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Corrosion evaluation of metallic materials for long-lived HLW/spent fuel disposal containers

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In present concepts for the disposal of spent fuel or vitrified HLW in geological formations such as rock salt, granite and clay the waste container is one of several barriers against the radionuclide release from the waste forms. The main requirement on such containers is their corrosion resistance under normal operating and altered conditions in the repository. In previous corrosion studies in various EU-laboratories on a large number of materials, two approaches were identified for the manufacture of long-lived containers. These are: the corrosion-resistant concept using passively corroding materials and the corrosion-allowance concept using actively corroding materials. For the corrosion-allowance concept, carbon steels are the most promising container material in all three rock formations. For the corrosion-resistant concept, two candidates, namely the alloy Ti 99.8-Pd for rock salt and stainless steels for granite and clay are under evaluation.
For a reliable assessment of the above mentioned materials for disposal containers, however, further investigations are needed. The objectives of the proposed corrosion programme are to clarify essential open questions which remained from the former programmes and to gain a better understanding of the corrosion mechanisms and the parameters affecting the corrosion processes. Essential aspects of the investigations in salt environments are the examination of the effect of salt impurities (e.g. B(OH)4-) and gamma radiolysis products on the corrosion of Ti 99.8-Pd and carbon steel at higher temperature, and the qualification of a welding technique for the container closure. Further important points are the evaluation of the influence of pH and slow strain rates on the corrosion resistance of the materials. In clay/bentonite, the effect of the Cl-, S2032- and O2 content on corrosion of carbon steel and stainless steels will be examined under aerobic and anaeobic conditions. In addition, corresponding studies will be performed on carbon steels, Ti-alloys and Ni-alloys to a less extent. Finally, corrosion of carbon steel and stainless steel will be studied in granite environments at various temperatures and slow strain rates.
The corrosion behaviour of the materials will be examined by a combination of chemical experiments (long-term immersion tests), electrochemical/radiochemical tests and slow strain rate tests. In addition, the results of the ongoing in-situ experiments in clay will be available and can be used for the interpretation of the laboratory-scale results. Evaluation of the materials for corrosion will be carried out by using various methods, e.g. measurements of pit depths, metallography, potentiostatic/ potentiodynamic measurements, impendance measurements, measurements of mechanical properties after corrosion etc. Additional information on the corrosion mechanisms will be obtained by XRD, SEM and XPS analyses.
The results of the investiqations can be used to describe source terms in the near field The project will be performed in close cooperation, between Forschungszentrum Karlsruhe (FZK) ENRESA, Madrid, FU Berlin and SCK.CEN Mol.

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FORSCHUNGSZENTRUM KARLSRUHE GMBH - TECHNIK UND UMWELT
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