The Swedish KBS-3 copper-cast iron canisters for geological disposal are in an advanced stage of its development. This reports deals with the cast iron insert that provides the mechanical strength of the canister. The ductility of manufactured inserts often fall below the initial requirements [1]. An issue then was what consequences low tensile properties and scatter may have on the long-term failure probability of the canisters. This paper describes an approach to assess the failure probabilities for given actual material data and how to derive acceptance criteria for manufactured canisters. A statistical test programme was implemented using three inserts to determine the tensile, compression and fracture properties. Tested specimens were investigated by micro-structural analysis to determine the microstructure and categorize and size defects. It was found that the material scatter and low ductility was caused by many defect types, but with slag defects as the most important ones. These data were then used to compute defect distributions and as direct input to FE-calculations of KBS-3 canisters. A large number of FE- analyses were performed at the maximum design load (44 MPa) covering distributions of material parameters and geometrical features of the canisters. The computed probabilities were very low for both failure modes even for material data with poor ductility.