ML20125C764

Discusses Technical Aspects of Rationale Used to Support Steam Generator Tube Interim Plugging Criteria for Plant & Independent Conclusions on Viability of Independent Plugging Criteria for One Fuel Cycle

ML20125C764
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	12/09/1992
From:	Shao L NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
To:	Beckjord E NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
References
NUDOCS 9212140066
Download: ML20125C764 (7)

Text

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8 n NUCLEAR REGULATORY COMMISSF pg y t .t W ASHINGTON, O. C. 20555

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MEMORANDUM FOR: Eric S. Beckjord, Director Office of Nuclear Regulatory Research FROM: Lawrence C. Shao, Director Division of Engineering Office of Nuclear Regulatory Research

SUBJECT:

INTERIM PLUGGING CRITERIA FOR TROJAN NUCLEAR PLANT The Division of Engineerir.g has provided a discussion of the key technical aspects of the rationale used to support steam generator tube interim plugging criteria (IPC) for the Trojan nuclear plant and to provide independent conclusions on the viability of IPC for one fuel cycle. The IPC apply only to the specific case of outer diameter stress corrosion cracking (ODSCC) and intergranular attack (IGA) at tube support plate (TSP) intersections in the steam generators. The technical rationale presented in the enclosure are based on data and analyses available from NRC research, Trojan plant operating 8 experience, and the technical literature. The enclosure also reflects staff technical experience and opini m s. The Office of Nuclear Reactor Regulation (NRR) has been consulted on technical details regarding IPC during the preparation of this document. The report endeavors to maintain a distinction between staff opinion and published data.

Based on the discussion presented in the enclosure, the Division of Engineering concludes that continued operatian of the Trojan plant for one fuel cycle is justified. This justification is based on:

(1) Examination of steam generator tubes removed from service at the -

Trojan plant which hr.s revealed cracks that are generally confined to the tube ssGport plate intersections.

(2) Burst test results from cracked tubes removed from service at the

• Trojan plant which showed burst pressures well in excess of main steam line braak (MSLP) prersure.

(3) Stress corrosion crack growth rate results which indicate that incremental growth of the cracks to a critical length beyond the tube support plate during one fuel cycle is unlikely.

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Eric S. Beckjord 2 DEC 0 sign (4) The probability of a main steain lir,e break, the key initiating event for a steam generator tube rupture is very low for one fuel 4 cycle.

(L /h Lawre ce C. Shao, Director Division of Engineering Office of Nuclear Regulatory Research

Enclosure:

As stated cc: J. Taylor J. Sniezek T. Speis J. Heltemes W. Minners T. Murley F. Miraglia W. Russell J. Richardson J. Strosnider L. Kokajko J. Fouchard

.- . .- . -- ~. ~

ENCLOSURE Discussion of Technical Rationale for Steam Generator Tube Interim Plugging Criteria (IPC) at The Trojan Nuclear Plant The purpose of this report is to provide a discussion of the key technical aspects of the rationale used to support steam generator tube interim plugging criteria (IPC) for the Trojan nuclear plant and to provide independent

onclusions on the viability of IPC for one fuel cycle. The IPC apply only to me specific case of outer diameter stress corrosion cracking (0DSCC) and intergranular attack (IGA) at tube supper' plate (TSP) intersections in the steam generators. The technical rationale presented in this report are based on data and analyses available from NRC research, Trojan plant operating experience, and the technical literature. The report also reflects staff technical experience and opinions. The Office of Nuclear Reactor Regulation (NRR) has been consulted on technical details regarding IPC during the preparation of this document. The report endeavors to maintain a distinction between staff opinion and published data.

The rationale presented in this report are based on technical considerations which we believe are adequate to justify IPC for one fuel cyM e. Cubsequent operation with IPC would require additional review after completion of one cycle and would require consideration of additional information developed at that time. Longer term technical considerations, suc3 as reliability and sensitivity of NDE techniques for steam generator tuba inspection, are the subjects of on-going and new NRC research which is being coordinated with NRR as part of an overall steam generator tube alternate plugging criteria (APC) action plan.

(1)

Background:

Steam generator tube structural integrity guidance provided in Regulatory Guide 1.121 has generally translated into a 40% through-wall

" plugging limit" for flaws in steam generator tubes as part of the plant technical specifications. However, evidence from pulled steam generator tubes at several plants has revealed numerous short cracks at TSP intersections which are greater than 40% through-wall and yet can withstand pressures in ex ess of three times cperating as required by Regulatory Guide 1.121. It has therefore been argued t,y the industry that the 40% plugging limit is conservative, at least for the case of short axial ODSCC/ IGA confined to TSP intersections. Burst testing of cracked tubes removed from service at the Trojan plant has resulted in burst pressures of at 1 east a factor of two in excess of main steam line break (MSLB) pressure, evei for through-wall cracks.' NRC research results on tubes with machined and chemically-induced flaws support the contention that the tubes retain ngnificant structural integrity even for up p through-wall cracks, providtd that the cracks are short. From this research "short" can be defined as less than 0.5 inches, which is the length of a near through-wall crack needed to burst for 7/8-inch diameter, 0.050 inch wall thickness tubing under MSLC differential pressure' (see Figure 1). The burst pressure is defined as the pressure required to penetrate the tube wall. Tube burst then, can result in either small or large leakage. T'ibe burst results when the differential pressure acts from the primary side. Tube rupture relates to a significant opening under Nrst 1

pressure with a consequent increast in leak rate and potential ductile crack advance. Burst failure is differentiated from collapse failure where the d1fferential pressure acts from the secondary side.

Based on the arguments presented previously and supporting analyses, the industry has proposed an alternative to the traditional 40% depth-based guidance, the so-called alternate plugging criteria (APC), for steam generator tubes. The APC are based on correlations between the voltage amplitude recorded during eddy current tube inspections with a bobbin coil and subsequent measurements of the tube burst pressures and leak rates. The APC are also currently restricted to 005CC at TSP intersections. A modified version of steam generator tube APC has been accepted by NRR for several licensees. These criteria have been termed interim plugging cri.eria (IPC).

(2) Trojan Service Experience: Examination of steam generator tubes removed from service at the Trojan plant has revealed cracks which were generally confined to the TSP intersections. Evidence from the Trojan pulled tube examinations' has shown that the outer diameter (OD) lengths of the cracks ranged almost up to the TSP thickness. Subsequent evaluation has revealed that 2 of the 21 ISP intersections examined had cracks which extended beyond the TSP thickness; these cracks extended 0.025 and 0.110 inches beyond the TSP.

(3) Steam Generator Tube Burst Test Results: Burst te:,t results on tubes removed from the Trojan steam operating or MSLB pressures.' generators showed noto When pressurized leakage failure,under normal the burst pressures measured for the tubes were in excess of the MSLB pressure by at least a f actor of two. NRC research results from burst tests of tubes with i

machined, chemically-induced and service-produced defects have also provided a I significant body of data on tube integrity. Equations which have been fitted to the burst test data for electric discharge machined (EDM) slots in steam generator tubes have shown that 0.5 inches would be the length of through-wall l

crack that would be expected to burst at MSLB pressures for 7/8-inch diameter, 0.050 inch wall thickness tubing' (see Figure 1). The equation for the through-wall EDM slot represents an extrapolation from data measured on up to 90% through-wall slots. NRC research has shown that the empirical equation developed from EDM slots provides a realistic estimate of remaining margin to failure for tubes with stress corrosion cracks.' An empirical equation fitted to data from burst tests of uniformly thinned steam generator tubes has also been developed.' This equation is contrasted with the EDM equation in Figure

1. It can be seen that the two equations are of similar form but that the uniform thinning equation provides more conservative estimates of tube burst pressures for flaw depths greater than a/t of 0.8, where a flaw depth and t tube wall thickness. Use of either equation to bound degradation up to a/t of 0.8 should yield similar results in terms of burst pressure. However, the l

EDM eouation provides a more accurate representation of stress corrosion cracking and should be used for flaw depths greater than a/t - 0.8.

(4) Stress Corrosion Crack Growth Rate: Growth of the 005CC tube cracks at the Trojan TSP intersections is not expected to be significant during one fuel cycle. For purposes of this report, significant can be defined as a through-2 I

wall crack extending on the order of 0.5 inches beyond the TSP intersection.

As described previously in (2), the Trojan cracks were generally confined to the TSP thickness; hence, growth beyond the TSP on the order of 0.5 inches would be required for these cracks to be considered critical from a MSLB pressure standpoint. Upper bound laboratory ODSCC growth rate data' indicate that crack growth of this magnitude would not be expected to occur during one fuel cycle. While a through-thickness, full TSP length crack would be expected to fail at MSLB pressure, the opening or rupture would be constrained by the tube support plate. True rupture for the portion outside of the TSP would be expected to occur at MSLB pressure only if the crack had grown on the order of 0.5 inches beyond the TSP intersection. Further, little or no movementoftheTSPwhichcouldpotentially" uncover"thecracksispredicted to occur for the MSLB condition.

(5) Probability of Main Steam Line Break: The probability of a MSLB, the key initiating event for a steam generator tube rupture, is very low. The MSLB would cause approximately a 2600 psi pressure, differential across the steam generator tubes. A MSLB has never occurred in a U.S. nuclear plant. Quoting from reference 5, "Under the Evaluation and imorovement of NDE for Inservice Insoection of Light Water Reactors Procram sponsored by the NRC, a team of experts estimated the median frequency of a MSLB to be 1.7 x 10" per reactor year for a volume of 50 gallons per minute. This extrapolates to a frequency estimate of 6.8 x 10" per reactor year for a four loop plant such as Trojan.

(6) Sumary and

Conclusions:

Based on a review of Trojan steam generator tube operating experience, on destructive examinations of tubes removed from the Trojan plant, stress corrosion crack growth rates and expert opinion concerning MSLB frequency, it is concluded that operation of the Trojan plant with steam generator tube IPC for one fuel cycle does not constitute a significant thre .t to public health and safety. Subsequent operation with IPC would require additional review after completion of one cycle and would include consideration of information developed at that time. In summary, the aca e conclusion is based on:

(1) Examination of steam generator tubes removed from service at the Trojan plant which has revealed cracks that are generally confined to the tube support plate intersectims.

(2) Burst test results from cracked tubes removed from service at the-Trojan plant which showed burst pressures well in e.vcess of main steam line break (MSLB) pressure.

(3) Stress corrosion crack growth rate results indicate that incremental growth of the cracks to a critical length beyond the tube support plate during one fuel cycle is unlikely. '

(4) The probability of a main steam line break, the ke; initiating event for a steam generator tube rupture is very low for one fuel cycle 4 3

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References:

1- " Trojan Nuclear Plant-Steam Generator Tube Repair Criteria forl Indications-at Tube-Support Plates, " Westinghouse Energy Systems, WCAP---

13129, _ Revision'1, December,1991, WESTINGH0VSE PROPRIETARY CLAS$2.

2- NUREG/CR-0718, Steam-Generato .Tubc Integrity Program, Phase I Report, USNRC,= September, 1979.

3- NUREG CR/2336, Steam Generator. Tube Integrity Program, Phase ~II Final" Report, USNRC, August, 1988.

4- HUREG'CR/5117, Steam Generator Tube Integrity / Steam Generator Group Project, Final Project Summary Report.:USNRC May,:1989. ,

5- Memorandum, C.J. Heltemes to F.P. Gillespie,. GI-163,-Multiple Steam Generator Tube Leakage, September 28, 1992.

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