Part 21 Report - 1997-860

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 JGW/dg enclosure copies: 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] FACILITY LICENSE R-52 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 alignment. 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. Conclusions 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. *** END OF DOCUMENT ***

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