Candidate Materials to Prevent Brittle Fracture – (186)

For heavy transport or dual purpose casks, selecting the appropriate materials for the body is a key decision. To get a Type B(U) approval, it is necessary to demonstrate that the mechanical strength of the material is good enough at temperature as low as –40°C so as to prevent the cask from any risk of brittle fracture in regulatory accident conditions. Different methods are available to provide such a demonstration and can lead to different choices. It should be noted also that the material compositions given by national or international standards display relatively wide tolerances and therefore are not necessarily sufficient to guarantee a required toughness. It is therefore necessary to specify to the fabricator the minimum value for toughness, and to verify it. This paper gives an overview of the different methods and materials that are used in several countries. Although the safety is strongly linked to the choice of the material, it is shown that many other parameters are important, such as the design, the fabrication process (multi layer, cast or forged body), the welding material and process, the ability to detect flaws, and the measured and/or calculated stress level, including stress concentration, in particular when bolts are used. The paper will show that relying exclusively on high toughness at low temperature does not necessarily deliver the maximum safety as compared with other choices. It follows that differences in approaches to licensing by different competent authorities may bias the choice of material depending on the country of application, even though B(U) licenses are meant to guarantee unilaterally a uniform minimum level of safety.