United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-18, Supplement 1: Failures of Undervoltage Output Circuit Boards in the Westinghouse-designed Solid State Protection System

                                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 

9109040234 
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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. 
.

                                                  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.  
.

                                                  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
 

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