ML19312E614
| ML19312E614 | |
| Person / Time | |
|---|---|
| Site: | MIT Nuclear Research Reactor |
| Issue date: | 05/27/1980 |
| From: | Bernard J, Lisa Clark MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE |
| To: | |
| References | |
| NUDOCS 8006050384 | |
| Download: ML19312E614 (2) | |
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.y-NUCLEAR REACTOR LABORATORY W,-
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- ..,-:o, g6171253-4211 tir+c:or ci =eac:ce c e a: :-s May 27, 1980 Mr. Scyce H. Gri'er, Director U.S. lluclear Regulatory Commission Region #1 631 Park Avenue Ring of Prussia, Pennsylvania 19406 Attention:
Mr. E. Green =an, Reactor Inspector Re: Reportable occurrence 50-20/80-2, License R-37:
Correction Gentle:en:
Please replace page 2 of Reportable Occurrence 50-20/80-2, dated May 23, 1980, with the enclosed revised page 2.
The purpose of the revision is to correct the second paragraph of section 7 which should have read " routinely measured every six months", not every six weeks. The blade drop ti=es are measured every six weeks, the blade thick-nesses every six onths.
The wording of the last few lines of the final paragraph os page 2 has also b.:en revised te clarify the description of the electromagn:. tic coupling of the blade and 1es purpose.
Sincerely, dk k
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GA-Lincoln Clark, Jr.
Director of Reactor Operations l
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Enclosure:
Page 2 (revised) 002 l
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Reportable Occurrence Report 50-20/80-2 May 23, 1980 Page 2 (revised) 6.
Description of Occurrence: A reactor startup was being conducted in accor-dance with approved operating procedures.
The startup was halted, in accor-dance with these procedures, when the shim bank position exceeded the esti-
=ated critical position by 0.50- inches.
Each of the six shim blades was then sequentially inserted while =enitoring the reactor power level.
The level decreased as expected for blades #2 through 06.
However, no decrease was observed-on insertion of shim blade #1.
Subsequent ceasurements were then conducted and it was determined that blade #1 was evidently only oper-ating from the full-in position to a point about 8.00 inches above the core bottom.
The reactor was then shut dcwn using the standard procedure. All blades inserted fully. Reactor power did not exceed 1.25 Kw during the above evolution.
A visual inspection was i= mediately cade of blade #1.
It was found that, as ' suspected from the observations =ade during the startup, blade #1 was being picked up by the electro =agnet that couples it to the drive rechanism.
Hewever, at a height of about 8.00 inches, the blade was separating from the magnet. Once separated the blade did not, as would be expected, fall back into its slot next to the core housing.
It did, hcwever, move in nor= ally whenever the drive mechanism was driven in so as to engage the blade.
7.
Description of Apoarent Cause of Occurrence: This occurrence was apparently due to the presence of a small particle, possibly more than one, of foreign
=aterial that impeded the motion of the blade.
Each shim blade rides in one of six 0.375" slots that border the outer perimeter of the core.
The blades are nominally 0.300" thick.
They are routinaly measured every six months (more frequently when initially in-stalled); thicknesses are in the range 0.320 - 0.340".
Measurements are made under water with a gage that fits over the full 7" width of the blade, which accounts for the values exceeding nominal.
There is no trend toward thickening.
Each blade is attached by a webbed offset plate to a weighted guide piece that moves vertically in a tube machined to a 1/16 inch clearance (see Figure 1).
The use of this weighted guide piece insures both that the blade will drop rapidly and that, should there be any misalignment with the drive, the resulting wear will not occur on the blade. An ar=ature, with a flat upper surface, sits atop the guide piece.
The blade may be raised or icw-ered by =ating the armature to the lower surface of an electretagnet that is connected to the drive rechanism. Hence, the only vertical support provided to the blade is via an electro =agnet.
This means that, in the event of any friction between the blade and its slot or between the weighted guide piece and its tube, the blade will readily separate from the drive mechanism during withdrawal.
The reason for this design is to provide for decoupling and in-sertion of the blade by gravity in the event of a scram signal.