United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #: 9711240143

Nuclear Reactor Laboratory    The University of    John G. Williams,

Engineering Building (20)         Arizona          Director

P.O. Box 210020                Tucson Arizona      e-mail:

Tucson, Arizona 85721-0020                         jgw@bigdog.


                                                   voice (520) 621-9729

                                                   FAX (520) 621-8096


November 13, 1997

U.S. Nuclear Regulatory Commission

Document Control Desk

Mail Stop P1-37                                   10CFR50.71

One White Flint North                             10CFR21.21

11555 Rockville Pike

Rockville, MD 20852

Subject:       Facility License No. R-52, Docket 50-113

               Information report, concerning malfunction of Mode

               Selector Switch

The occurrence documented on the following pages was reported to the NRC

by telephone and fax on 5 November 1997.  This report supplement that

information and includes analysis, conclusions and remedial actions that

will be reviewed by the Reactor Committee for this facility.

John G. Williams, Director

Director, Nuclear Reactor Laboratory




Non-Power Reactors and Decommissioning Project Directorate, USNRC

Mr. Marvin Mendonca, USNRC

Dr. M. Cusanovich, Vice Pres. for Research, University of Arizona

Reactor Committee Members

[Illegible print]


Malfunction of Mode Switch in Pulse Mode

          November 13, 1997:

Description of the Occurrence

On November 4, 1997, a pulse was performed with the mode switch in its

Pulse High setting and with the transient rod anvil set for a $1.75

pulse.  The transient rod operated normally, fuel temperature rise was

recorded showing energy deposition in the range expected for a normal

pulse and the timer scram operated normally.  No valid indication

appeared, however, for the peak reactor power or the integrated pulse

energy.  The right safety channel indicated a trip on reactor power.

A digital data acquisition system connected to the right safety channel

showed that the signal on that channel displayed a normal reactor period

of approximately 11 ms, but then saturated This showed that the channel

was connected to a valid indication of reactor power level from the ion

chamber that is normally connected in steady state mode, not the high

range ion chamber which should be selected in pulse mode.  ne trip on the

right safety channel was attributed to power in excess of 110 kW, which

is the steady state limit.

Testing of the mode switch revealed that changing the switch from its

manual to pulse positions did not reliably switch between the normal and

high range signals.  The problem was corrected after removal of the rear

casing of the mode switch and application of switch cleaning fluid. 

After reassembly, the system appeared to behave normally.  The reactor

was then operated at 95 kW steady state while the high range channel was

calibrated and shown to behave normally.  A pulse was then performed and

normal indications were found on all channels.

Analysis and diagnosis of the cause

A malfunction of the mode switch was the cause.  It resulted in failure

to pick a relay which switches from the normal to the high range channel

when pulse mode is selected, and which also switches in the nvt circuitry

that Indicates Integrated pulse energy.

The following symptoms demonstrate that the high range channel was not

engaged, but Instead the normal range uncompensated ion chamber was

operating: (a) the reactor tripped on a high power scram from the right

safety channel, even though the calculated peak power was less than 100

MW, and the trip was set at 1000 MW for pulse mode; (b) fuel temperature

Indications showed a peak temperature of 186 C, in the normal range for a

$1.75 pulse, corresponding to energy deposition of 9.3 MW and peak power

of 92.4 MW; (c) a digital data collection system connected to the channel

(after the switching

relay) saturated, though it should not have done so below 1100 MW; (d)

prior to saturation, the same data showed an exponential increase with a

period of 11 ms, normal for a $1.75 pulse, an indication that the channel

was connected to a true power indication, though not the high range one.

On November 10, 1997, the mode switch was removed from the console for

inspection and cleaning (see attached memo and part drawing).  As a

result of this inspection it was determined that the problem had actually

been caused by a mechanical misalignment of the rear wafer of the switch,

not by dirt.  This was attributed to wear and aging that had loosened the

nuts holding the rear case.  This also loosened the wafer assembly which

is held together by the same nuts.  The problem was corrected not by the

cleaning fluid, as Initially supposed but by the retightening of these

nuts that occurred on reassembly.

Corrective and remedial action

The immediate problem was corrected before the reactor was placed back in

service on November 4.  Tightening of the nuts holding the casing of the

mode switch also secured the switch wafers in their correct operating


To prevent a recurrence, the Electronics Technician and the Reactor

Supervisor have proposed a small modification in the switch assembly, so

that the wafer assembly is held together independently of the nuts which

secure the rear casing.  Details are given in the attached memo from Mr.

Lohmeier.  These proposed changes will be submitted for approval to the

Reactor Committee before implementation.


A malfunction that was attributed in part to aging was found in the Mode

Selector Switch (General Atomics part number ELD 239-4300C).  It was

corrected by tightening two nuts.  To prevent a recurrence, a minor

change in the assembly has been proposed.  Licensees of other facilities

using this GA part may wish to consider the applicability of this

experience to their systems.

The system failed safe causing actuation of reactor scram at 110 kW.  The

reactor did not operate outside its normal envelope, and there was no

threat to health or safety of the public.

J. G. Williams, Reactor Laboratory Director

Date:          11/13/97

To:            Dr. John Williams

From:          Wayne Lohmeier

Subject:       Mode Switch Problem

Dr. Williams,

On November 4, the day of the pulse problem, the Mode Selector Switch

(P/N ELD 239-4300C) rear case was removed and contact cleaner was sprayed

on the switch contacts. The case was then re-installed and the reactor

functioned normally.

On November 10, the switch was removed from the reactor console and

cleaned.  Cleaning involved removing a foam like residue from the spray

contact cleaner.  There was no obvious "dirt".  The switch wafers are

constructed with clear plastic in front of and behind the switch

contacts.  Although not hermetically sealed, the contacts are protected

from the outside elements.

The following observations were made in the switch construction.  There

are two screws approximately 3 inches long that hold the switch frame,

the front case.  the wafers, and the rear case.  The assembly is secured

by the screws and a washer, a lock washer.  and a nut on the outside of

the rear case.

My recollection of removing the rear case on November 4 is, the rear case

was not aligned with the front case (off by approximately 10 degrees) and

the nuts holding the rear case were not very tight.

The wafer that picks the "Pulse High/Lo" relay is the "D wafer" located

at the rear of the assembly. I suspect the problem was, the rear wafer

was lagging the mode switch knob because of the mis-alignment and nuts

not being tight.

Harry could feel a binding in the switch when going to the Pulse High/Lo

positions prior to removing the rear case and cleaning.  When the

reassembly was completed, it felt smooth, and an audible sound of the

switch engaging at each position is now present.

Corrective Action:

1.   Shorten the rear spacers between the rear wafer and rear case by the

     thickness of a lockwasher and nut.

2.   Assemble the switch wafers to the switch frame and front case using

     a lockwasher and nut.  Wafer assembly will now not rely on rear

     cover for mechanical support.

3.   Attach rear case using a lock washer and nut.

Figure "-1 ASSEMBLY and, DETAIL ITEM 9" omitted.

Figure "ELD 239-4300C Mode switch assembly" omitted.


Page Last Reviewed/Updated Monday, March 30, 2020