ML19241A601

Discusses Failure of Nuclear Instrumentation Channel 4.On 790531,while Operating at 10 Mw,Channel 4 Indication Failed Downscale.Caused by Failure of Picometer.Corrected by Installation of Spare Picometer Module

ML19241A601
Person / Time
Site:	University of Missouri-Columbia
Issue date:	06/29/1979
From:	Brugger R, Mckibben J MISSOURI, UNIV. OF, ROLLA, MO
To:	NRC COMMISSION (OCM)
References
NUDOCS 7907050322
Download: ML19241A601 (2)

Text

Research Reactor Facility UNIVERSITY OF MISSOURI pese3,e p3,x Cc%mbia, usnuo 65201 June 29, 1979 Te'eonore oto ee2 42n Director of Licensing U. S. Nuclear Regulatory Commission Washington, D. C. 20555

Reference:

Doc %t 50-186 University of Missouri micense R-103

Subject:

Failure of Nuclear Instrumentation Channel 4 Descriotion On May 31, 1979, while operat1:tg at a ste.ady state power of 10 MW, channel 4, one of three Nuclear Instrumentation (N.I.) Power Range Channels, indication failed downscale. Both channels 5 and 6 were indicating no change in reactor power. Each power range channel is required to provide a rod run-in at 115% of full power and a rear. tor scram at 125% of full power. The failure of channel 4 indication caused its associated rod run-in and scram trips to be inoperable while the reactor wa, operating, which is a deviation from Technical Specifications 3.3.a and 3.4.c.

Analysis At 1641 on May 31,1979, with the reactor operating in manual control at a power level of 10 MW; N.I. channel 4 indication failed downscale; while electronic technicians were seating a circuit board during wcrk on the regu-lating blade automatic control circuit. The reactor operator with the control room console watch promptly manually scrammed the reactor to comply with Technical Specifications. With the reactor shutdown, N.I. channel 4 was in-vestigated and revealed the picoammeter module had failed. The picoammeter is the output module of channel 4 and its output signal is sent to the regu-lating blade automatic control circuit, safety trip circuits, and power indication meters.

In attempting to seat the circuit board, the outpt.. of the picoammeter had been siiorted to ground which overloaded the last stage transistor in the picoammeter caus N the failure. The spare piccarrmeter module was installed qQ ?907050 w 6 4x0 acesen COLUMBIA KANSAS CITY ROLLA ST, LOUIS b l lo 4, g,. 4 h an eaua ccccr umy -c taan 310 1 c/ /!

Director of Licensing June 29, 1979 Page 2 and start-up checks of N.I. channel 4 were completed with no additional problems.

There was no failure of the safety system, since N.I. Power Range Channels 5 and 6 were operational and would have provided the required rod run-in or scram pro-tection.

Corrective Actions The reactor was shut down immediately when N.I. channel 4 failed downscale and the problem was corrected by installing the spare picoammeter module. To prevent reoccurrence of this problem, the input resistor (10 megohm) for the N.I. Channel 4 output to the automatic control circuit was moved from the circuit board to the piccarceter side of the circuit board connector pin. The 10 megohm resistor now protects the picoammeter from being overloaded by a short at either the con. tor pin or in the circuit board. Additionally, insulating material was udded around the automatic control circuit board con-nectors to prevent grounding.

Sincerely, i- k,f -

J. C. McKibben Reactor Manager

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Robert M. Brugger '

Director JCMK:vs cc: Directorate of Regulatory Operations, Region III Reactor Advisory Comittee Reactor Safety Subcommittee

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