ML20205N881

Ro:On 880929,Tech Spec Deviation Occurred Due to Intermediate Range Monitor Channel 3 Indicating Downscale & Associated Short Period Rod run-in & Scram Trip Inoperable. Caused by Breakdown of Cable Insulation.Cabling Replaced

ML20205N881
Person / Time
Site:	University of Missouri-Columbia
Issue date:	10/28/1988
From:	Alger D, Mckibben J MISSOURI, UNIV. OF, COLUMBIA, MO
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 8811070026
Download: ML20205N881 (3)

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4 Research Reactor Facility UNIVERSITY OF MISSOURI g,,, pg October 28, 1988 cuo<a ussui ss211 Teepnone (314) 882 4211 Director of Nuclear Reactor Regulation U. S. Nuclear Regulatory Corrmi*.sion Mail Station P1-137 Washington, D.C. 20555

REFERENCE:

Docket 50-186, University of Missouri Research 'leactor License R-103

SUBJECT:

Report as required by lechnical Specification 6.1.h(2) concerning reactor operation with one of two intermediate range monitors inoperable.

DESCRIPTION:

't 0100 on September 29, 1988, wnile operating at 10 MW in automat h control, the duty operator observed that Intermediate Range Monitor (IRri) channel 3 was indicating downscale. The duty operator .mmediately shut down the reactor to investigate the problem. Reviaw of the IRM channel 3 chart recorder showed that the indication had been downscale approximately 30 minutes before it was detected. With the IRM channel 3 log level indication f ailed downscale, the associated short period rod run-in and scram trips were inoperable, which is a deviation from Technical Specifications 3.4.c and 3.3.a.

ANALYSIS:

Intermediate Range Monitor (IRM) channal 3 is one of two intermediate channels which provide short period rod run-in and scram functions to assure protectior, of fuel elements from a continuous startup rod withdraw 31 accident (Hazard Summary Report, Addendum 5 Section 5). Each intermediate range channel provides a rod run-in at a 10 second (minimun) period and a reactor scram at an 8 second (minimun) period.

When the IRM instrument failed, the associated rod run-in and scram trips were inoperable, which is a de 'iation from Technical Specifications 3.4.c and 3.3.a. There was no fa'lu.*e of the reactor safety system, since the redundant IRM channel 2 was operable and would have provided the required period scram and/or rod run-ir if it had been required. If a power transient had occurred, the three power range instruments scran and rod run-in trips would have shut down the reactor before a high period trip could occur due to the reactor beitig at 10 MW.

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Director of Nuclear Reactor Regulation Page 2 October 28, 1988 The reactor safety system is dependent on the short period trip p ri tion during a reactor startup. A downscale failure of an IRM instrumenc w~ . be immediately detected since the operator is closely monitoring the reactor period during the startup.

The failure of IRM channel 3 was determined to be due to breakdown of the cable insulation for the channel 3 compensated ion chamber detector. The detector and cabling were replaced and the IRM channel 3 was tested and source checked and found to be operational.

Previous failures attributed to degradation in cabling insulation are typically characterized by progressively erratic log level indication and spurious period scrams or rod run-ins. However, the mode of failure in t s instance--an immediate downscale indication--cannot be documented to have 4

previously occurred.

The downscale failure mode is detected on the powe range monitors l (channel 4, 5, and 6) by annunciating a "downscale" alarm from each monitor's -

trip unit. Intermediate Range channel 2 and 3 do not have downscale trip alarms because all four trip functions available in each monitor da already utilized to provide: 1) short period scram, 2) short period rod run-in, 3) -

regulating blade automatic permissive and 4) drawer inoperative. The drawer I inoperative condition is tripped by any of three causes: 1) any module removed from monitor, 2) selector switch S1 placed in any position other than ,

_0P_ ERA _T_E; or 3) when the High Voltage Power 5upply output drops below some 4 predetermined minimum voltage.

CORRECTIVE ACTION:

1 The reactor was shutdown immediately when the duty operator d'!tected [

the IRM channel 3 indication downscale. The detector and cebling was re- L placed, tested and its operability was verified, t

' One corrective action considered was the installation of a downscale alarm. However, the intermediate range trip units cannot accommodate the t change without sacrificing an already existing trip function. Since this mode t of failure (an immediate downscale failure) is not a corson one for the IRM channel and wou'Id be quickly observed during a reactor St.artip, it was deemed

not necessary to displace any existing trip functionf tr p ovide a new downscale trip.

The nuclear instrument detector cables p eviously installed have been [

experiencing short working lifetime. The electronics technicians have  ;

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l Director of Nuclear Reactor Regulation Page 3 i October 28, 1988 l l

i recently ordered and begun installing more irradiation-resistant cabling when detectors need replacing. The new type of cabling was installed on the IRM channel 3 detector on September 29, 1938.

Sincerely, L' ' {A-J. C. McKibben Reactor Manager Endorse:aent :

'eviewed and Approved D. M. Alger Assistant Director, Nuclear Technology and Operations xc: Reactor Advisory Comittee Reactor Safety Subcommittee

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