ML20090D744

Ro:On 710830,following Scram,Steam Flow Indication of One of Two Main Steam Lines Low.Investigation Revealed Erroneous Pressure Differential Readings Produced by Primary Measuring Device.Caused by Open Crack in Tubing

ML20090D744
Person / Time
Site:	Nine Mile Point
Issue date:	12/13/1971
From:	Burt P NIAGARA MOHAWK POWER CORP.
To:	Morris P US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20090D748	List: ML20090D744
References
5507, NUDOCS 8303020334
Download: ML20090D744 (2)

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NI AG AR A MOH AWK POWER CORPORATION NIAG ARA MOHAWK Nine Mile Point Nuclear Station Post Office Box 32 Lycoming, New York 13093

,,6,# 4Ir s December 13, 1971 /

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Dr. Peter A. Morris, Director '

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Division of Reactor hicensing ( [g y 4.rf.,f'W]d /

United States Atomic Energy Commission - ec$

Nashington, D. C. 20545 44YO/'2^r *di

Dear lir. !! orris :

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Re: Provisional Operating License DPR-17 Docket No. 50-220 Nine Mile Point Nucicar Station After returning to power following a " scram" on August 30, 1971, it was observed that the steam flow indication of one of the tuo main steam lines was only two thirds the value of the other. Investigation revealed erroneous pressure differential readings being produced by the primary measuring device, a Builders Iron Foundry 24 (21.564) X 12.45 Venturi insert no :le, Model NZ1W.

In this design, a casting having a smooth contour to the " throat" is welded at one end only, fastening it to the inner circumference of the steam pipe and permitting expansion at the unwelded end. A cavity is formed between the steam pipe inner surface and the casting at the throat. Therefore, it is necessary to carry the static pressure at the throat, K2 , through this cavity in tubing.

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The Icading edge of the casting is the unwelded one and therefore, the outside of the tubing carrying the K2 Pressure is surrounded by K, pressure. A Icah in the tubing would reduce the differential pressure and cause the steam flow meter to read low.

Shortly thereafter, with the unit shutdown, examination disclosed an open crack in the tubing at a point where a bend had been provided for expansion. Subsequent examinatica of the similar{'

tube in the other steam line venturi revealed a tight crack in the same location. (lloweve r, it had not apparently started to leak.)

The failed tubes were seamless type 304 per AST'l-A269-63T with nominal dimensions of 7/S" 0.D. and .035" wall thickness. All j,f3 other parts of the assembly, except a stainless steel insert at the f, +hroat, were of carbon steel.

3 Yi ' IIetallurgical analysis of the failed tubes revealed that Il failure had occurred by stress corrosion cracking of heavily sensi-Q}i tired material that had been cold worked prior to sensitization.  ;

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Dr. Peter A. Morris, Director Page 2 Division of Reactor Licensing In manufacture, the entire assemble was welded together and then subjected to a normalizing heat. It is then readily apparent as to how the stainless steel tubes became sensitized.

Replacement of both tubes was made with Inconel tubing, non-sensitized and of slightly heavier wall. The slight bend was not included as stress analysis indicated a lower stress level with straight configuration.

Very truly yours,

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P. A. 13urt Station Superintendent Pall:pw

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