Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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.
8507230389
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IN 85-63
July 25, 1985
Page 2 of 2
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
Attachment:
List of Recently Issued IE Information Notices