SIMILARITY ASPECTS FOR CLOSURE SYSTEMS IN SMALL SCALE PACKAGE DROP TESTING

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 small-scale models. This has to be especially considered for bolted closure systems. On the one hand, the sizes of the bolts (diameter, pitch of the thread, stress cross section) 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. Due to nonlinearity 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 small-scale model. Additional problems can also result from a dispersion of friction conditions at the contact areas of the bolted joints (in threads and under bolt heads). This dispersion as well as an imprecision of the bolt tightening technique lead to more or less considerable scatter of the bolt pre-tension. The minimum pre-tension creates more severe conditions in a drop test with regard to the seal function (higher probability of the lid opening and sliding). The maximum pre-tension 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. For instance, in some cases a correction of the theoretical similarity relation between tightening torques for the bolts of the original cask and its tested small-scale model may be necessary to make this transfer correct. In this paper some recommendations and examples are described concerning the modelling of closure systems based on BAM experience in the approval assessment of transport casks for radioactive materials.