NUMERICAL ANALYSIS OF CASK ACCIDENT SCENARIOS IN STORAGE FACILITIES

Mechanical drop test scenarios for Type B (U) packages according to the IAEA regulations have to be carried out onto the so-called “unyielding target” (usually with cask impact limiters) and onto the puncture bar respectively. They are predefined and do not require any further investigation of scenarios that really could happen on transportation routes. Cask accident scenarios in the framework of approval procedures for interim storage sites are derived from a detailed analysis of the handling procedures necessary from arrival of cask at the site to its storing position. In that case casks are usually handled without impact limiters. Dependent on possible drop heights, drop positions and underground, conservative cask accident scenarios are derived for further safety proofs. According to the mechanical assessment concept of the considered approval procedure numerical calculations have to be provided by the applicant to demonstrate mechanical cask safety. Stresses and strains in the cask body as well as in the lid system have to be identified and assessed. Using the example of a 3-m-vertical-drop of a transport and storage cask for spent fuel elements onto the floor construction made of damping concrete covered by screed, BAM developed a finite element model. The finite element code ABAQUS/Explicit™ [6] was used. Additional experimental investigations are not provided and therefore parametric studies are necessary to identify the sensitivity of the finite element model to significant parameters and to verify the finite element models according to the requirements of the BAM GGR-008 [3] (Guidelines for Numerical Safety Assessments…). The paper describes the investigation of modeling the material behaviour and attachment of bottom side cask components. Questions concerning the modeling of crack length limiting reinforcement in the screed layer are discussed. The influence of the mesh density of the screed layer and its strength is considered as well. Finally, the developed finite element model could be used for an independent safety assessment. It can help to understand the complex mechanisms of the interaction between the cask components and floor construction.