Information Notice No. 80-45: Potential Failure of BWR Backup Manual Scram Capability
SSINS No.: 6835
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
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
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.
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