United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Circular No. 81-12, Inadequate Periodic Test Procedure of PWR Protection System

                                                          SSINS No.: 6830  
                                                          Accession No.:   
                                                          IEC 81-12        

                               UNITED STATES 
                           WASHINGTON, D.C. 20555 
                               July 22, 1981 

                         PROTECTION SYSTEM 

Description of Circumstances: 

On November 30, 1980, ground isolation procedures were being carried out to 
locate an electrical ground in the 125V dc bus No. 1 at the St. Lucie 
Nuclear Power Plant. Part of the procedure requires deenergizing the dc 
control power to the reactor trip circuit breakers (TCBs). These breakers 
are designed to trip (fail-safe mode) on undervoltage upon loss of dc 
control power. Referring to Figure 1, the four TCBs (TCB 1,3,5, and 7) that 
are supplied control power from dc bus No. 1 did not trip immediately 
(because of binding) on undervoltage when control power was deenergized. The 
operators did verify that the shunt trip for each of the four TCBs was 
functioning. Operators also verified that the redundant TCBs from dc bus No. 
2 (TCB 2,4,6, and 8) tripped on similar undervoltage conditions, and, in 
fact, inadvertently caused a plant trip while testing. Investigation into 
the problem by the licensee identified the cause of failure as an 
out-of-adjustment condition in the linkage mechanism of the undervoltage 
trip device. This adjustment problem, together with the lack of cleaning and 
periodic relubricating of the trip shaft mechanism, is the subject of IE 
Bulletin 79-09 which was issued April 17, 1979. While shutdown, the 
undervoltage trip mechanism on each of the above four faulty TCBs was 
adjusted and verified to operate satisfactorily. 

Investigation by the licensee revealed that the reactor protection system 
(RPS) periodic test procedure in use at the time did not verify the trip 
function of the undervoltage trip coil independent of the shunt trip coil. 
Referring to Figure 1, it can be seen that during a reactor trip test at 
this facility, these coils operate simultaneously causing the trip opening 
of the associated TCBs. This arrangement of the RPS and the trip test 
procedure may be similar for other PWRs. 

BWRs may use similar circuit breakers in safety systems; therefore, similar 
inadequacies in their circuit breaker test procedures may exist. 

Following adjustment of the linkage mechanism of the undervoltage trip 
device the licensee instituted a revised surveillance test procedure to 
check the undervoltage and shunt trip devices independently and to insure 
proper breaker operation. Although not shown in Figure 1, the undervoltage 
trip coil and the shunt trip coil are separately fused so that test 
procedure changes resolved the problem at St. Lucie. 

                                                           IEC 81-12      
                                                           July 22, 1981  
                                                           Page 2 of 2    

Recommended Action for Holders of Operating Licenses and Construction 

It is recommended that holders of operating licenses and construction 
permits review for applicability the specific items presented in the 
"Description of Circumstances." It is further recommended that the procedure 
for surveillance testing of trip circuit breakers be reviewed and revised as 
necessary to provide independent testing of each trip function, including 
position verification to ensure that the breaker actually trips. 

If the trip circuit breakers do not have provisions for independent testing 
of each trip function, including position verification, then appropriate 
modifications should be made to include such features (e.g., local 
pushbuttons for the shunt and undervoltage trip coils, separately fused 
circuits for the shunt and undervoltage trip coils, etc). 

No written response to this circular is required. If you need additional 
information with regard to this subject, please contact the director of the 
appropriate NRC Regional Office. 

1. Figure 1 
2. Recently issued IE Circulars 

Page Last Reviewed/Updated Tuesday, September 01, 2015