Influence of Iron(II) and Iron(III) Ions on the Corrosion of MTR Fuel Element Claddings in Repository-Relevant Brines
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*Forschungszentrum Jilich, 52425 Jilich, Germany, [email protected] **Forschungszentrum Jilich, 52425 JRlich, Germany, [email protected]
ABSTRACT After the USA decided in 1988 to no longer accept spent fuel elements from German material test reactors (MTR), a national back-end fuel cycle alternative was sought in the Federal Republic of Germany [1]. The aim is their direct final disposal in deep, stable geologic formations. The corrosion of material test reactor (MTR)-fuel element claddings (aluminium) in repository-relevant brines was examined. Before the aluminium cladding material can corrode, the POLLUX cask, containing the fuel elements, must be corroded. In this case, iron(II) and iron(III) ions are present in the brine. These ions decisively influence the corrosion of the MTR fuel element cladding material, therefore the mechanism responsible for this phenomenon should be identified. Tests were performed in which Fe(II) and Fe(III) salts were added to the brines. In these experiments, the percentage mass decrease of the aluminium cladding, the iron content of the brine, as well as the pH value were determined. As expected the results provided the information about the corrosion mechanism. The higher the concentration of iron ions in the brines, the higher the aluminium corrosion rate was for all three brines. Identical redox equilibria between Fe(II) and Fe(III) were formed in the brine, irrespective of whether Fe(II) or Fe(III) salt had been added. It is assumed that the acceleration of the corrosion rate is based on the fact that Fe(II) is reduced to metallic iron by absorbing the electrons produced during the oxidation of aluminium to Al(III). The aluminium cladding material does not function as a barrier for the release of radionuclides from the fuel elements. The results of this study show that the 0.38 mm thick aluminium cladding will corrode through after approximately four
weeks. INTRODUCTION After the USA decided in 1988 to no longer accept spent fuel elements from German material test reactors (MTR), a national back-end fuel cycle alternative was sought in the Federal Republic of Germany [1]. The aim is an initial interim storage of the fuel elements in CASTOR MTR casks for approximately 40 years before their direct final disposal without reprocessing [2]. Geologically stable salt rock formations are regarded as possible repositories. The fuel elements could be finally stored in a suitable manner in POLLUX MTR casks in the drifts or boreholes of a salt mine [3]. From a safety-engineering point of view, the penetration of low-salt-content waters through slightly fissured anhydride formations is conceivable as an accident scenario for this type of final disposal. This would produce highly concentrated, extremely corrosive brines, which would first come into contact with the storage casks, then penetrate into the cask and finally act on the fuel elements. The fuel element consists of a metallic uranium-aluminium alloy, which is clad with aluminium. The POLLUX cask, made of nodular gre
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