ML20090D533

Ro:On 720809,main Fuel Grapple Failed Over Spent Fuel Storage Pool.Caused by Failed Grapple Cable & Sheared Lower Support Pins.Thrust Bearing Lacked Lubrication.Repair of Grapple Head Will Occur When Acceptable Method Approved

ML20090D533
Person / Time
Site:	Nine Mile Point
Issue date:	11/03/1972
From:	Burt P NIAGARA MOHAWK POWER CORP.
To:	Skovholt D US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20090D535	List: ML20090D533
References
6162, NUDOCS 8303020127
Download: ML20090D533 (3)

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NIAGARA MOHAWK POWER CORPORATION t-NIAGARA MOHAWK 1

Nine Mile Point Nuclear Station Post Office Box 32 Lycoming, New York 13093

- . November 3, 1972 -

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Mr. Donald J. Skovholt -

N Assistant Director for Reactor Operations h Division of Reactor Licensing (64/ ' /,%g b ed. ' ,

United States Atomic Energy Commission '

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Dear Mr. Skovholt:

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Re: Provisional Operation License: DPR -

/p' @ %F Docket No.: 50-220 On August 9,1972, a failure of the main fuel grapple occurred. Although the failure occurred over the spent fuel storage pool, the grapple was not loaded and no fuel bundles were damaged or upset.,The grapple was not carrying fuel at the time.

After transferring a fuel bundle from the spent fuel storage racks to the fuel preparation machine, the operator placed the grapple in the full-up or retracted position. As the operator was moving the bridge to the east side of the pool,-he heard the contacts on the grapple and assumed that he had accidently hit the down lever on the grapple. Normal reaction to this was to reach with the other hand and place the icver in the up position to return the grapple to its retracted position. This was done without shifting his eyes from the direction that the crane was moving. Moments

/l /C later, there was a loud bang.

jdv When the operator stopped the crane and looked, the grapple was resting on I the cab floor and the fuel storage rack in the pool below. The operator NN anchored the grapple to the bridge trolley with a rope in order to prevent it fron dropping into the pool. The operator did not observe anything f1' peculiar just prior to or during the grapple failure, therefore, could not g)? give anyother information concerning the problem. $

' Inspection showed that the grapple cabic had failed and the lower support pins between the first section of the grapple and swivel section had sheared. Failure of the cabic allowed the telescoping section to extend out until the grapple head rested on the fuel storage rack in the pool.

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November 3, 1972 Mr- Donald J. Skovholt -

U. S. Atomic Energy Commission I

Shearing of the support pins allowed the first section to drop, first striking the power control panel support structure, and second, hitting the trolley floor. The grapple control boxes attached to the sides of the grapple came to rest on the trolley grating thus supporting the upper sec-tion of the grapple. The grapple was removed from the pool and dismantled.

Investigation showed that the grapple head haci cracks in the welding which This damage was probably due to attached the head to the tubular section. ,

the impact received during this incident.

Three parts of the sheared pins were found. The pins showed that they had sheared due to an impact in the downward direction. The diameter of the pin at the shear was 3/8". 1/8"-wear had taken place between the pin and gimbal section.

The bushings in the grapple that housed the pir.s were worn in an oval shape and had cracks at the 12:00,,3:00 and 9:00 o' clock positions indicating over stress. ,

One arm on the grapple that held the bushing and pin was bent in. This cas caused when the grapple fell and struck the power control panc1 support structure.

The control pancis attached to the grapple were undamaged, but the connec-tion attaching the control cable to the right side control box was crushed between the box and trolley floor. Wires in the cable were damaged.

The stops on the first section of the grapple showed markings where the second section made contact but.no damage was found.

Further investigation showed that the support pins between the thrust bearing and ginbal were worn as badly as the failed pins and impact load could have sheared these as well.

The thrust bearing was disassembled and found to lack lubrication. No pro-vision was supplied to apply any lubricant to this bearing. The balls and races were worn and had frettic corrosion.

The cam type limit switch which operates the hoist drum and limits both the fully extended and fully retracted grapple positions was found in the permissive position although the hoist drum was wound up to a position above the retracted position. It is felt that this condition can casily occar C as the dynamic breaking effects from jogging the hoist near its upper limit can retract the grapple slightly above its upper linit. If the grapple is far enough beyond its upper limit, the limit switch becomes inoperable and ,

the grapple hoist can be raised or lowered from this position.

Another limit switch was nounted on the top of the grapple and is called the overtravel switch. This switch was removed when the grappleAlthough was removed the from the pool and could not be checked for proper adjustment. .)

switch was operable cicctrically, it is assumed that this switch was out of adjustnent.

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9 9 November 3, 1972 Mr$ Donald J. Skovholt .

U. S. Atomic Energy Commission t The load cell was checked for operability with a 1650 lb. Ioad. Although the load cell did interrupt the hoist circuit at 1650 lbs., the hoist continued coasting for-a short period of time until the mechanical brake stopped travel. No dynamic breaking occurs when the grapple is beyond its overtravel limit. The load on the cable was found to be 2J00 lbs.,

The vendor 1.as been contacted conce'rning thi_ " matter. They have recommended the following system modificatie"a for preventing this incident in the , _

future:

1. Installation of a torque limiter between the hoist motor and the gear box to the hoist drive.

2. Change the dynamic breaking timer from a one second duration to 0.5 second duration. '

. 3. Reconnect main hoist master switch wiring such that full hoisting speed is reduced ~to half hoisting speed electrically when any vertical limit trip is indicated _. , - .

These modifications have not yet been accepted by Niagara Mohawk. The major area of concern is the reliability of torque limiting devices.

Further analysis of system modifications are underway.

On the matter of weld crack damage to the grapple head, the vendor has not yet resolved a final recommendation. There is concern over in plant re-plating methods after welding repairs. Testing of nickel plated specimens similar to the material used on the grapple are underway. Repairs-to the grapple head will be initiated as soon as an acceptable method is approved. . . .

Very truly yours, A) m d . 8 8 Is. E M .,,g/, ,

P. Allister Burt General Superintendent Nuc1 car Generation .

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