FEM Simulation of Extreme Thermal and Mechanical Accident Loads on Screwed Spent Fuel Cask Lid Structures

Abstract

The complex analysis of screwed spent fuel cask lid structures under extreme thermal and mechanical loads is very important for the evaluation of cask integrity and leak tightness under such conditions. The interest of such problems has been increasing since the terrorist attacks from September 11, 2001. Due to extension experiences of BAM in calculation and experimental testing of transport and storage casks for radioactive materials, BAM in this context has developed new methods to estimate the safety margins of transport and storage spent fuel and high-level waste casks under extreme thermal and mechanical loads resulting from aircraft crashes. In case of thermal loads, a thermal heat transfer analysis has to be made, which gives the time-dependent temperature distribution of the casks. But this is not enough, while the extreme kerosene fire scenario creates a strong transient heating of the cask body and its lid system. This causes elastic and plastic deformation of the cask body, the decrease of screw forces and especially great relative displacements between the seals and its contacting flanges. This results in an elevated leak rate. To cover this case so-called thermo-mechanical analyses had been carried out. One of the most critical mechanical loads on the cask is a central impact onto the lid-seal-system. This can be caused by direct aircraft crash or its engine as well as by a following impact of building structures of a nuclear facility like a storage hall. In this situation dynamical analyses had been carried out. Although it is currently not possible to calculate the leakage rates from deformation analysis directly, for the present it is possible to estimate the behaviour of the seal based on the calculated relative displacements at its place and the behaviour of the lid bolts under the thermal and mechanical impact loads respectively, in combination with experiments where the leakage rate of the seals had been measured after radial and axial shifting of the flanges. Except of the lid bolts, the geometry of the cask and the thermal/mechanical loads are axial-symmetric, which simplified the analysis considerably by using two dimensional finite element models and parameter studies are possible. The lid bolts had been “smeared” with a special technique as two-dimensional plane-stress bolt model, which has been verified with three dimensional bolt calculations. Experiments and calculation studies show that the German transport and storage casks for radioactive material have sufficient safety margins even by extreme thermal/mechanical loads. This paper will present the methodologies developed for the studies. Some of the calculation results will be presented.