This paper will discuss the ASME Code Committee activities involved in the incorporation of surface stress improvement (SSI) into ASME Code Cases N-770-4 and N-729-5. ASME Code Cases N-770 [1] and N-770-1 introduced several mitigation approaches for dissimilar metal weld (DMW) locations in PWR primary system piping and provided inspection relief for locations that were mitigated. The initial approaches contained in N-770 and N-770-1 included mechanical stress improvement and weld overlay methods that have a global stress relief effect to achieve a very low tensile surface stress state or a compressive stress state at the weld inside surface to halt crack initiation, as well as growth of acceptably sized cracks. The weld overlay mitigation methods are also effective because they introduce PWSCC-resistant material, i.e., Alloys 52, 152, or their variants. (The initial approaches also included Alloy 52/152 weld inlay and weld onlay, methods that do not require stress improvement but do require access to the weld inside surface.)
While the mechanical stress improvement and weld overlay methods address the majority of the DMW locations in the primary piping system, there are locations that cannot be treated by these approaches due to the weld geometry or access limitations for the needed equipment. Additionally the dissimilar metal J-groove welds in the reactor pressure vessel head penetration nozzles (RPVHPN) could not be addressed at all by the approaches developed for DMW locations. To address the industry need to mitigate the unfavorable DMW geometries and locations along with the RPVHPN locations, the use of surface stress improvement (SSI) was studied and documented in EPRI reports Materials Reliability Program (MRP)-267 [2], “Technical Basis for Primary Water Stress Corrosion Cracking by Surface Stress Improvement,” and MRP-335 [3], “Topical Report for Primary Water Stress Corrosion Cracking by Surface Stress Improvement.” These reports formed the technical basis for the SSI-related changes made in Code Cases N-770-4 and N-729-5. Along with the technical bases noted, support from the international community in terms of operational experience with SSI in their power plants was invaluable in providing the necessary understanding, context, and confidence to committee members.
The ASME “Task Group High Strength Nickel Alloy Issues” (TGHSNAI) was assigned the task of revising the existing Code Cases, N-770 [1], “Alternate Examination Requirements and Acceptance Standards for Class 1 PWR Piping and Vessel Nozzle Butt Welds Fabricated With UNS N06082 or UNS W86182 Weld Filler Material With or Without Application of Listed Mitigation Activities” and N-729 [4], “Alternate Examination Requirements for PWR Reactor Vessel Upper Heads With Nozzles Having Pressure-Retaining Partial-Penetration Welds.” To incorporate the SSI approach into these Code Cases, the first action was to determine whether the SSI process was considered to be a peening process as defined by ASME Section III NB-4422 criteria. This required the submittal of an Interpretation of NB-4422 to determine if SSI techniques were considered a peening process under ASME Section III. The interpretation (Interpretation III-1-13-03), documented in ASME File 12-1192 [5], specified that SSI was not considered peening by Section III. This interpretation provided the framework by which SSI could be directly applied to ASME Section XI inspection criteria without the need to first revise ASME Section III NB-4422. SSI (peening) was first incorporated into Code Case N-770 [1] to provide a mitigation alternative for locations unable to be addressed by the methods addressed thus far. The revision to Code Case N-770 [1] does not provide guidance for the application of SSI activities but rather, it provides the process performance criteria and the inspection guidance following the application of SSI and establishes the pre-application inspection acceptance criteria. Following the approval of SSI in Code Case N-770 [1] addressing the DMW in the primary coolant piping system, the SSI approach was applied to the partial penetration dissimilar metal J-groove welds in RPVHPNs in Code Case N-729 [4]. The application to RPVHPNs provides the industry with a valuable asset preservation tool while significantly lowering the safety risks associated with primary water stress corrosion cracking (PWSCC) and degradation from borated water leakage for the RPVHPNs.
