ML20112E109

Informs of Possible Unsafe Incident in Operation of Reactor. on 701010,reactor Scram (Doors Open) Received.Shim Blade 4 Failed to Drop Into Core.Licensee Believe That Failure of Shim Blade Caused by Malfunction in ball-nut Mechanism

ML20112E109
Person / Time
Site:	Neely Research Reactor
Issue date:	11/04/1970
From:	Kirkland R Neely Research Reactor, ATLANTA, GA
To:	US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20112E009	List: ML20112E021 ML20112E086 ML20112E088 ML20112E096 ML20112E109 ML20112E128 ML20112E146 ML20112E171 ML20112E184
References
NUDOCS 9606050151
Download: ML20112E109 (2)

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'T Attention: Division of Reactor Licensing I

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Reference:

Docket 50-160, License R-97 Gentlemen:

This is to inform you of a possible unsafe incident in the operation of the Georgia Tech Research Reactor. At 1254 hours0.0145 days <br />0.348 hours <br />0.00207 weeks <br />4.77147e-4 months <br /> on October 10,

' 1970, a reactor scram (Doors Open) was received. Shim blade No. 4 failed to drop into the core; it remained at 36 where it was prior to the scram. The remaining 3 shim blades inserted normally and l

the reactor was shut down. An immediate investigation was begun to determine the cause, l

This is the third shim blade drop failure at the GTRR. The first occurred on February, 1970, when No. 2 did not drop. On August 12, i 1970, No. 4 shim failed t o drop.

A number of attempts were made to repeat the " sticking" of the No. 4 blade at 36 . Unlike the previous two blade drop failures, we were unable to reproduce a " stuck blade" at 36 . Before disassembly of the i

drive mechanism, the reactor core was examined using a recently l acquired boroscope. We were able to verify that no mechanical interference existed between No. 4 shim blade and any fuel or experimental position extensions.

Disassembly of the drive mechanism did not reveal any readily apparent mechanical defect. Some small metal chips were found on the ball-nut screw. We contacted the manufacturers of the ball-nut, j Saginaw Steering Gear Division of General Motors in Saginaw, Michigan.

l It was their opinion that, after six years of operation, some wear would occur on the ball-nut and failures such as we have recently experienecd could be expected. In the manufacturing process, the ball-nut is intentionally made less hard than the screw so that wear occurs'on the ball-nut first. Additionally, any particulate

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materialthatmihhtgetintotheball-nutmechanismcouldcausea /

malfunction.

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AEC PU6UC DOCUtM ROCW U. S. Atomic Energy Commission November 4,.1970 We believe that this failure of the No. 4 shim blade - and very possibly the August 12 failure - was caused by a malfunction in the ball-nut mechanism. We have ordered replacement ball-nuts for all four of the drive mechanisms. They will be scheduled for installation soon after receipt. Additionally, we are going to install a flexible boot on each drive to prevent any particulate matter from reaching-the ball-nut.

On October 20, 1970, the No. 4 drive mechanism was installed on the-reactor and a series of 25 test drops was initiated. There were yo problems noted, and, at 1345 hours0.0156 days <br />0.374 hours <br />0.00222 weeks <br />5.117725e-4 months <br />, the reactor was started up.

The above matters were discussed with staff members of the USAEC Atlanta office on several occasions. We believe that the problem will be satisfactorily resolved when the new ball-nuts are installed.

Should any information subsequently develop to alter the above conclusions, we will' advise you.

Very truly yours, E

b .

Robert . Kirkland Reactor Superv.isor RSK:jh cc: Nuclear Safeguards Committee Mr. John G. Davis, USAEC, Atlanta i

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