Information Notice No. 83-42: Reactor Mode Switch Malfunctions
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
June 23, 1983
Information Notice No. 83-42: REACTOR MODE SWITCH MALFUNCTIONS
All boiling water reactors holding an operating license (OL) or construction
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
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.
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
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
June 23, 1983
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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
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