ML20085M552

RO 50-267/75/17:on 750912,orifice Valve Assembly Associated W/Control Rod Drive Observed to Have Fallen Vertically Downward from Installed Position.Caused by Improper Final Assembly of Control Rod Drive Unit

ML20085M552
Person / Time
Site:	Fort Saint Vrain
Issue date:	10/02/1975
From:	Hillyard H, Mathie F, Swart F PUBLIC SERVICE CO. OF COLORADO
To:	Howard E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
References
RO-50-267-75-17, NUDOCS 8311080628
Download: ML20085M552 (7)

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O P. O. Box 361, Platteville, Colorado 80651 October 2, 1975 i

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,.s Nuclear Regulatory Commission 02[.g Mr. E. !! orris Howard, Director

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Region IV Office of Inspection and Enforec=ent

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Suite 1000 nuington, Texas 76012 P.EF: Facility Operating License Nn. DFR-34 Docket No. 50-267

Dear Mr. Howard:

Enclosed please find a copy of Unusual Event Report. No. 50-267/75/17, Preliminary, submitted pcr the requirements of the Technical Specifi-Cations.

Very truly'yours,

,Frederic E. Svart Superintendent, Nuclear. Production

~rort St. Vrain Nuclear Generating Station FES/alk Mr. Roger S. Boyd

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8311080628 751002 PDR ADDCK 05000267 S

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_ 9 Unusual DISTRIBtTfIONS Events and Abnormal Unusual Operating Monthly occurrence Occurrence

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REPORT DATE:

October 2, 1975 UNUSUAL EVENT 75/17 Page 1 of 5 OCCURRENCE DATE:

September 12, 1975 FORT ST. VRAIN NUCLEAR GENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO P. O. BOX 361 PLAITEVILLE, COLORADO 80651 REPORT No. 50-267/75/17 Preliminary IDENTIFICATION OF OCCURRENCE:

At approximately 1030, September 12, 1975, the orifice valve assembly associated with control rod drive serial nomber 010 was observed to have fallen vertically downward from its nor=al, install'ed position on the control rod drive unit while

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it was positioned in a special work stand located over an equipment storage well This control. rod drive unit was in position in the work on the refueling floor.

stand

• for approximately two days and ha'd not been moved prior to the orifice valve becoming disengaged.

This has been identified as an unusual event per AC 7.6', Non1 outine' Reports, R

Section C.2 of the' Fort St. Vrain Technic,a1' Specifications.

CONDITIONS PRIOR Routine Shutdown TO OCCURRENCE:

Steady State Power Hot Shutdown

' Routine Load Change Other (specify)

X Cold Shutdown Reft $eling Shutdown Routine Startup The major plant parameters at the time of the event were as follows:

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Wth Power RTR E E'CT We psig Secondary Coolant Pressure

'F Temperature f/hr.

Flow Primary Coolant Pressure psig

• P Core Inlet-Temperature

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IMUSUAL EVENT 75/17 Page 2 of 5 CONDITIONS PRIOR TO OCCURRENCE (continued):

'F Core Outlet Primary Coolant Temperature (continued) f/hr.

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• No bearing on this event.

DESCRIPTION OF OCCURRENCE:

Vrain are in the process of modifica-All of the control rod drives at Fort St.

Control rod. drive tion to eliminate internal gas flow leakage paths (CWP-51).

serial number 010 had been removed from region five of the PCRV and placed in a special work stand on the refueling floor for implementation of the necessary All operations and handling associated with this control rod modifications.

drive were routine and normal.

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a functional Upon completing the prescribed approved modifications to this unit,All tests resulted.

operating test was performed using established procedures.Just prior to preparing in normal response from the control rod drive unit.

this unit for return to the PCRV, the orifice valve was observed to have fallen from its normal position at the lower end of the control. rod drive unit.

The dust cover (6) which protects the bolted support assembly for the orifice -

All of the parts were re-valve and actuator shaft was removed for inspection.

covered in tact: 1 self-locking. nut (42), 6 belleville washers (41),.1 bearing w:asher (49), and 1 stop plate (15) (see drawing SLR D1201-400G attached). The

control rod drive unit was then further disassembled as required to allow addi-tional internal inspection of the orifice drive mechanism.

APPARENT CAUSE Unusual Service Cond.

OF OCCURRENCE:

Design Including Environ.

Component Failure.

Manufacture Installation /Const.

X Other (specify)

Improper final assembly of the Operator Procedure control rod drive unit at time _

of manufacture.

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The apparent cause of this occurrence is failure to fully eng ge the lock nut The locking action of _ the

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with the threaded portion of the support shaf t'.

lock nut requires full engage.ient so that this improper assembly allowed the lock nut to disengage from the shaft.

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O o UNUSUAL EVENT 75/17 Page 4 of 5 ANALYSIS OF OCCURRENCE: All of the bolted support parts (12, 42, 49, 41, and 15) (SLR-D1201-400) were No' abnormal indi-removed from the control rod drive and closely inspected. The orifice valve support shaf t and ac-cations of any kind could be found. tuator were removed from the control rod drive unit and carefully inspected, with considerable emphasis being given. to the threaded end of the stiaft to All threads looked normal; no signs of which the bolted support is attached. All of the parts were then reassembled distress or abnormality could be seen. This was done to determine that the physical mating of all parts was normal. Further, tihe self-satisfactorily and 'all parts fit properly and smoothly. (42) threaded down to its proper position and tightened snugly, locking nut This is nor-locked such that a wrench was required to remove it. was in fact mal and proper. The only conclusion which can be drawn in view of the observed facts is that the self-locking nut had never been tightened down properly during initial Because the nut was not properly tightened, vibrations assembly of the unit. and pulsing of the nearby orifice drive motor could have caused the nut to This threaded support is back off sufficiently to release the orifice valve. essentially unstressed during normal installation and operation of.the control rod drive which would allow the nut free movement if it were,not preperly tightened. CORRECTIVE ACTION: Seven spare control rod drives plus the 25 units still to be removed from the PCRV will be inspected to verify that the orifice valve support bolt is properly assembled, and that the retaining nut is tightened to the proper It is felt that this unusual event is an position on the mating threads.If any additional discrepancies are found, the additional isolated case. units, presently installed in the PCRV, will be removed for inspection. FAILURE DATA /SIMILAR REPORTED OCCURRENCES: None PROGRAMfATIC IMPACT: None ,0DE IMPACT: ?- l e, v

O O ~ ~ UNUSUAL EVENT 75/17 Page 5 of 5 kav C. # a _/'n-Submitted By: Harvey W. Hillyard, Jr/ Technical Services Supervisor l-Reviewed By: Frank M. Mathie Superintendent Maintenance f? e I Approved By: / pederic E. Swart Superintendent, Nuclear Production 9 e S e 0 a t e e B e l i D}}