United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Circular 77-09 - Improper Fuse Coordination in BWR Standby Liquid Control System Control Circuits


CR77009 

                               UNITED STATES 
                      NUCLEAR REGULATORY COMMISSION 
                         WASHINGTON, D. C. 20555 

                               May 25, 1977 

J. P. O'Reilly, Director, Region I 
N. C. Moseley, Director, Region II 
J. G. Keppler, Director, Region III 
E. M. Howard, Director, Region IV 
R. H. Engelken, Director, Region V 

IE CIRCULAR 77-09 - IMPROPER FUSE COORDINATION IN BWR STANDBY LIQUID CONTROL SYSTEM CONTROL CIRCUITS 

Enclosed is IE Circular 77-09 for distribution to all BWR nuclear power 
reactor facilities with an operating license or construction permit for 
information only. 

This Circular should be forwarded by mail or other means within five working
days of receipt of this memorandum by-your office. A draft copy of the 
transmittal letter is enclosed also. 


                                        H. D. Thornburg, Director 
                                        Division of Reactor Operations 
                                             Inspection 
                                        Office of Inspection  
                                             and Enforcement 

Enclosures: 
1. IE Circular 77-09 
2. Draft transmittal letter
.

(Transmittal letter for IE Circular 77-09 to each,holder of a BWR Operating 
License or Construction Permit.) 

Addressee: 

The enclosed IE Circular, 77-09 is forwarded to you for your information. No
written response is required. 

Should you have any questions concerning this matter contact the Director of
the NRC Regional Office. 


                                        Signature 
                                        (Regional Director) 

Enclosure: 
IE Circular 77-09 
.

                                                       IE Circular 77-09  
                                                       Date: May 25, 1977  
                                                       Page 1 of 2 

IMPROPER FUSE COORDINATION IN BWR STANDBY LIQUID CONTROL SYSTEM CONTROL 
CIRCUITS 

Description of Circumstances: 

On October 9, 1976, the Standby Liquid Control System at the Cooper Nuclear 
Station failed to operate during performance of the required surveillance 
test of an explosive valve. The system failure resulted from a blown fuse in
the main control power circuit. The licensee found that error in selection 
and coordination of fuses in the Standby Liquid Control System could render 
the system inoperable. 

The firing circuit for the Standby Control System explosive valves receives 
electrical power from the system pump starter cabinet through a control 
power transformer. The main control power circuit is fused as is each of the 
two detonators for each explosive valve. These fuses must be selected so 
that the fuses for the explosive valves open before the main fuse in the 
control power circuit when a fault develops. Operating experience has shown 
that system electrical faults are common to this application because the 
metal fragments produced by detonation of the explosive valve are likely to 
short to ground during a firing operation. 

Following the event of October 9, 1976, the licensee discovered that the 
control power fuse rated at 2 amperes did not have sufficient transient 
current capability to ensure that the 2 ampere rated fuses in series with 
the explosive valves would open first and protect the firing circuit if the 
valve wires short circuited upon firing. The fuses found in both Standby 
Liquid Control starter circuits were sized to protect the 0.25 KVA control 
power transformer. Corrective action has included the replacement of the 
control power transformer with one of larger KVA capacity and the 
replacement of the control power fuse with a 10 ampere rating and MDL 10 
characteristics. The firing circuit fuses were changed to 2 ampere FNM2 
types (i.e., a fuse having faster slow blow characteristics). Following 
these modifications a total system operational test was conducted 
successfully including detonation of both explosive valves. 
.

                                                       IE Circular 77-09 
                                                       Date: May 25, 1977  
                                                       Page 2 of 2 

Based on the above, licensees of boiling water reactor facilities may wish 
to review the Standby Liquid Control System drawings and actual installation
to ensure proper fuse coordination. Following any modifications to the 
system consideration may also be given to performing a total system, 
operational test to verify that the firing circuit fuses blow before the 
main control power fuse. 

No written response to this Circular is required. If you require additional 
information regarding this matter, contact the Director of this NRC Regional
Office. 


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