-1173
N. Merriweather, RII
(404) 331-5577
H. Garg, NRR
(301) 492-0929
Attachment: List of Recently Issued NRC Information Notices
