ML20207H152

Safety Evaluation of Topical Rept TR-108708, BWRVIP Vessel & Internals Project,Underwater Weld Repair of Nickel Alloy Reactor Vessel Internals (BWRVIP-44), Sept,1997.Rept Acceptable

ML20207H152
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Issue date:	06/09/1999
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U.S. NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION SAFETY EVALUATION OF "BWRVIP VESSEL AND INTERNALS PROJECT, UNDERWATER WELD REPAIR OF NICKEL ALLOY REACTOR VESSEL INTERNALS (BWRVIP-44),"

1 EPRI REPORT TR 108708, SEPTEMBER 1997

1.0 INTRODUCTION

L 1.1 Background By letter dated October 27,1997, the Boiling Water Reactor Vessel and Intemals Project (BWRVIP) submitted the Electric Power Research Institute (EPRI) propriety Report TR-108708, "BWRVIP Vessel and Intemals P .sct, Underwater Weld Repair of Nickel Alloy Reactor Vessel intemals (BWRVIP 44)," Satember 1997.

The BWRVIP-44 report provides the technical basis for utilizing the underwater wet flux-core arc welding (FCAW) process in repairing reactor intemal components at a water depth of up to 50 feet without an environmental chamber. This welding procedure would provide high-quality repair of Alloy 600.and Type 304 stainless steel welds in accordance with the Code accepted criteria established in Code Case N-516, " Underwater Welding,"Section XI, Division 1 and Section IX of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, " Welding and Brazing Qualifications." The report providas background information based on prior underwater wet welding technology developed by EPRI, equipment evaluation, welding parameter development, and mock feasibility test results for the repair of specific components.

1.2 Purpose The staff reviewed the BWRVIP-44 report to determine whether the underwater flux-cored arc welding process developed by the EPRI Repair and Replacement Applications Center (RRAC) will provide an acceptable level of weld quality based on the established guidelines contained in Code Case N-516 and Section IX of the ASME Boiler and Pressure Vessel Code. The i review considered the welding process parameters, weldment fabrication and testing, and the demonstration of underwater weld repair applications.

1.3 Organization of this Report Because the BWRVIP report is proprietary, this SE was written so as not to repeat information contained in the report. The staff does not discuss in any detail the provisions of the , i guidelines nor the parts of the guidelines it finds acceptable. A brief summary of the contents .

of the BWRVIP-44 report is given in Section 2 of this SE, with the evaluation presented in l Section 3. The conclusions are summarized in Section 4. The presentation of the evaluation is structured according to the organization of the BWRVIP-44 report.

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SUMMARY

OF BWRVIP-44 REPORT The BWRVIP-44 report addresses the following topics:

1. Weldina Process / Parameter Develooment - Studies were conducted to: 1) select welding process parameters, filler materials, and power supplies for welding on inconel and stainless steel materials at depths up to 50 feet; and 2) establish material properties that ,

will meet the provisions of ASME Code Case N-516 and Section IX. Weld development '

was performed on Alloy 600 and Type 304L base materials. Alloy 625 filler metal was l

used for all test welds. A flux-cored arc welding (FCAW) process was developed to '

address welding at greater depths including seledion of welding power supplies,  ;

adjustment / refinement of the flux formulations and fill ratios of the welding consumables, j and welding parameter selection. A supplemental gas shleiding technique was introduced to improve bead quality and heat input characteristics for multi-pass welds. <

The shielding cup design consisted of a purge or evacuation gas (compressed alr) and l an intemal shielding gas (argon) in order to evacuate the arc environment. Parameter selection was evaluated for out-of-position welding applications including vertical, horizontal, overhead, and flat positions. All underwater welding was performed in the ,

hyperbaric chamber at RRAC, which accurately simulates depth via applied pressure.  !

l Completed weldments were evaluated metallographically and mechanically to determine ,

the suitability of the welding process.  ;

2. Weidment Fabrication and Tgatirig - Based on the results of the prior section, additional

_ I welding joint configurations were produced to address the majority of in vessel repairs and provide confidence that both stainless steel and inconel base materials can be i effectively welded. Samples underwent a series of mechanical tests, including bend tests and tensile tests, in addition to detailed metallographic evaluation to document microstructure, weld penetration, and dilution. The weldment consisted primarily of groove weld configurations to support ASME Section IX mechanical testing provisions and Code Case N-516 qualification acceptance criteria. Samples were welded in four  ;

positions to address all position welding qualifications as stipulated in Code Case N-516. i Radiography, ultrasonic testing (UT), and visual inspections (VI) were also performed to i evaluate the overall quality of typical repair weldments per ASME Section IX.

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3. Demonstration of Underwater Weld Reoair Aoolications - Upon the successful completion of section 2 discussed above, component specific repair approaches were evaluated.

Mock ups representing potential repair locations were used for the testing procedures.

The emphasis of this task was to demonstrate that the underwater welding repair techniques are suitable for repair of nickel-alloy and stainless steel reactor materials with typical repair configurations at depths of up to 50 feet. Detailed metallographic testing plus non-destructive examination (NDE) were also performed on these demonstration -

samples.

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3.0 STAFP EVALUATION

' With the exception of the issues described below, this review finds that the results of the underwater welding process presented in the subject report to be acceptable for the following reasons:

High quality underwater welds, meeting all ASME Section IX provisions and Code Case N-516 acceptance criteria, can be produced.

Quality underwater flux-cored arc welding (FCAW) repairs meeting ASME Section IX -

provisions can be made.

i Adequate inspections of underwater repair welds can be performed utilizing current techniques such as ultrasonic testing (UT).

Item 1. Susceptibility to IGSCC The BWRVIP-44 report presents a study of the development of an underwater flux-cored arc welding procedure to provide high-quality welds for repair of Alloy 600 and Type 304 stainless steel in limited access locations. Industry experience has shown that such materials which comprise many intemal components of a BWR vessel are susceptible to intergranular stress corrosion cracking (IGSCC). IGSCC is a time-dependent degradation process which is known to be caused and accelerated by the presence of corrosive environments, creviges, residual stresses, material sensitization, cold work and irradiation.

Although the BWRVIP study describes the results of bend and tensile tests performed on the weld specimens, the staff believes that a funher discussion should be provided with regards to IGSCC susceptibility for the welds subjected to this underwater repair procedure.

Item 1. Degree of Residual Stresses Throughout welding processes, the base metal, heat affected zones (HAZ), and underlying weld passes experience stresses due to thermal expansion and contraction. These residual stresses can also increase the IGSCC susceptibility in the stainless steel material located at the weld joint. The BWRVIP needs to adequately characterize the stresses introduced by this underwater welding method on the surfaces of the welded section and their potential contribution to IGSCC on the me,terials examined.

. .' Item 2'. Reinspection Strategy The BWRVIP-44 report states the ultrasonic inspection provides a viable technique for the inspection of underwater weld repairs. However, a summary of the reinspection scope and frequency for this procedure should also be addressed.

4.0 CONCLUSION

The NRC staff has reviewed the BWRVIP-44 report and finds that the results of the qualification and demonstration tests indicate that the flux core process is a viable technique l

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for weld repair of stainless steel and inconel components at depths up to 50 feet, except where the staff's conclusions differ from the proposed guidance, as discussed above. The results of the licensee's welding procedure are acceptable because it followed the Code acceptance i criteria established in Code Case N-516, " Underwater Welding" and Section IX of the ASME Boller and Pressure Vessel Code, " Welding and Brazing Qualifications." The staff requests that the BWRVIP review and resolve the issues raised above, and incorporate the staff's conclusions into revised BWRVIP-25 report. Please inform the staff in writing as to this resolution.

5.0 REFERENCES

1. Carl Terry, BWRVIP, to USNRC, *BWR Vessel and Intemals Project, Underwater Weld Repair of Nickel Alloy Reactor Vessel Intemals (BWRVIP-44)," EPRI Report TR-108708, September 1997, dated October 27,1997.

2. Case N-516 of the ASME Boiler and Pressure Vessel Code, " Underwater Welding,"

Section XI, Division 1, dated August 9,1993.

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3.Section IX of the ASME Boiler and Pressure Vessel Code," Welding and Brazing Qualifications," dated July 1,1989.

Principal Contributor: T. K. Misra