United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 80-45: Potential Failure of BWR Backup Manual Scram Capability

                                                          SSINS No.: 6835  
                                                          Accession No.:   
                                                          801216002        
                                                          IN 80-45         

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT
                          WASHINGTON, D.C. 20555  
                                     
                             December 17, 1980 

Information Notice No. 80-45:   POTENTIAL FAILURE OF BWR BACKUP MANUAL 
                                   SCRAM CAPABILITY 

The primary purpose of this Notice is to advise BWR licensees that a 
mechanism exists which could defeat one of the backup manual scram (reactor 
trip) features normally available to the reactor operator. In some 
GE-designed reactors, placing the Mode Selector Switch (MSS) in "Shutdown" 
should actuate a scram. This feature is used at some facilities as part of a 
normal reactor shutdown, and for certain plant situations the Technical 
Specifications require placing the MSS in the "Shutdown" position as a means
of attaining immediate shutdown.  As described below, a situation may arise 
that blocks this scram.  

The Pilgrim Nuclear Power Station has reported that following the loss of 
power from a vital M-G set and manual transfer to the alternate power 
source, the annunciator "Shutdown Scram Reset Permissive" came on and stayed 
on. Since such an annunciator may not be provided for all BWRs, some 
facilities may not be able to detect this condition. Subsequent 
investigation revealed that when one RPS bus power supply is lost and the 
MSS is in the "Run" mode, relay K-17 is immediately deenergized and after 
two seconds one of two series bypass contacts in the MSS scram circuit is 
closed. Upon restoration of RPS bus power, it is possible for relay K-16 to 
be energized before K-17 (i.e., a relay race). If this occurs, the following 
conditions will result: (1) the normally open K-16 contact in the MSS scram 
bypass circuit is closed, thereby bypassing the MSS in "Shutdown" Scram; and 
(2) relay K-17 will be kept deenergized. 

Even with this condition (i.e., the bypass circuit made up and the MSS in 
"Run") a Scram should occur if the MSS is turned in a deliberate manner to 
"Shutdown" with a pause in either the "Startup" or "Refuel" mode; however, 
if the MSS is turned to "Shutdown" quickly such that relay K-17 is not 
energized, a Scram due to MSS manipulation may not occur. 

We understand that General Electric has been informed of this potential for 
bypassing the MSS and that General Electric is preparing a Service 
Information Letter (SIL) containing recommendations for preventing such 
bypassing. 

This Information Notice is provided as an early notification of a possibly 
significant matter that is still under review by the NRC staff. Recipients 
should review the information for possible applicability to their 
facilities. No specific action or response is requested at this time; 
however, if NRC evaluations so indicate, further licensee actions may be 
requested or required. 

If you have any question regarding this matter, please contact the director 
of the appropriate NRC Regional Office. 

Page Last Reviewed/Updated Tuesday, November 12, 2013