APPLICATION LIMITS OF LOW-DUCTILE CAST IRON FOR RADIOACTIVE WASTE CONTAINERS

Cost-optimized containers for radioactive waste are needed especially for final disposal of nonheat generating low active waste. In Germany such containers should be made of low-ductile cast iron with increased contents of metallic recycling material from decommissioned and dismantled nuclear installations. Nevertheless, they have to fulfil IAEA regulations, repository acceptance criteria and interim storage requirements. Due to the reduced fracture toughness of this material, the influence of postulated crack-like material defects must be carefully assessed. Hence, the application limits of this new material must be investigated, and a series of drop tests with cubic container-like models (mass approx. 4 Mg) onto real targets was performed. The test objects were partially equipped with artificial crack-like defects. These tests examined the influence of different concrete targets on stresses in the cask body and fracture behaviour. Extraregulatory tests were done until failure of the components. The drop tests were simulated with dynamic three-dimensional finite element calculations. Crack-like defects inside the structure were assessed under quasi-static load as well as dynamic impact conditions. Maximum drop height depended on material quality (expressed by the dynamic fracture toughness), stresslimiting constructive measures, real target strength, and crack size and shape. Limitations of the new cast iron material quality for safety relevant applications were found on the basis of these results. The numerous tests showed that, depending on the requirements, containers for final disposal can be built of low-ductile iron with fracture toughness less than half lower limit for material quality licensed at the moment. Material application limits are thereby also determined through the opportunities granted by safety assessment methods. This project justifies the application of brittle fracture proof transport and storage packages for radioactive materials as recommended in App. VI of the Advisory Material for the IAEA Regulations for the Safe Transport of Radioactive Material (IAEA No. TS-G-1.1).