ML20086E189

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Ao:On 740206,during Closure of Reactor Vessel,One Bolt on Control Rod Drive 30 in Penetration 11 Failed to Engage Threads of Flange.Caused by Failure of Three Cap Screws & Two Dowel Pins.Procedure Sequence Revised
ML20086E189
Person / Time
Site: Fort Saint Vrain Xcel Energy icon.png
Issue date: 02/12/1974
From: Swart F
PUBLIC SERVICE CO. OF COLORADO
To: Oleary J
US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20086E183 List:
References
NUDOCS 8312060250
Download: ML20086E189 (4)


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Information Report No. 1 Facility: Fort St. Vrain Nuclear Generating Station

Subject:

Control Rod Drive Serial No. 30 - Mechanical Failure Report Date: February 12, 1974

1. Scope This report is submitted to describe the mechanical failure in Control Rod Drive Serial No. 30 and the conditions leading up to the discovery of the failure. This report will also describe the steps taken to determine the cause of the mechanical failure. and a change in an associated procedure made to prevent one possible cause of the failure from occuring in the future, i 2. Description During the early evening hours of Wednesday, February 6,1974,

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activities preparatory to closing the reactor vessel were proceeding on.

the reactor refueling floor. One of these activities was bolting and torquing the primary closures in the refueling penetrations. As the bolts were being run in .on the No. 30 Control Rod Drive (CRE) in the No.11 penctration, one bolt failed to engage the threads of the flange in the penetration.

To investigate the cause of this problem, it was necessary to remove the CRD from the penetration. To facilitate.its removal the breaker to the drive motor was racked in and the special by-pass box, required for CRD removal, installed in the instrument cabeling at the penetration. The control rod pair was withdrawn, per procedures, to the normal out-limit, at 190" withdrawn. At this point, the normal up-limit was bypassed and the control rods were withdrawn to the extreme up-limit,~236". At this time the " shipping stop" tool was inserted to lock the control rods in position to prevent their moving during.CRD transport to storage.

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4 To check proper shipping tool insertion into the drive mechanism, the brake fuse was removed and the instrumentation checked for control rod movement. The position instrumentation was observed for movement for approximately two (2) minutes with no movement detected. At this time the breaker was racked out and both the power and instrument cable were dis--

connected. Because of the hour, the crew went-to dinner,' leaving.the CRD in_the penetration. Approximately 45 minutes later, as the CRD was being hoisted from the penetration, one of the men noticed the control rods were fully' extended -from the drive and not withdrawn as they were supposed to be.

Withdrawal of the CRD was immediately's. topped.

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To be assured the extended control rods were still in the control rod channels in the core, one man entered the reactor vessel to investigate.

The control rods were found to be engaged. The CRD was then lowered into the penetration to its normal position and all electrical cables reattached.

Position instrumentation indicated the control rods were fully inserted. The shipping tool was removed and the brake fuse inserted and an attempt made to withdraw the control rods. The rods withdrew approximately 0.5" when the out-limit light came on. The control rods were then reinserted.

The reactor remained in a shut down condition through the entire exercise.

3. Cause Determination and Resultant Damage On Thursday, February 7,1974, a procedure was developed to verify proper gear train rotation and to verify position indications. Gear train rotation was verified but position indication proved to be erratic and unreliable.

The control rods were then withdrawn for a given length of time which would locate the top of the control rods within the guide tubes of the CRD. The shipping tool was inserted and the CRD hoisted approximately two feet by the reactor building crane. Core entry was made for' visual inspection.

It vis found that the control rods were still fully inserted. The CRD was lowered to its normal position.

It was decided to remove the CRD with the control rods extended.

Necessary control room / refueling floor communications were established and the CRD removed using the reactor building oyerhead crane. Upon removal, the control rods were disconnected and stored and the CRD positioned for disassembly. A spare CRD was installed in penetration No. 11 and the control rods inserted.

Upon partial disassembly of the CRD drive end, it was found that the three bolts and two dowel pins that mount the gear that engages the shipping tool to the cab' e drum hub casting were broken, allowing the cable reel to turn with the shipping tool engaged. The coupling between the position indicating potentiometers and limit switches and the drive mechanism was also broken.

4. Analysis of Damage The potentiometer coupling failure _is explained as follows:

As the control rods fell into the core, their.

travel was not controlled by dynamic braking because the CRD had been disconnected electrically.

.As the rods reached the bottom of their travel, the drive mechanism continued.to rotate and rewind the cables in reverse on the drum, driving the wiper of the POT's against their stop, causing the coupling

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to fail.

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Closer visual investigation of the gear that was sheared loose revealed one cracked tooth.

The failure of the three cap screws and two pins and the impli-cations of the failure is still under investigation.

The supplier of the CRD's (General Atomic) has been requested to make an analysis of the failed area'to determine if the cap screws and dowel pins would fail in shear should the CRD drive motor be energized with the shipping tool in place. An independent analysis will also be made.

General Atomic has also been requested to consider a redesign of the shipp'ing tool so that it would fail before any part in the gear train.

J A review of the procedure used to remove and insert the CRD s in the penetrations nas made and the procedure sequence revised to prevent possible energizing of the CRD drive motor with the shipping tool inserted.

We are also inspecting a representative number.of CRD's that'have been through the Preltsinary Operating Test program for any sign of damage, as an assessment of the condition of other rods and to ensure that we do not experience a generic problem.

Our analysis of the incident indica'tes the health and. safety of the public was at no time compromised by the failure. The reactor remained in a shut down condition throughout the incident. The mechanical failure _

in the CRD did not prevent the control rods from being inserted into the core nor was the CRD rendered inoperable.

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February 13, 1974 Mr. John O' Leary, Director Directorate of Licensing United States Atomic Energy Commission Washington, D. C. 20545

Subject:

Information Report No. 1 Control Rod Drive Serial No. 30 Mechanical Failure Fort St. Vrain' Station

Dear Mr. O' Leary:

The attached report is submitted as an informational report only to keep the AEC apprised of the status of Fort St. Vrain.

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Very truly yours, j':

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(7 m Nw )s. $wdh Frederic E. Swart Superintendent

. Fort St. Vrain Nuclear Generating Station FES:11 cc: Director, Region IV, Denver r

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