United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 83-42: Reactor Mode Switch Malfunctions

                                                            SSINS No.: 6835 
                                                            IN 83-42       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                                June 23, 1983

Information Notice No. 83-42:   REACTOR MODE SWITCH MALFUNCTIONS 

Addressees: 

All boiling water reactors holding an operating license (OL) or construction
permit (CP) 

Purpose: 

This information notice is provided as an early notification of potentially 
significant events concerning mode switch malfunctions while changing 
operating modes. It is expected that recipients will review the information 
for applicability to their stations. No specific action or response is 
required  at this time. 

Description of Circumstances: 

Susquehanna 1 Events: 

On March 22, 1983, Susquehanna Unit 1 was being prepared for a startup with 
all control rods fully inserted. The reactor mode switch was placed in the 
startup position in order to perform a surveillance test. Following the 
completion of the surveillance test the mode switch was returned to the 
shutdown. position which design produces a full scram signal. However, this 
time, the reactor protection system (RPS) channel "B" failed to actuate, and
only a half scram signal was produced. The operator verified that all 
control rods remained fully inserted, and the operator depressed the reactor 
scram buttons as a precautionary measure. The manual scram actuated both 
channels of the RPS. 

The mode switch was test cycled several times and proper operation was 
intermittent. The switch was removed and a replacement mode switch was 
acquired from the NSSS supplier. Testing showed that there was improper 
contact continuity and this replacement switch was also deemed unacceptable.
Four additional mode switches were acquired and all four mode switches 
successfully passed a bench test. One was used as a replacement. 

On May 16, 1983, Susquehanna Unit 1 was again being prepared for a startup. 
The reactor mode Switch was placed in the refuel position and immediately a 
scram signal and a group 1 isolation signal were generated. When the mode 
switch was placed in the startup position, the signals for a scram and a 
group 1 isolation were again produced. The reactor mode switch was a 
recently installed modified version of the type used in the March 22 event. 
The startup was, suspended until an acceptable replacement could be 
obtained. 

8305110479 
.

                                                           IN 83-42 
                                                           June 23, 1983 
                                                           Page 2 of 3 

Dresden 2 Event:  

On December 17, 1982, power at Dresden Unit 2  was being decreased to a hot 
standby condition so that a crack could be repaired in an emergency drain 
line of a feedwater heater. When the reactor power was less than one percent 
the reactor mode switch was moved from the run position to the startup 
position. With the mode switch in the startup position, the group 1 
isolation signal from low main steam line pressure (850 psig) should be 
bypassed. When the reactor pressure decreased to less than 850 psig a group 
1 isolation occurred. The closure of the main steam line isolation valves 
resulted in a reactor scram. Similar events  had previously been experienced 
at Dresden Unit 2 and at Quad Cities Unit I. All had resulted in group 1 
isolations from a low main steam line pressure signal after the mode switch 
was placed in a position that normally bypasses this signal. 

Discussion: 

Following the March 22 event at Susquehanna, Pennsylvania Power and Light 
contacted a research laboratory to test the failed reactor mode switch. The 
test consisted of a verification of contact positioning during both smooth 
and uneven switch rotation. Upon completion of the testing for contact 
positioning the reactor mode switch was disassembled so that the individual 
parts could be examined. 

The test results revealed numerous contact positioning errors that appear to
be the result of significant irregularities in the mode switch cam shaft 
parts and large design clearances resulting in imprecise operation of the 
cam followers. 

General Electric was contacted and confirmed that problems existed with the 
mode switch used at Susquehanna General Electric identified this mode switch
by the model name RUDEL-GOULD, manufactured by Gould for General Electric. 
Currently Susquehanna is the only operating station that uses this type of 
switch. General Electric intends to issue a Field Disposition Instruction 
(FDI) to alert other potential users of this switch about its problems and 
required modifications. The mode switch that was in place at Susquehanna on 
May 16 was a newly modified mode switch. General Electric inspected the 
modified mode switch and found a switch shaft cam improperly oriented and 
cam surface irregularities. This mode switch was temporarily replaced with a 
switch that was successfully tested following the March 23 event. 

The reactor mode switches that failed at Dresden Unit 2 and Quad Cities Unit 
are identified by the model number SB-1 and are commonly used on BWR 2s 
through BWR 5s. It appears that because of years of service the SB-1 switch 
has sticking contacts. Both stations replaced the mode switch during the 
last refueling outage. 

BWR owners should be unexplained isolation or scram signals that may have 
coincide with mode switch movement while changing operating states. It may 
be prudent for licensees to change their manual scram procedures to require 
the use of the scram push buttons before moving the mode switch to the 
shutdown position. 
.

                                                              IN 83-42     
                                                              June 23, 1983 
                                                              Page 3 of  3 

No written response to this notice is required. If you have any questions 
regarding this matter, please contact the Regional Administrator of the 
appropriate NRC Regional office, or this office. 


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

Technical Contact:  Paul R. Farron, IE
                    (301) 492-4766

Attachment:
1.   List of Recently Issued IE Information Notices
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