ML19352B210

Ro:On 810409 & 14,during Reactor Prestartup Checks,Interlock Used to Prevent Withdrawal of Shim Safety Control Rods Found Not Functioning.Caused by Threshold Change Due to Change in Solid State Switching Device.Capacitor Installed

ML19352B210
Person / Time
Site:	05000128
Issue date:	04/24/1981
From:	Randall J TEXAS A&M UNIV., COLLEGE STATION, TX
To:	Madsen G NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
References
NUDOCS 8106030372
Download: ML19352B210 (2)

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TEXAS ENGINEERING EXPERIM ENT STATION THE TEXAS A&M UNIVERSITY SYSTEM

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COLLEGE STATION, TEXAS 77843 q# '[

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24 April 1981 NUCLEAR SCIENCE CENTER 713/845-7551 0

Mr. G. L. Madsen d'

\f Office of Inspection and Enforcement . ( ,

Region IV U.S. Nuclear Regulatory Commission

' p Dgo -1 611 Ryan Plaza Drive, Suite 1000 g b' Arlington, Texas 76012 r 8 Rafcronca: Docket No. 50-128 N w Lo

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Subject:

Failure of Shim Safety Control Rod Source Count Interlock Reported 4-14-81

Dear Mr. Madsen:

Enclosed is a final report of a reportable occurrence observed during reactor pre-startup checks at the NSCR. This report is submitted in compliance with Section 6.4 and 1.8.c of change No. 11 of the Technical Specifications, Facility License No. -83 for the Nuclear Science Center, Texas A&M University.

S nSerely i -

J hn D. Randall irector JDR/ym cc: R. R. Berg, Chairman Reactor Safety Board Texas A&M University Enclosure

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1 RESEr*aCH AND DEVELOPMENT a:OR MANKIND

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Final Report of Failure of Shim Safety Control Rod Low Source Count Interlock Reps **aole Occurrence On April 9, 1981 and April 14, 1981 during reactor pre-startup checks of the NSCR it was determined that the interlock to prevent withdrawal of shim safety control rods for low source level was not functioning properly. In the mal-f unction on April 9,19ol the circuit provided a low level interlock alarm but failed to prevent the withdrawal of shim safety control rode 1 and 4.

Corrective maintenance was performed on the circuit and the pre-startup check performed. In the Lalfunction on April 14, 1981 the circuit again provided a low level interlock relay were not properly closing. After the relay was was replaced the circuit performed satisfactoruly and normal operations were resumed.

Af ter the April 14 malfunction, the circuit was again checked by an electronics technician. It was determined that the failure was intermittant which presented difficulties in diagnosis. After a comprehensive analysis it was determined that 22..VAC was superimposed on the +12 VDC control rod drive control sirnal.

This voltage did not originate in the power supply but was apparently picked up by inductance somewhere in the console. A proposal was presented te an NSC Staff Review that a lufD capacitor be installed between the positive and negative leads of the power supply for the control rod drive signal but would reduce the 22 VAC. Since all parts of the circuit apt aared to function normally it was deduced that the AC voltage had always been on the DC line.

However, a slight change in the solid state switching device or in other circuit components had apparently caused a threshold change that occassionally caused triggering by the AC, placing the circuit in the improper mode. Since the filtering action of the capacitor would reduce the AC component by a factor of twenty it was asserted that the problem would be alleviated. The circuit modification was approved by the NSC Staff Review.

Corrective Actign The ha fD capacitor was installed and the circuit tested. The AC voltage was reduced to approximately 1 VAC and the interlock and control rod drive circuits functioned normally. Due to the intermittant nature of the malfunction the fact that testing indicated proper operation was not deemed conclusive. The circuit was watched very carefully for the next several days. Since no failure has been observed for the last 128 days it is quite probable that the defect has been corrected.

+ - s 12 VDC I U)# COUNT RAEE - IM N ER -

SUPPLY -

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INTERLOCK SHM SAFETY SETATE CONTROL SWITCH ROD DRIVE F1LTER CAPACITOR !!% STALLED TO REMOVE THE 22 VAC.

FIGURE I

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