United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-63: Potential for Common-mode Failure of Standby Gas Treatment System on Loss of Off-site Power

                                                            SSINS No.: 6835 
                                                            IN 85-63       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555 

                                July 25, 1985

Information Notice No. 85-63:   POTENTIAl FOR COMMON-MODE FAILURE OF 
                                   STANDBY GAS TREATMENT SYSTEM ON LOSS OF 
                                   OFF-SITE POWER 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 

Purpose: 

This information notice is being provided as a notification of a potentially
significant problem pertaining to the standby gas treatment system (SBGT). 
This problem could result in the loss of all SBGT trains following a loss of
all off-site power. This supplements information provided in IE Information 
Notices 83-25 and 84-81 on other SBGT problems. It is expected that 
recipients will review the information for applicability to their facilities
and consider actions, if appropriate, to preclude a similar problem from 
occurring at their facilities. However, suggestions contained in this 
information notice do not constitute NRC requirements; therefore, no 
specific action or written response is required. 

Description of Circumstances: 

On May 14, 1985, Carolina Power and Light (CP&L) reported a logic design 
error in the SBGT at Brunswick 1 and 2 which would result in the 
inoperability (i.e., failure to automatically start) of both trains of the 
system following a loss of off-site power. Design of Brunswick's SBGT 
requires automatic initiation when any of the following signals are 
received: high radiation in the reactor building ventilation exhaust, high 
pressure in the drywell, or low reactor water level. The system is designed 
to operate from either on-site or off-site power sources. However, because 
of a design error, the instrument that senses the SBGT heater temperature 
deenergizes a relay on loss of off-site power, seals in the signal, and 
prevents automatic initiation of SBGT until manually reset. 

At Brunswick, indications and reset are available in the control room. CP&L 
is considering a modification to the circuit to install a time-delay relay 
to allow time for the system to switch to on-site power. This effectively 
provides automatic reset for situations where a cause for lockout does not 
exist. 



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                                                             IN 85-63     
                                                             July 25, 1985 
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Discussion: 

The signals that trip SBGT are unique to each plant; however, automatic 
lockouts are generally associated with the electric heaters. The events 
described in IE Information Notices 83-25 and 84-81 provide examples where 
heater trips have locked out systems and rendered them inoperable. Based on 
a review of several licensee event reports, it appears that this design 
feature is fairly common. 

For designs such as Brunswick, with indication and reset in the control 
room, the operators might be able to diagnose and correct a problem. If 
indication and reset are not available in the control room, significant 
difficulties might be encountered if an accident requiring filtration were 
accompanied by a loss of off-site power. In either event, however, the 
requirement for automatic start generally necessitates the ability to start 
automatically even after a loss of off-site power. 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 


                                   Edward L. Jordan Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  Mary S. Wegner, IE
                    (301) 492-4511

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