A Probabilistic Methodology to Determine Acceptance Criteria and Failure Probabilities for the KBS-3 Ductile Cast Iron I
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A Probabilistic Methodology to Determine Acceptance Criteria and Failure Probabilities for the KBS-3 Ductile Cast Iron Inserts Karl-Fredrik Nilsson1, Claes-Göran Andersson2, Fred Nilsson3, Peter Dillström4, Mats Andersson5 and Philip Minnebo1 1 European Commission, DG-JRC, Institute for Energy, Petten, The Netherlands 2 Swedish Nuclear Fuel and Waste Management Co, SKB, Stockholm, Sweden 3 Royal Institute of Technology, Dep. of Solid Mechanics, Stockholm, Sweden 4 DNV Technical Consultants, Stockholm, Sweden 5 ÅF Industriteknik, Stockholm, Sweden ABSTRACT 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 FEanalyses 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. INTRODUCTION The KBS-3 system relies on a multi-barriers concept with the canister as key barrier and the buffer material (bentonite) and the crystalline rock as the other main barriers. The canister consists of a ductile cast iron insert and a copper shielding, Figure 1. The copper protects the canister from corrosion whereas the ductile cast iron insert provides the mechanical strength. The insert is cast around a steel cassette which provides the channels where the spent fuel is placed. The canister should inhibit release of radionuclides for at least 100 000 years. The 50 mm copper is sufficient to ensure that corrosion cannot lead to leakage of radionuclides. Canisters will be loaded in compression by the hydrostatic pressure and the swelling pressure from the bentonite, giving a total pressure of 14 MPa. Several ice ages are expected with a maximum ice-sheet of 3 km, giving an additional pressure of 30 MPa. The maximum design pressure is therefore 44 MPa [2]. A relatively large number of canisters have been manufactured as part of SKB’s development programme. An issue that caused some concern wa
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