ML20236U614

Safety Evaluation Authorizing Use of Alternative Alloy 690 Welds (Inco 52 & 152) as Substitute for Other Weld Metal

ML20236U614
Person / Time
Site:	Farley
Issue date:	07/23/1998
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20236U602	List: ML20236U600 ML20236U614
References
NUDOCS 9807300346
Download: ML20236U614 (4)

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l SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST TO USE ALTERNATIVE MATERIALS IN THE FABRICATION AND INSTALLATION OF STEAM GENERATORS SOUTHERN NUCLEAR OPERATING COMPANY. INC.. ET AL.

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i JOSEPH M. FARLEY NUCLEAR PLANT. UNITS 1 AND 2 DOCKET NOS. 50-348 AND 50-364

1.0 INTRODUCTION

By letter dated May 22,1998, the Southern Nuclear Operating Company, Inc. (SNC) requested approval under the provisions of Title 10 of the.C. ode of Federal Regulations (10 CFR)

Section 50.55a(a)(3) to use the American Society of Mechanical Engineers (ASME)Section IX

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Code Cases 2142-1 and 2143-1 during the impending fabrication and installation of replacement steam generators (SGs) for the Joseph M. Farley Nuclear Plant (FNP), Units 1 and 2.

These two Code Cases introduce and classify new nickel base weld metals that closely match and are intended for welding Alloy 690. Code Case 2142-1 establishes welding classifications and other requirements for a bare wire filler metal. Code Case 2143-1 establishes welding classifications and other requirements for a coated electrode.

The subject Code Cases were approved by the ASME on June 5,1995, and published in the 1995 Edition of the ASME Boiler and Pressure Vessel Code (ASME Code), Code Cases Supplement No.1. However, since the 1995 Edition of the ASME Code has not been incorporated by reference into the regulations, these Code Cases cannot be used without prior NRC review and approval.

SNC intends to use Alloy 690 tubing and components in the fabrication and installation of i

replacement SGs for FNP, Units 1 and 2. SNC believes that use of the new weld metals will enhance the service life of the replacement SGs. Industry studies indicate that these new weld metals are less susceptible to intergranular stress corrosion cracking (IGSCC) than the other nickel based weld metals currently applied.

I Use of Code Cases 2142-1 and 2143-1 is advantageous to SNC because it eliminates the I

burden of requiring qualification of separate welding procedures for each weld metal, as is the case for non-Code welding materials.

l 9807300346 980723 PDR ADOCK 05000349 P

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l l Thus, this relief request incorporates two issues:

1. Use of Alloy 690 type weld metals in Code Class 1 construction, and,

2. The use of two ASME Code Cases that group the new weld metals in the same welding categories as other commonly employed nickel based weld metals. This allows the use of appropriate existing welding procedures and performance qualifications with the new weld metals.

Section 50.55a(a) requires that systems and components of boiling and pressurized water cooled nuclear power reactors must meet the requirements of the ASME Code as specified in 10 CFR 50.55a(b) through (g) and that components be designed and fabricated to qualify standards commensurate with their safety function. Section 50.55a(3) states that alternatives to the requirements of 50.55a(c) through (h) may be used, when authorized by the NRC, if (i) the proposed alternatives would provide an acceptable level of quality and safety or (ii) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

2.0 DISCUSSION Due to the extensive history of IGSCC problems in Alloy 600, the industry has sought an attemative alloy. Currently, Alloy 690 is the industry material of choice. This choice is the result of numerous laboratory studies that show that Alloy 690 has little or no susceptibility to IGSCC in environments that simulate pressurized water reactor (PWR) and boiling water reactor (BWR) plant conditions. The NRC staff's review of the laboratory test results has resulted in the staff position that, based upon the available technical evidence, there is no technical reason to disallow the use of Alloy 690 base materialin nuclear plant construction.

Alloy 600 type weld metals (such as inco 82 and 182) were widely used during the construction of nuclear power plants. Operating experience showed that inco 182 was also susceptible to IGSCC, although primarily in BWR environments. Weld metals matching Alloy 690 have also been tested in simulated PWR and BWR environments. Commercial development of these weld metals lagged that of the Alloy 690 base metal. No matching weld metal has been commercially available until now. Thus, staff evaluation of these weld metals has not been previously conducted.

Corrosion studies examing the susceptibility of weld metals to IGSCC in SG environments are scant compared to the voluminous base metal studies. This is because the base metal performance is a strong indicator of the expected performance of a matching weld metal.

Results of the principle study, which included weld metals, are found in the Electric Power Research Institute (EPRI) report NP-5882M, titled, " Stress Corrosion Cracking Resistance of Alloys 600 and 690 and Compatible Weld Metals in BWRs." Two experimental Alloy 690 weld metals were tested and found to be immune to IGSCC in pure water environments. However, since these were laboratory simulations of a BWR environment, the results are only an indicator, and not a guarantee, of the weld metals' performance in a PWR environment.

4 in the EPRI report, the designations R-127 and R-135 were used for the experimental weld

. metals. These were the inco designations for the developmental weld metals that became i

inco 52 and 152, respectively. Inco 52 is the commercial filler metal (tig wire) described in i

ASME Code Case 2142-1. Inco 152 is the coated electrode described in Code Case 2143-1.

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Another paper, *lnconel 690: A New High Nickel Alloy for Corrosive Environments at Elevated Temperature," by A. J. Sedriks, et al., of the inco Research and Development Center, included tests of a matching filler metalin a wide variety of environments. The two most interesting tests were conducted in simulated SG environments: desersted ammoniated and borated water at 316' C. The test results showed the welds and weld metal were highly resistant to gw.sral corrosion.

SCC susceptibility was tested by exposing welds to a variety of chloride environments. The controls used in these tests were Alloy 800 (not 600) and type 304 stainless steel Both of these alloys are known to crack in elevated temperature chloride environments. in all cases, Alloy 690 was tested for periods significantly longer than the time to crack Alloy 800 (the more resistant of the two control alloys), in no case did the Alloy 690 welds crack despite test i

durations 8 times longer than that of the control alloys.

Additional testing for IGSCC susceptibility in pure water environments was conducted. Another group of Alloy 690 welds plus control alloys were exposed to aerated water at elevated temperatures in the presence of a crevice.t Cracking was readily initiated within the controls.

.None of the Alloy 690 welds cracked despite testing durations 24 times longer than for Alloy 600 and 12 times for Alloy 800 and type 304 stainless steel. Thus, use of the Alloy 690 welds provides reasonable assurance of structuralintegrity.

Code Case 2142-1 (inco 52) lists the American Welding Society (AWS) specification AWS A5.14 and Unified Numbering System (UNS) designation UNS N06052 for a filler metal similar to inco 52. The Code Case establishes the F No. of this weld metal as F-No. 43 for both procedure and performance qualification purposes. Code Case 2143-1 (inco 152) lists appropriate AWS and UNS specifications for a coated electrode matching inco 152 and establishes F-No. 43 for this material for welding purposes. By this wt of specifications and F No. assignments, these materials are completely described for welding purposes as similar to other presently used nickel-based weld metals. Thus, existing welding procedures may be i

used with the inco S2 or 152 type weld metals without requalification.

3.0 CONCLUSION

The staff concludes that, based upon the available technical evidence, use of Alloy 690 welds (Inco 52 and 152) as a substitute for other weld metals as requested by SNC in its May 22, 1998, letter will provide reasonable assurance of structuralintegrity. Furthermore, the staff finds that Code Cases 2142-1 and 2143-1 appropriately identify and classify these two weld metals for welding purposes, and there is no need to follow the special procedure and perform 6nce qualifications for non-Code materials.

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Pursuant to 10 CFR 50.55a(a)(3)(i), the proposed attemative to employ the aNemative welding i

materials and the classification categories of Code Cases 2142-1 and 2143-1 is authorized.

Principal Contributor: J. Zimmerman Date:

July 23,1998

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