ML20198B349

Safety Evaluation Accepting Relief Request to Use ASME Section XI Code Cases 2142-1 & 2143-1 for Replacement SGs

ML20198B349
Person / Time
Site:	South Texas
Issue date:	12/15/1998
From:	NRC (Affiliation Not Assigned)
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ML20198B342	List: ML20198B339 ML20198B349
References
NUDOCS 9812180145
Download: ML20198B349 (4)

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g Q NUCLEAR RE2ULATORY COMMISSION t WASHINGTON, D.C. 2066H001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION l REQUEST TO USE ALTERNATIVE MATERIALS IN THE FABRICATION AND l

JMSTALLATION OF STEAM GENERATORS l STP NUCLEAR OPERATING COMPANY SOUTH TEXAS PROJECT (STPL UNITS 1 AND 2 DOCKET NOS. 50-498 AND 50-499

1.0 INTRODUCTION

By letter date:I July 2,1998, the STP Nucioar Operating Company (STPNOC, or the licensee) l requested approval under the provisions of 10 CFR 50.55a(a)(3) to use the American Society of Mechanical Engineers (ASME)Section IX Code Cases 2142-1 and 2143-1 during the impending fabrication and installation of replacement steam generators for the STP Electric Generating Station, Units 1 and 2 (STP-1 and STP-2). These two Code Cases introduce and classify 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 bare filler metal. Code Case 2143-1 establishes welding classifications and other requirements for coated electrodes. The subject Code Cases were approved by ASME on June 5,1995, and published in the 1995 Edition of the ASME Boiler and Pressure Vessel Code, Code Cases Supplement No.1.

10 CFR 50.55a(c), requires, in part, that components of the reactor coolant pressure boundary be designed, fabricated, erected, and tested in accordance with the requirements for Class 1 componants of Section 111 of the ASME Boiler and Pressure Vessel Code. More specifically,10 CFR 50.55a(c)(3)(iv) requires that ASME Code Cases to be applied to components of the reactor coolant pressure boundary must have been determined suitable for use by the NRC.

ASME Code Cases related to materials that have been determined suitable for use by the staff are listed in NRC Regulatory Guide (RG) 1.85. The subject Code Cases are not included in RG 1.85; thus, the licensee requested authorization pursuant to 10 CFR 50.55a(a)(3).

Pursuant to 10 CFR 50.55a(a)(3), alternatives to the requirements of 10 CFR 50.55a(c) through (h) may be used when authorized by the NRC if the licensee demonstrates that the proposed attematives would provide an acceptable level of quality anc' safety [10 CFR 50.55a(a)(3)(i)] or that compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety [10 CFR 50.55a(a)(3)(ii)).

STP intends to use alloy 690 tubing and components in the fabrication and installation of replacement steam generators for STP-1 and STP-2. The licensee believes that use of the new weld metals will enhance the service life of the replacement steam generators. Industry studies 9812180145 981215 PDR ADOCK 05000498 P PDR

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indicate that these new weld metals are less susceptible to intergranular stress conosion cracking (IGSCC) than other nickel base weld metals currently applied.

The licensee states that use of Code Cases 2142-1 and 2143-1 is advantageous to STP

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

This relief request incorporates two issues:

1. Use of alloy 690 type weld meta!s in Class 1 construction, and,

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

2.0 DISCUSSION Due to the extensive history of IGSCC problems associated with alloy 600, the industry sought an alternative alloy. Currently, alloy 690 is the material of choice in place of alloy 600 for newly  ;

fabricated steam generators. This choice is the result of numerous laboratory studies which l 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. NRC staff review of these laboratory test results has resulted in the staff position that, based upon the ,

available evidence, there is no technical reason to disallow the use of alloy 690 base material in nuclear plant construction.

Although the commercial development of alloy 690 weld metals lagged that of the alloy 690 I base metal, the weld metals matching alloy 690 have also been tested in simulated PWR and BWR environments. Corrosion studies examining the susceptibility of inco 52 and 152 (the matching alloy 690 weld metals) to IGSCC in steam generator 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 principal study which included weld metals are found in the Electric Power Research Institute (EPRI) report NP-5882M, " Stress Corrosion Cracking Resistance of Alloys 600 and 690 and Compatible Weld Metals in BWRs (Reference 1)." Two experimental alloy 690 weld metals were tested. Results showed that both of the alloy 690 weld metals are 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. In this 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 inco 52 and 152, respectively. Inco 52 is the commercial filler metal (TIG wire) described in ASME Code Case 2142-1. Inco 152 is the coated electrode described in Code Case 2143-1.

Another paper,"inconel 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

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[ Reference 2], included tests of a matching filler metal in a wide variety of environments. The two most interesting tests were conducted in simulated steam generator environments:

deaerated ammoniated and borated water at 316*C (601*F). Test results showed the welds were highly resistant to general corrosion. IGSCC 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 of 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 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 underaerated water at elevated temperatures in the presence of a crevice. 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 304 stainless.

Not considered in either study is the effect, if any, of heat to heat variations in the weld metal compositions. Such variations were found to play a substantial role in the IGSCC susceptibility of alloy 600. The strong performance of alloy 690 suggests there would be minimal effect.

Other more recent references also indicate acceptable performance of alloy 690 weld metal 1 with respect to IGSCC resistance in a simulated PWR operating environment as well as in a chloride faulted environment [ References 3,4). The results of the laboratory studies are supported by U.S. operating experience to date. Since 1989,14 U.S. plants have replaced

, steam generators using tubing fabricated from alloy 690 and compatible weld metals. To date, industry experience has not identified any service-induced corrosion of alloy 690 base metal or its associated weld metal.

Code Case 2142-1 lists the American Welding Society (AWS) specification A5.14 and Universal Numbering System (UNS) designation N06052 for a filler metal conforming to inco 52. It establishes this weld metal as F-No. 43 for both procedure and performance qualification purposes. Code Case 2143-1 lists appropriate AWS specification A5.11 and UNS specification W86152 for a coated electrode conforming to inco 152 and establishes F-No. 43 for this material for welding purposes. By these set of specifications and F-No. assignments, the materials are completely described for welding purposes as similar in their welding characteristics to many other Code nickel base weld metals. These materials are similar in composition, mechanical and thermal properties to approved metals; thus, existing welding procedures may be used with the inco 52 or 152 type weld metals.

3.0 CONCLUSION

The staff concludes that based upon the available technical evidence, the subject weld metals may be used as a substitute for other weld metals. The metals have sufficient material integrity to provide an acceptable level of weld integrity, quality and safety. The licensee has determined that their use could enhance the safety of the replacement steam generators.

Further, the staff finds that the Code Cases appropriately identify and classify these same two weld metals for welding purposes.

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4 Tho staff, therefore, concludes that pursuant to 10 CFR 50.55a(a)(3)(i), relief is authorized to employ the alternative welding materials of Code Cases 2142-1 and 2143-1 for the replacement steam generator project at STP-1 and STP-2. Use of Code Cases 2142-1 and 2143-1 is authorized until such time as the Code Cases are approved by reference in RG 1.85. At that time, if the licensee intends to continue to implement these Code Cases, the licensee is to follow all provisions in Code Cases 2142-1 and 2143-1 with limitations issued in RG 1.85, if any.

4.0 REFERENCES

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1. " Stress Corrosion Cracking Resistance of Alloys 600 and 690 and Compatible Weld Metals in BWRs," Electric Power Research Institute, NP-5882M, July 1988.

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2. A.J. Sedriks, et al., "inconel 690: A New High Nickel Alloy for Corrosive Environments at Elevated Temperature," Boshoku Gijutsu, Vol. 28, No. 2,1979.

3. Crum, J.R. and T. Nagashima, " Review of Alloy 690 Steam Generator Studies," Eighth intemational Symposium on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, p. 127-137,1997.

4. Psaila-Dombrowski, M.J., et al., " Evaluation of Weld Metals 82,152,52 and Alloy 690 Stress Corrosion Cracking and Corrosion Fatigue Susceptibility," Eighth Intemational Symposium on Environmental Degradation of Mate.-ials in Nuclear Power Systems - Water Reactors, p. 412-421,1997.

PrincipalContributor: S. Coffin Date: December 15, 1998 l

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