United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-53: Failure of Remote Shutdown System Instrumentation Because of Incorrectly Installed Components

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                          WASHINGTON, D.C.  20555 

                             September 4, 1991 


NRC INFORMATION NOTICE 91-53:  FAILURE OF REMOTE SHUTDOWN SYSTEM 
                               INSTRUMENTATION BECAUSE OF INCORRECTLY 
                               INSTALLED COMPONENTS


Addressees 

All holders of operating licenses or construction permits for nuclear power 
reactors. 

Purpose  

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to an event where the remote shutdown system at a 
nuclear power plant could have failed to perform certain functions because 
associated components were installed incorrectly.  It is expected that 
licensees 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 May 18, 1991, Entergy Operations, Inc., the licensee for the Waterford 
Steam Electric Station, performed post-modification testing on a main steam 
isolation valve (MSIV) at the Waterford 3 reactor with the reactor in Mode 
5. The testing included closing the subject valve using fire isolation 
switch FR-4 in the auxiliary relay room to ensure that the control wiring 
changes on the MSIV did not affect the function of the switch.  If the 
control room is evacuated because of a fire, the switch is to be used to 
isolate control circuits for the MSIV and several other components from the 
control room and to ensure that power is supplied to the components to 
maintain them in the desired position.  When the switch was operated, it 
travelled past the isolation position, allowing two of the adjacent 
make-before-break contacts to connect, which shorted the positive contact of 
one circuit to the negative contact of another circuit.  The short circuit 
caused the power supply breaker for the switch to trip, but the switch was 
severely damaged by smoke and fire.  The licensee quickly extinguished the 
fire.  While replacing the switch, the licensee determined that stop screws 
that should have been installed when the switch was installed were missing.  
This allowed the switch to rotate past the intended stopping position.  The 
licensee replaced the failed switch and successfully tested the replacement.  
The switch is a multideck rotary switch manufactured by Electroswitch.  The 
licensee determined that the stop screws 


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                                                          IN 91-53
                                                          September 4, 1991
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were supplied with the switch, but installation instructions did not specify 
that the stop screws be installed in this application.  The licensee 
inspected several other isolation switches in adjacent panels and found 
another switch with missing stop screws.  Further information may be found 
in NRC Inspection Report 50-382/91-18. 

Discussion 

Fire in certain plant areas such as the control room or the cable spreading 
room can affect the ability to achieve safe shutdown of the plant from the 
control room.  In addition, the control room can become inaccessible for 
reasons other than a fire.  These situations warrant the use of a remote 
shutdown system to achieve safe shutdown of the plant from a remote shutdown 
panel or emergency control stations that are independent of the control 
room.  The remote shutdown system includes switches to isolate the control 
circuits of required safe shutdown equipment from the control room and to 
transfer controls to either the remote shutdown panel or emergency control 
stations.  Therefore, licensees provide required instrumentation and 
controls for the remote shutdown system in accordance with the requirements 
of General Design Criterion (GDC) 19 of Appendix A and Appendix R to Part 50 
of Title 10 of the Code of Federal Regulations (10 CFR Part 50).  The 
significance of the problem described above is that the remote shutdown 
capability for the plant may be compromised if the control circuits and 
transfer switches (for shutdown equipment) fail because they are installed 
incorrectly.  If these switches fail, certain instrumentation and control 
functions for the remote shutdown system may be lost.  

Licensees may not recognize that stop screws have not been installed in the 
fire isolation (transfer) switches unless the operator challenges the 
mechanical stops of these switches.  Although the switches were tested for 
MSIV operability during pre-operational testing at Waterford, the effective-
ness of the mechanical stops of these switches was apparently never tested. 

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:  T. Chandrasekaran, NRR 
                     301-492-0859 
                 
                     C. Doutt, NRR
                     301-492-0847

Attachment:  List of Recently Issued NRC Information Notices 
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