[en] The Nuclear Waste Management Organization (NWMO) is implementing the long-term Adaptive Phased Management program for Canada’s used nuclear fuel. The used fuel bundles will be contained in Used Fuel Containers (UFC), and safely stored and isolated in a deep geological repository. The UFC is a key containment boundary to ensure that the short-lived radionuclides decay in-situ. The UFC is composed of a cylindrical shell welded to two hemispherical heads. A special partial penetration closure weld was designed considering the primary hydrostatic external pressure load in the repository and optimization for closure operations in a hot cell environment. The Canadian nuclear regulations require that some beyond design basis conditions shall be analyzed to understand their consequence as part of the safety assessment. The structural performance of the weld under one of such conditions, i.e., a significant one-directional lateral compressive load, is therefore investigated by the finite element method. Two extreme conditions are considered in the analysis using the real material properties obtained from mechanical testing. The performance of the weld is also demonstrated by flattening test under extreme external pressure. (author)