ML20086H808

Final Ro:On 911119,safety Channel 2 Meter Flickered & Dropped Out Hard Downscale (Pegged).Caused by Intermitten Fault in Fine Adjust Potentiometer of Circuit. Poteniometer Replaced & Amplifier Card Reseated

ML20086H808
Person / Time
Site:	05000083
Issue date:	12/03/1991
From:	Vernetson W FLORIDA, UNIV. OF, GAINESVILLE, FL
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II)
Shared Package
ML20086H810	List: ML20086H808 ML20086H815
References
NUDOCS 9112090417
Download: ML20086H808 (4)

Text

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. NUCLEAR ENGINEERING SCIENCES DEPARTMENT Nuclear Reactor Facility University of Florido-mu. .ueo. .u.. .

GeewateJimewa 3N11 n <mm un s.. um December 3,1991 Potential Violation of Technical Specifications:

Safety Channel #2 Circuit Failure - Final 14-Day Report Nuclear Regulatory Commission Suite 2900 101 Marietta Street, N.W.

g Atlanta, GA 30323

- Attention: Regional - Administrator, Region II Re: University of Florida Training Reactor Facility License. R-56. Docket No. 50-83

- Gentlemen:

Pursuant to the reporting requirements of paragmph 6.6.2(c)and (g) of the UFTR Technical-Specifications, a description of a potential violation of the Technical Specifications was reported--by telephone /telecopy(Attachment I) on 20 November 1991 and a final 14-day

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written report is submitted with this letter to include occurrence scenario, NRC notification, evaluation of consequences, Lcorrective action and current status. The potentially -promptly reportable occurrence involved the failure of the Safety Channel #2 meter _ circuit which represented a _ loss of the Safety Channel #2 overpower trip function for a brief(about '20 i

seconds) period of time, following and during which an unscheduled shutdown was in progress.

L- - Scenaria l

l On 19 November.1991, after the second startup of the day was begun at'1340 hours0.0155 days <br />0.372 hours <br />0.00222 weeks <br />5.0987e-4 months <br /> and after 32 minutes of operation at full power and. irradiation of several samples in the rabbit L system, .the Safety Channel #2 meter was noted- to flicker and then drop out hard l- downscale(pagged). Because this event represented a loss of Safety Channel #2 overpower trip capabiliti an unscheduled reactor shutdown was commenced immediately at 1432 hours0.0166 days <br />0.398 hours <br />0.00237 weeks <br />5.44876e-4 months <br /> l

l with the reactor shut down and secured at 1433 hours0.0166 days <br />0.398 hours <br />0.00237 weeks <br />5.452565e-4 months <br />. The event . c.cred immediately y'RO G.W. Fogle.with SRC D. Simpkir.s present to observe the shutdown and system L sponses. All operator responses in the event w2re proper. During the shutdown with-ower at about 10 kW some 20 seconds or so aner commencing the unscheduled shutdown, the Safety Channel #2 meter was noted to telurn to read normal.

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Nuclear Regulatory Commission Safety Channel #2 Circuit Failure Page 2 December 3,1991 After completion of the unscheduled shutdown, Maintenance Log Page #91-61 was opened and the test trip of Safety Channel #2 was noted to be operating normally. Because of the hard downscale nature of the channel failure and no indication on any other monitoring, recording, or trip channel, the fault was isolated to the Safety Channel 2 meter circuit which contains two ampliners whose failure was initially thought to be a possible cause of the event- At this point in agreement with input from several members of the Reactor Safety Review Subcommittee (RSRS), reactor management decided to report this event to NRC Region 11 as a potential Tech Spec violation.

Subsequently, during extended bench tesing and checks of the meter circuit assembly, an intermittent fault in the fine adjust potentiometer of the circuit was isolated. Although it was not the source of the Safety Channel failure, the Gne adjust potentiometer obviously needed replacement. Per 10 CFR 50.59 Evaluation and Determination No. 91-09(Safety Channel #2 Calibration Module), both the coarse and line gain potentiometers were replaced with scaled potentiometers to provide better resistance to environmentally-driven

. degradation. The coarse gain potentiometer was replaced with aa identical component, only r scaled for better cuironmental response; the fine gain potentiometer was also replaced with a sealed canister pctentiometer but with a 2500 versus a 2000 adjustable potentiometer.

This change was evaluated to represent only about 0.33% change in sensitivity with the circuit response left unchanged - only the adjustable setting will be slightly different to provide the proper full power calibrati on from an unchanged voltage input to this point.

Extensive additional analysis and checks were performed on the meter and related circuits.

Subsequently, the Safety Channel #2 amplifier card was rescated and further checks performed including circuit run checks, heat and cold tests and checks of all Safety Channel

1. 2 harness assemblies and connectors with no further faults noted.

Since oxidation / corrosion on contacts has o. - sionally been a problem with the instrumentation in this console and since this intermittent type failure could have been caused by such oxidation of contacts, it was evaluated that the cleaning of the contacts by reseating the Safety Channel #2 amplifier card had corrected the fault with no further repair or maintenance needed especially in light of the extensive checks that had be.en run.

On 25 November 1991 the RSRS Executive Committee met to review the occurrence and corrective actions taken prior to approving restart. In particular, the specifics of the occurrence were reviewed per the completed Unscheduled Shutdown Review and Evaluation (UFTR Form SOP-0.6B). They also reviewed the event and concluded it to 1

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Nuclear Regulatory Commission Safety Channel #2 Circuit Failure Page 3 December 3,1991 be a potential abnormal occurrence and a potentially reportable occurrence per UFTR Technical Specifications, Section 6.6.2 delineating requirements for special reports and per SOP-0.6,Section 3.2.3.3.3 indicating certain safety system failures are promptly reportable.

The communications with Region Il as documented in the prompt notificatian letter (Attaciunent 0 and in a telephone conversation on November 20, 1991 with Mr. Ed McAlpine were also reviewed. Subsequently the unrelated corrective action to replace both the coarse and fine gain potentiometers in the Safety Channel #2 calibration module with scaled potentiometers for better environmental protection was reviewed and approved under 10 CFR 50.59 Evaluation and Dete mination No. 91-09 to include replacement of the fine gain potentiometer with a 250 0 adjustable potentiometer. Based on the extensive circuit checks, the nature of the failure indicating the probable cause to be failure in the meter circuit anet the corrective action invo~lved in cleaning the meter circuit and other contacts, all members approved restart subject to resetting the meter circuit assuming the channel calibration was unchanged, performing a valid preoperational check and providing NRC Region 11 with notification of restart and subject to the recommendation to observe the safety channel for a period with an extra person following reaching power which UFER management agreed to do.

Following completion of ali checks and restart to full power performed by a reactor operator with a second SRO observing for the first two hours at full power, the Safety Channel #2 responded properly with no further problems noted.

NRC NnilDn1119n NRC Region II was informed of this event per a telephone conversation on 20 November 1991 with Mr. Ed McAlpine relative to the brief loss of Safety Channe1_ #2 trip capability.

The situation was confirmed in a following telecopy (Attachment D. Subsequently after the RSRS Executive Committee gave approval to restart subject to certain conditions and completion of all maintenance and tests, NRC Region ll(Craig Bassett) was notified of the intent to restart with a commitment of a second reactor ogrator to be present for the first 3 two hours at full power to assure noticing any recurrence of the failure. Subsequently the restart was successful as Safety Channel #2 responded properly. The UFTR was then considered to be returned to normal operations. The fuct of the successful restart was communicated to NRC Region II (Craig Bassett) on 2 December 1991.

-Nuclear Regulatory Commission Safety Channel'#2 Circuit Failure December 3,1991

- Page 4

- Evnluntion/ Corrective Action The cleaning of contacts is considered to have corrected the cause of this Safety Channel

1. 2 failure. The occurrence has been evaluated as a potential abnormal occurrence; however, the loss of the trip function on Safety Channel #2 was brief, the reactor was promptly shutdown and secured and the other reactor protection system channels were all operable. Therefore, the event is considered to have negligible effect on reactor safety and no e. T2t on the health and safety of the public.

- Current Stntus/Conseanences -

The Reactor Safety' Review Subcommittee (RSRS) Executive Committee met late on November 25, 1991 to review this event and subsequent corrective actions and planned activities. The committee essentially agreed with actions taken and with the staff evaluation that the occurrence represented a violation of the Technical Specifications. The Executive Committee also agreed that the UFTR was in full compliance with the Technical Specifications at that point and approved the restart of the- UFTR and subsequent return to normal operations. Reactor Management and the RSRS Executive Committee agreed there has been no significant compromise to reactor safety in the occurrence and no impact on the health and safety of the public. Other than.considering the event in the P:xt regular RSRS meeting, this occurrence is now considered closed.

-If further information is needed, please advise.

Sincerely,

'I h William G. Vernetson Director, Nuclear Facilities-

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Notary PublicC E ~ rJ  : Dati:

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cc: R. Piciullo l Reactor Safety Review Subcommittee

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