ML12171A512

Pre-Filed Hearing Exhibit NYS000378, History of Westinghouse Model 44 Steam Generators (ML013100106)(from NRC Website (Nov. 2, 2000))

ML12171A512
Person / Time
Site:	Indian Point
Issue date:	06/19/2012
From:	Office of Information Services
To:	Atomic Safety and Licensing Board Panel
SECY RAS
Shared Package
ML12171A508	List: ML12171A511 ML12171A512 ML12171A513 ML12171A514 ML12171A516 ML12171A517 ML12171A518 ML12171A519 ML12171A520 ML12171A521 ML12171A524 ML12171A525 ML12171A526 ML12171A527 ML12171A528 ML12171A529 ML12171A530 ML12171A533 ML12171A534 ML12171A536 ML12171A537
References
RAS 22624, 50-247-LR, 50-286-LR, ASLBP 07-858-03-LR-BD01
Download: ML12171A512 (2)

Text

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NYS000378 Submitted: June 19, 2012

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Generators

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Plants Commercial Operation Dates Replacement Dates Ginna 07/1970 06/1996 Point Beach 1 12/1970 03/1983 H. B. Robinson 03i197i i0ii984 Point Beach 2 10/1972 12/1996 Turkey Point 3 12/1972 04/1982 Turkey Point 4 09/1973 05/1983 Indian Point 2 08/1974 N/A Indian Point 3 08/1976 06/1989 uegradation Mechanisms in Westinghouse Modei 44 Steam Generators Model44 Steam Generators (SGs) use alloy 600 material for tubing. The tubes are supported vertically by the thick tube sheet at the bottom of the steam generator and laterally by drilled-hole tube support plates which are made of carbon steeL Corrosive products (typically magnetite) tend to accumulate in the crevice bet"'Neen tube a.'1d tube support plates because of the d..ri!!ed-hole configuration and Lhe susce-ptibility of the carbon steel to corrosion. The corrosive products have caused tube denting which resulted in axially a.Tid drcum.ferentially oriented stress corrosion cracking in tubes. Th.is degradation rnechanism was the predominant reason for tube plugging in model 44 SGs.

The tubes in Model 44 SGs were mechanically rolled for only a few inches into the tubesheet starting at the primary (lower) face of the tubesheet. As a result, a crevice exists between the tube and the tubesheet for the portion of the tube within the tubesheet that is not expanded. Since the tube was mechanically rolled into the tubesheetj the expansion transition is conunonly referred to as a roll transition. Plants with SG tubes t.~at are or..ly roll expanded for a srr..all distance within t.lJ.e tubesheet :are referred to as partial-depth hardroll plants. The crevice region has been a site for corrosion. Degradation in the crevice andior at the roll transition has led some plants to re-roll the tubes above the original rolled region to provide a new pressure boundary free of any detectable defects. By so doing, the licensees have been able to apply alternate repair criteria, such as the F-star criterion, in which tubes with degradation in a certain specified distance below the top of the tubesheet or the bottom of the roll transition (whichever is lower),

~ay remain in service. Tubes accepted for continued service on the basis of the F-star criterion have structural !Llld lea...lcage integrity consistent with regulatory criteria.

In general, plants wit.1. SGs that have only been pw.1:ially expanded i...'1.to the tubesheet have exlllbited verj liitle circumferentially oriented degradation either at the expansion transition or at the top of the tubesheet.

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}v1ost of these pla."lts have exhibited axially oriented primary water stress corrosion craddng at Lhe roll trartsition, axially oriented outside diameter stress corrosion cracking in the tubesheet crevice region, andior intergranular attack in the tubesheet crevice region.

The small u-bend regions of the tubes have exhibited axial and circumferential cracking. The degradation is caused by excess ovality during tube fabrication process, residual stresses, and tube denting at the top tube support plate. However, some plants had preventively plugged the row of tubes with the smallest u-bends (innermost row)_

There were wastage problems in early operation of mcdel44 SGs (see tube rupture events below). With improved secondary side cherr.dstry, the tube wastage problem was essentially elirrJnated.

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