INFLUENCE OF THE MECHANICAL STRENGTH OF THE ASSEMBLIES ON THE DESIGN OF CASK BASKETS.

Spent fuel casks must be designed to resist drops in the horizontal position. The drop tests with mock-ups do not always accurately reproduce either the lodgements of the internal basket, or the fuel assemblies. Measured accelerograms are then used to calculate the behavior of these components. For calculation purposes, the assemblies are usually modeled as lumped masses. That is equivalent to stating that the forces exerted by the assemblies on the basket are always proportional to the accelerations of the basket. The forces obtained in this way show unrealistic peaks that require either a reinforcement of the basket or a reduction of the cask capacity. To mitigate these consequences, the IAEA authorizes to apply a limited mathematic filtering of these peaks (Safety series 37). This paper presents simulations showing a more refined evaluation of the forces applied on the basket. These simulations were run with a recent version of the CLASH computer program. This software has been used since the early eighties to analyse the non-linear seismic behavior of reactor cores and spent fuel racks. It shows how the assemblies crash progressively against the wall of their lodgement. The mixing grids buckle plastically and the fuel rods bend between these grids. The outer rod layers hit the lodgement walls and nearly all the rods end up leaning on each other. This progressive compaction of the assembly significantly damps the loading. The assembly finally bounces against the opposite side of the lodgement using up the small amount of kinetic energy that remains. The program also evaluates the bending stresses in the rods and gives the axial distribution of the load on the walls. It is to be noted that there is no load near the grids. Large and sustained loads only appear below the grids. Direct shocks of rods on the walls, between the grids, generate very large but short lasting loads. The presented analysis leads to the conclusion that the load on the basket could be significantly lower than that obtained based on the assumption that the assemblies are merely distnbuted masses.