Structural analysis of spent nuclear fuel dry storage casks

Abstract

The structural analysis of the accidental handling drop onto an unconfined concrete floor of a spent nuclear fuel (SNF) interim dry storage cask (IDSC) has been successfully carried out at the transportation stage, using representative material properties for the (IDSC) without shock absorption. The IDSC system consists of a transportable storage cask (TSC) held within a transfer cask (TC), the latter equipped with handles for transportation of the system. This system is used in many nuclear power plants to provide offsite storage of the spent nuclear fuel assemblies. As it can easily be exposed to a handling drop at the transportation stage, this current study has made use of the handling drop as a loading case to determine if it is suitable to use a specific grade of steel and stainless steel to construct the IDSC at the transportation stage while falling on a concrete floor. The objective was to establish the structural response of the IDSC under an impact force initiated from a nine-metre height. This height is considered the worst case in accidental handling drops in nuclear waste transport operations. Finite element (FE) simulations were performed using ABAQUS/Explicit. The analysis was carried out for three drop orientations: vertical drop and oblique drops at 45 degrees and 60 degrees to the horizontal. The results showed that the highest stresses experienced by the IDSC occurred with an oblique drop at an angle of 60 degrees to the horizontal. For the particular IDSC design considered, the maximum von Mises equivalent stresses obtained on the outer-shell were beyond the elastic limit of the material for all test cases. This implies that more energy is being absorbed by the outer-shell as compared to the inner-shell. The vertical drop modelling technique was selected to be tested over two different base surfaces. This included a drop test onto both a deformable (flexible) and a rigid base. The energy absorbed by the IDSC over a rigid base was extremely severe in that the IDSC sheared between the sealing. Four finer mesh density ratios were chosen for the mesh convergence study. The four mesh density ratios were associated with the ABAQUS/EXPLICIT default mesh sizes, that is, 1:6, 1:8, 1:10, and 1:12. After running the analyses at different mesh ratios, it was established that the results converged adequately at the last three mesh ratios. Therefore, the most preferred mesh density was 1:12 based on the convergence study, which resulted in stable energy transfers. This meant that once the kinetic energy decreased, the internal energy increased slightly, thus rendering the results acceptable. The parameters of the unconfined concrete structure are those of a 50 MPa Grade Concrete based on similar studies conducted previously. The results also illustrated that a large deformation can be captured at a mesh density ratio of 1:12. Regions with different high stress concentrations were detected along the height of the TSC at a position of 2 m from the bottom edge (midway) and along the full length of the TC. The equivalent plastic strains (PEEQ) and Von Mises stresses obtained during the vertical drop were lower in magnitudes than those for the two oblique angles of 45 degrees and 60 degrees. The results obtained were validated against a similar existing study in which LS-DYNA3D and ABAQUS/Explicit were used.