ML19207B457

Discusses Irregularities in Regulatory Rod Movements.Cause Is Unknown But Short Range Binding in Neighborhood of 25 Degrees May Result from Faulty Bearings on Shim Arm Shaft. Further Tests Are Planned.Reactor Will Remain Shut Down

ML19207B457
Person / Time
Site:	National Bureau of Standards Reactor
Issue date:	08/15/1979
From:	Rozier Carter NATIONAL INSTITUTE OF STANDARDS & TECHNOLOGY (FORMERL
To:	Reid R Office of Nuclear Reactor Regulation
References
NUDOCS 7908290490
Download: ML19207B457 (2)

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- - UNITED STATES DEPARTMENT OF COMMERCE

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August 15, 1979 4

Mr. Robert Reid

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Chief, Operating Reactors Branch No. 4

U. S. Nuclear Regulatory Commission j Washington, DC 20555 Subject. Docket No. 50-D

Dear Mr. Reid:

On the af ternoon of August 7,1979, with the reactor operating at 10 MW, the NBSR shim arms were being routinely adjusted to compensate for the movement of the regulating rod which had reached its upper control limit as a result of normal burnup and xenon buildup. The adjustment began by withdrawing shim No.1 which was at 28.5*. After commencing withdrawal, the operator noted no downward movement of the regulating rod and no power deviation as normally would be expected.

Glancing at shim No.1 indicator, it was then reading about 28.8*.

Subsequently he noted a jump in the indicator reading to approximately 29.2*. This was followed by an alarm and momentary rundown. A rundown takes place at 115% of power on two of three instruments. Reactor scram occurs at 125% of power on two of three instruments. Examination of relevant recorder charts showed a momentary power spike. One of the three safety channel (NC-8) selected to be recorded at the time showed a spike from about 105 to 132 re: order indication. The automatic control channel (NC-5) which was also being recorded showed a momentary rise f rom about 94 to 109. The fact that only a rundown occurred and a fast scram did not occur indicate that another safety channel reached 115 to cause the rundown but not 125 to cause a scram. There was no change indicated in the thermal pouer or AT. The reactivity associated with shim No. 1 movement from 28.S* to 29.2* is approximately 10 cents. The entire safety system including trip points had been checked the day before as part of prece nrational checks required prior to startup.

Following the short rundown which restored power to normal, shim No. I was tested and was found to be normal and smooth in the affected region.

The previous occurrence could not be duplicated.

The following morning, further checks were made by comparing the response of the regulating rod to the movement of each of the four shims in the affected red ion. While the response of the regulating rod to the movement of each of the shims was different as expected, that for shim No.1 appeared to be considerably different indicating a possible problem kOh 790829097[)

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P in shim No.1 movement in that region. The reactor was shutdown for more detailed checks.

To date, the following additional checks have been made.

1. The withdrawal and insertion times of shim No. I were measured and compare well with previous times.

j 2. The shLs was scrammed to measure release and drop times and to examine the drop trace. Both the times and the trace were normal.

! 3. The electronics associated with shim No.1 and with the auto-matic control system were checked and found to be normal.

4. Finally a trace of the movement of each of the shims as they were being driven up and down throughout their length of travel was made. The trace for shim No. 1 showed irregularities in certain areas, i.e. , it was not completely uniform over its length of travel. The irregularities can best be described as hesitations. Both the withdrawal and insertion times compared well with previous times. (Shim speed is about 2 3/4' per minute) . The traces for the other three shims appear to be uniform over their length of travel.

S. Shim No. 1 drive unit was removed and checked. Everything was found to be normal. Certain components in the drive that could possibly cause movement irregularities s te changed and another trace of the movement of shim No.1 was obtuined. There was no significant change in the trace with the same irregularities indicated.

None of the above positively identified the problem. Subsequent to the preparation of this letter, additional information was obtained as a result of continuing checks including viewing shim No. 1 movement with a periscope. There appears to be a definite binding in the movement of shim No. 1 over a short range in the neighborhood of 25*. The cadmium blade appeared to be normal. It is suspected that the problem may be in one of the bearings on the shim arm shaft.

Further, tests, checks and examinations are planned to positively identify the problem and to correct it. The reactor will continue to be shutdown.

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y ,y- , 4 i m i-Robert S. Carter Chief, Reactor Radiation Division

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