ML20085M662

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RO 50-267/75/17A:on 750912,orifice Valve Assembly Associated W/Control Rod Drive Observed Falling Vertically from Normally Installed Position.Caused by Improper Final Assembly of Control Rod Drive Unit
ML20085M662
Person / Time
Site: Fort Saint Vrain 
Issue date: 12/18/1975
From: Hillyard H, Mathie F, Swart F
PUBLIC SERVICE CO. OF COLORADO
To: Howard E
NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML20085M657 List:
References
P-75030, R)-50-267-75-17, RO-50-267-75-17, NUDOCS 8311090159
Download: ML20085M662 (6)
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v P. O. Box 361, Platteville, Colorado 80651 December 18, 1975 Fort St. Vrain IEFILE COPY Unit No. 1 P-75030 3

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Mr. E. Morris Howard, Director Nucicar Regulatory Co= mission Region IV R

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Suite 1000 Arlington, Texas 76012

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REF: Facility Operating L No. DPR-34 Docket No. 50-267

Dear Mr. Howard:

Enclosed please find a copy Unusual Ehrent Report No. 50-267/75/17A, Final, submitted per the requirements of the Technical Specifications.

Very truly yours, Frederic E. Swart Superintendent, Nuclear Production Fort St. Vrain Nuclear Generating Station FES/alk cc: Mr. Roger S. Boyd

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REPORT DATE:

December 18, 1975 UNUSUAL EVENT 75/17A Page 1 of 5 OCCURRENCE DATE:

September 12 1975 FORT ST. VRAIN NUCLEAR CENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO P. O. BCX 361 PLATTEVILLE, COLORADO 80651 REPORT No. 50-267/75/17A Final IDENTIFICATION OF OCCURRENCE:

At approximately 1030, September 12, 1975, the orifice valve assembly associated with control rod drive serial number 010 was observed to have fallen vertically downward from its normal, installed position on the control rod drive unit while it was positioned in a special work stand located over an equipment storage well on the refueling floor. This control rod drive unit was in position in the work stand,for approximately two days and had not been moved prior to the orifice valve becoming disengaged.

This was identified cs an unusual event per AC 7.6, Non-Routine Reports, Section

. C.2 of the Fcrt St. Vrain Technical Specifications.

CONDITIONS PRIOR TO OCCURRENCE:

Steady State Power Routine Shutdown Hot Shutdown Routine Load Change I

Cold Shutdown Other (specify)

Refueling Shutdown Routine Startup The major plant parameters at the time of the event were as follows:

Power RTR MWeh ELECT MWe Secondary Coolant Pressure psig Temperature

'F Flow

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Primary Coolant Pressure psig Temperature

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  • F Core Outlet
  • No bearing"on this event.

Flow f/hr.

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UNUSUAL EVENT 75/17A Page 2 of 5 DESCRIPTION OF j

OCCURRENCE:

All of the control rod drives at Fort St. Vrain were in the process of modifica-tion to eliminate internal gas flow leakage paths (CWP-51). Control rod drive 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 i

modifications. All operations and handling associated with this control rod l

drive were routine and normal.

Upon completing the prescribed approved modifications to this unit, a functional operating test was performed using established procedures. All tests resulted in normal response from the control rod drive unit.

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

The dust cover (6) which protects the bolted support assembly for the orifice valve and actuator shaf t was removed for inspection. All of the parts were re-covered intact: ' 1 self-locking nut (42), 6 believille washers (41),1 baaring washer (49), and 1 stop piste (15) (see drawing SLR D1201-400G attached). The contr'ol rod drive unit was then further disassembled as: required to allow addi-tional internal inspection of the orifice drive mechanism.

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APPARENT CAUSE 0F OCCURRENCE:

Design Unudual Service Cond.

Including Environ.

Manufacture Component Failure Installation /Const.

X Other (specify)

Ing_reper final assembly of the-Operator r

Procedure control rod drive unit at time of manufacture.

The apparent cause of this occurrence was failure to fully engage the lock nut with the threaded portion of the support shaf;.,during initial fabrication. Tha locking action of the lock nut requires full engagement.

Improper assembly allowed the lock nut to disengage from the shaft.

ANALYSIS OF OCCURRENCE:

All of the bolted support parts. (12, 42, ;4h, = 41, and 15) (see attached SLR-D1201-400)~ were removed from the control rod drive'and closely inspected.

No abnormal indications of any kind could be found. The orifice valve' support 3

shaf t and actuator were removed from the control rod drive. unit and carefully I

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UNUSUAL EVENT 75/17A Page 4 of 5 ANALYSIS OF OCCURRENCE (centinued):

inspected, with considerable emphasis being given to the threaded end of the shaft to which the bolted support is attached.

All threads looked normal; no signs of distress or abnormality could be seen. All of the parts were then reassembled to determine that the physical mating of all parts was normal.

This was done satisfactorily and all parts fit properly and smoothly.

Further, the self-locking nut (42) threaded down to its proper position and tightened snugly in such a way that a vrench was required to remove it.

All these actions were normal and proper.

Twenty-nine control rod drive assemblies remaining to be modified were inspected for proper installation of the orifice valve retaining lock nut.

Of the twenty-nine inspected, twenty-seven were inspected by an x-ray photograph, and the two remaining were inspected visually. None were observed to be improperly installed (reference GA FIR 4687, dated Decembe.r 4, 1975).

It was concluded that the self-locking nut had never been tightened down properly during initial assembly of the unit, allowing vibrations and pulsing of the nearby orifice drive motor to cause the nut to, back off sufficiently to release the orifice valve.

This threaded support is essentially unstressed during normal installation and operation of the control rod drive allowed the nut. free move-ment.

CORRECTIVE ACTION:

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Because subsequent inspection of the twenty-nine control rod drive assemblies showed that they were properly assembled, it was concluded that the loose lock nut of serial number 010 represents a single, random failure.

Since the opera-bility of the orifice valve was in no way compromised by this failure, and that no potential threat to the health or safety of the public was involved, no further corrective action is anticipated.

FAILURE DATA /SIMILAR REPORTED OCCURRENCES:

,m None PROGRAl2fATIC IMPACT:

None CODE IMPACT:

5 None

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UNUSUAL EVENT 75/17A Page 5 of 5 Submitted by:

t## [s H. W. Ill11yadi, Jr. /

Technical Se"rvices Supervisor Reviewed by:

Ard Frank M. Mathie Superintendent, Maintenance Approved by:

F/ederic E. Swart

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Superintendent, Nuclear Production 7

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