Application of a fracture toughness analysis for ferritic steel components of transport/storage casks using an adapted EUROCODE 3 approach

The choice of a structural steel material for a component of a transport and storage cask for RAM is strongly determined by the demand that the cask has to withstand a free fall from 9 m height without losing its integrity and leak tightness. In terms of fracture mechanics this means that instable crack growth must not occur even under the conditions of high amplitude dynamic loading at temperatures of -40 deg. C. In the course of harmonization of European design guidelines, the Eurocode 3 has been developed which contains a fracture mechanic based concept for the steel selection to avoid brittle fracture, called Annex C. This method combines fracture mechanics tools like the failure assessment diagram (CEGB-R6-procedure) with fracture mechanics life time assessment procedure for fatigue loaded structures. The required toughness in terms of the stress intensity factor KI is related to the T27J Charpy transition temperature by means of a master curve and by a correlation between the fracture mechanics transition temperature TK100 and the Charpy transition temperature T27J. Both relations have been proved to be valid for structural steels in the range of 235 to 960 MPa yield strength. Besides that a semi-probabilistic safety approach that takes account of the model inaccuracies by calibration of large scale tests has been applied to derive a safety element for a risk of failure of pf 10-5. The fracture mechanic concept of Eurocode 3 has been adopted to calculate critical failure lengths for lids made from ferritic steels of transport and storage casks. The safety requirements of Appendix VI of the IAEA Advisory Material have been taken into account. It has been shown that the adopted Eurocode 3, Annex C, method allows an economical calculation of critical failure length on a high level of safety. A failure probability of pf = 8 . 10-7 is reached by applying lower bound estimates of fracture toughness and an additional additive safety factor ΔTa of 20 deg. C. In comparison to the ASME KIR-reference curve also the strain rate approach of the new concept proved to be on the safe side. (authors)