Welding Residual Stress Simulation for Large Spent Fuel Storage Container Containment Closure Welds

Abstract

Used fuel from CANDU reactors are stored in large storage containers for long term waste management. To provide leak-tight containment of the used fuel these containers are sealed with multi-pass closure welds, however, post weld heat treatment to relieve the generated welding residual stresses (WRSs) is generally not performed due to the negative effects of high temperatures on concrete shielding material. Typically, used fuel storage containers are fabricated and inspected in accordance with the requirements of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessels Code (BPVC) and are subject to inservice inspections in support of long term waste management programs. Per the current requirements of Section XI of the ASME BPVC¹, structural integrity evaluations of detected pre- and inservice inspections flaws shall consider the effects of WRS when determining their acceptability. For cylindrical geometries analytical estimates of WRS are provided in various codes, however, for non-cylindrical geometries only basic flat plate estimates exist and therefore WRSs are either assumed to be equal to room temperature yield strength or must be determined from experimental or numerical methods. Due to the size of used fuel storage containers detailed WRS simulation of the entire welding process is not often practical. Additionally, results from numerical WRS analysis may be required to establish initial material and stress conditions in subsequent structural integrity evaluations, such as those required for the evaluation of dynamic accident events. This paper presents a procedure for improving the overall efficiency of WRS analysis of used fuel storage containers using a hybrid static/moving heat source approach.