ML20205D471

SER Accepting Util Proposed Alternative to Employ Alternative Welding Matls of Code Cases 2142-1 & 2143-1 for Reactor Coolant System to Facilitate Replacement of Steam Generators at Arkansas Nuclear One,Unit 2

ML20205D471
Person / Time
Site:	Arkansas Nuclear
Issue date:	03/26/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20205D468	List: ML20205D465 ML20205D471
References
NUDOCS 9904020212
Download: ML20205D471 (4)

Text

1. % UNITED STATES

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j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555 4001

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR RELIEF FROM ASME CODE REQURIEMENTS ENTERGY OPERATIONS. INC.

ARKANSAS NUCLEAR ONE. UNIT NO. 2 DOCKET NO. 50-368 l

1.0 INTRODUCTION

By letter dated September 9,1998 (2CAN099804), Entergy Operations, Inc. (the licensee) requested approval under the provisions of 10 CFR 50.55a(a)(5)(i) to use the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code),Section IX Code Cases 2142-1 and 2143-1 during the impending fabrication and installation of replacement steam generators for Arkansas Nuclear One, Unit-2 (ANO-2). These two Code Cases classify nickel base weld materials (weld filler metal and welding electrode) thct closely match and are intended for welding Alloy 690. Code Case 2142-1 establinhas welding classifications and other requirements for a bare wire 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 American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code), Sode Cases Supplement No.1.

10 CFR 50.55a(c) requires, in part, that components of the reactor coolant pressure boundary be designed, fabrlcated, erected, and tested in accordance with the requirements for Class 1 components of Section 111 of the ASME Code. More specifically,10 CFR 50.55a(c)(3)(iv) requires that ASME Code Caser applied to components of the reactor coolant pressure boundary must have been determined suitable for use by the Nuclear Regulatory Commission (NRC). ASME Code Cases related to materials that have t,een 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 has requested authorization pursuant to 10 CFR 50.55a(a)(3).  ;

I Pursuant to 10 CFR 50.55a(a)(3), alternatives to the requirements of 10 CFR 50.55a(c) through i (h) may be used when authorize 1 by the NRC if the licensee demonstrates that the proposed alternatives would provide an acceptable level of quality and safety [10 CFR 50.55a(a)(3)(i)) or that compliance with the specified requirements would result in hardship or unusual difficulty without compensating increase in the level of quality or safety (10 CFR 50.55a(a)(3)(ii)).

Although 10 CFR 50.55a references only the applicable portions of Section lll and XI of the ASME Code, ASME Section 111 requires in NB-2400 that all welding material used in the construction or repair of ASME Code Class 1 components and material conform to the requirements for other welding material as permitted in Section IX.

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, l that compliance with the specified requirements would result in hardship or unusual difficulty without compensating increase in the level of quality or safety [10 CFR 50.55a(a)(3)(ii)].

l Entergy Operations, Inc. intends to use alloy 690 tubing and components in the fabrication and l installation of replacement steam generators for ANO-2. The licensee believes that use of the l new weld metals will enhance the service life of the replacement steam generators. Industry  !

i studies indicate that these new weld metals are less susceptible to intergranular stress

] corrosion cracking (IGSCC) than other nickel base weld metals currently applied.

The licensee states that use of Code Cases 21421 and 2143-1 is advantageous to ANO-2 I because it eliminates the burden of requiring qualification of separate welding procedures for each weld metal, as is the case for non-Code welding materials.  ;

This relief request incorporates two issues: l

1. Use of alloy 690 type weld metals in Class 1 construction, and, j

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 materials.  !

2.0 EVALUATION Due to the extensive history of IGSCC problems associated with alloy 600, the industry sought I 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 show that alloy 690 has little or no susceptibility to IGSCC in environments that simulate pressurized water (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 the  ;

nuclear plant construction. l Although the commercial development of alloy 690 weld metals lagged that of alloy 690 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 weld 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 parformance 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 Report NP-5682M," Stress Corrosion Cracking Resistance of Alloys 600 and 690 and l 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 ind'cator, and not a guarantee, of the weld metals performance in a PWR environment. In this report the designations R-127 and R135 were developmental weld metals that became inco 52 and 152, respectively. Inco 52 is the commercial filler metal (TlG wire) described in Code Case 2142-1. Inco 152 is the coated electrode described in Code Case 2143-1.

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Another paper, "inconel 690: A New High Nickel Alloy for Corrosion Environments at Elevated Temperature," by A. J. Sedriks, et al., of the inco Research and Development Center

[ Reference 2], included tests of matching filler metal in a wide variety of environments. The two most interesting tests were conducted in simulated steam generator environments: dearated 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 of 24 times longer than for alloy 600 and 12 times longer for alloy 800 and 304 stainless steel.

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 with respect to IGSCC resistance in a simulated PWR operating environment as well as in a chloride faulted environment [ Reference 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 21421 lists the Unified Numbering System (UNS) designation (UNS N06052) for a weld filler metal commercially known as "inco 52," conforming to the American Welding Society (AWS) Specification AS.14. It establishes the F No. of this weld metal as F-No. 43 for both procedure and performance qualification purposes. Code Case 2143-1 lists the UNS designation (UNS W86152) for a coated electrode commercially known as "inco 152,"

conforming to AWS specification A5.11. It establishes the F-No. of this material as F-No. 43 for both procedure and performance qualification purposes. By this set of specifications and F-No.

assignments, these materials are completely described for welding purposes as similar in their welding characteristics to many other nickel based metals approved by the code. These metals are similar in composition, mechanical and thermal properties to approved metals; thus, existing welding procedures may be used with Inco 52 or 152 type weld metals. ,

3.0 CONCLUSION

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

p The staff has reviewed the' licensee's request and supporting information and concludes thct pursuant to 10 CFR 50.55a(e)(3)(i), the propoced alternatbe to ernploy the alternative welding materials of Code Cases 2142-1 and 2143-1 for the re ctor coolant system to facilitate the replacement of steam generators at ANO-2, is authori7.ed. Use of Code Cases 2142-1 and 2143-1 is authorized until such time as the Code Casos 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 lirnitations issued in RG 1.85, if any.

4.0 REFERENCES

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

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

3. Crum, J. R. and Nagashima," Review of Alloy 690 Steam Generator Studies," Eighth International 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 Materials in Nuclear Power Systems -

Water Reactors, p. 412-421,1997.

Principal Contributor: Chris Nolan Date: March 26, 1999