Structural Weld OverLay (SWOL) for NPP Krško

SiPRO prepared the design modification package and the 10CFR50.59 licensing package for the implementation of structural weld over-lays over the Krško NPP pressurizer nozzles. The technical problem is the bi-metallic dissimilar metal welds (DMW), which are used in nuclear coolant systems to connect the stainless steel pipe fittings to the equipment made of carbon steel or low alloy steel. Such welds are often implemented using Cr-Ni Alloy 82/182, which ghas been demonstrated thru the use as such sensitive to the Primary Water Stress Corrosion Cracking (PWSCC).

SWOL is a process by which the PWSCC resistant material is deposited by special welding over the outside surface of the susceptible material. Thus, at the same time provide structural reinforcement, joint stress mitigation and new pressure boundary. SWOL is applied on pressurizer nozzles (surge, safety, relief and spray nozzles), which is classified as ASME Code III Class 1 component. Design is provided by AREVA, USA. It includes sizing, residual stress analyses, crack growth analyses, Leak-Before-Break (LBB) concept analyses, piping stress evaluations and code reconcilaiations. Design meets the requirements of original ASME Code III 1971 Edition with Addenda Winter 1972, 1974 Edition Appendix F for Faulted Conditions and Code Case N-740-2. Special technical report titled “Weld Overlay Alternative Repair Technique For Pressurizer Alloy 82/182 Butt Welds for NE Krško” justifies the path of conformance from Code Case N-740-2 to the ASME Section XI Code Cases N-504 “Alternative Rules for Repair of Class 1, 2 and 3 Austenitic stainless Steel Piping” and N-638 “Similar and Dissimilar Metal Welding Using Ambient Temperature Temper Bead Technique” that provides the references. Installation was performed by Numip d.o.o. in outage 2010.


Figure 1 - Surge line SWOL on the pressurizer bottom


Figure 2 - Top head SWOL locations 


Figure 3 - Surge line nozzle (partial section) without SWOL


Figure 4 - Surge line nozzle (partrial section) with SWOL


Project Manager: Mladen Debeljak, B.Sc.M.E