Similarity aspects for closure systems in reduced scale package drop testing

Abstract

An exact scaling of all structural components of a package for radioactive materials and their mechanical characteristics is not always possible in drop tests with reduced scale models. This has to be especially considered for bolted closure systems. On the one hand, the sizes of the bolts cannot be scaled with the same geometrical scale factor. On the other hand, the possibilities of an accurate scaling of seal characteristics are very limited. Owing to non-linearity of the force–compression relationship of a gasket, it is, for instance, impossible to simulate the maximum compression force and permissible elastic decompression of a metallic gasket simultaneously on the same scale model. Additional problems can also result from a dispersion of friction conditions in the bolted joints (at threads and under bolt heads). This dispersion, as well as an imprecise bolt tightening technique, leads to more or less considerable scatter of the bolt pretension. The minimum pretension creates more severe conditions in a drop test with regard to the seal function (higher probability of the lid opening and sliding). The maximum pretension is usually conservative for the total bolt stress (the sum of the initial tension and additional load due to the drop test). These circumstances should be considered in planning drop tests as well as regarding the interpretation and transfer of test results to the original package design. In this paper some recommendations are described concerning the modelling of closure systems based on Federal Institute for Materials Research and Testing (BAM) experience in the approval assessment of transport casks for radioactive materials.

Keywords:

• TRANSPORT CASK
• DROP TEST
• SCALE MODEL
• CLOSURE SYSTEM
• BOLTED JOINTS