Comparison of the Structural Integrity of Ductile Iron and Stainless Steel Casks for Transport of Radioactive Material

Abstract

The results are presented of 9 m (30 ft) drop simulations of three different types of transport casks, a monolithic ductile iron (DI) cask, a monolithic stainless steel (SS) cask, and a lead-shielded stainless steel (SS/Pb) sandwich cask. Each simulation involves two casks, one lying horizontally on an unyielding surface and the other positioned 9 m (30 ft) above the top surface of the lower cask. The top cask then free falls onto the lower cask, resulting in a more severe impact than the standard drop test required by the Nuclear Regulatory Commission (NRC). The drop tests were simulated using DYNA3D, a non-linear, explicit, three-dimensional finite element code for solid and structural mechanics. The results show that the monolithic casks are much stiffer than the stainless steel/lead sandwich cask. The largest difference was observed between the DI cask and the SS/Pb sandwich cask. Although the SS/Pb cask experiences considerable plastic deformation, none of them experiences failure by rupture, and they all perform within the requirements of Regulatory Guide 7.6, Revision 1 and IOCFR71. The better to compare the results, stress- and strain-based factors of safety were calculated for all of the simulations. These calculations show that the DI cask has a larger margin of safety than the SS/Pb sandwich cask, while the monolithic SS cask has a larger margin of safety than the monolithic DI cask. Finally, to address the concern over the brittleness of the DI casks, critical flaw sizes were calculated. All flaws required for crack propagation were larger than those detectable by current inspection techniques. Overall, the results of this study indicate that DI has sufficient strength, ductility, and fracture toughness to be considered as a structural material for transport casks.