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Effect of Wet Air Radiolysis on Zircaloy Corrosion


Congress title :Migration '07
Congress location :Munich
Congress date :27/08/2007

Summary

In France, 80% of electricity is produced by nuclear energy. Zirconium alloys such as Zircaloy-4 (Zy 4) are widely used as cladding materials of light water and heavy water reactors. During reactor operation, the Zircaloy cladding tubes get activated and oxidised. The oxidised layer is several micrometers thick. It is a non-homogeneous in thickness and in composition (hydrides, amorphous precipitates). Its structure is strongly strained. During the reprocessing operations of the irradiated fuel, the cladding tubes are cut up in “hulls” and washed with nitric acid to dissolve the uranium oxide fuel. The hulls are then rinsed and dried before being compacted in a wafer form. They are then placed in a steel container. The containers mostly contain activated oxidized metal pieces which can contain traces of fission and activation products and also actinides. Before being emplaced in the disposal vaults of a deep geological disposal facility, such containers are expected to be introduced into preformed concrete over-packs. In a geological disposal, these containers will be exposed simultaneously to humid air and to irradiation during the first hundred years after emplacement. In a radioactive ambiance, air radiolysis will occur and is likely to accelerate corrosion processes. Considering that most of the radionuclides contained in these containers are present in the bulk of metallic pieces, it is important to study the corrosion of zircaloy and the behaviour of the zirconia layer in this conditions in order to increase knowledge of possible release phenomena that may be induced by radiolytic corrosion. A fundamental study has thus been defined on the effects of wet air radiolysis on Zircaloy corrosion, the radiolysis being induced by MeV proton irradiation. In order to control the thickness of the oxides, we have synthesized zirconia layers using thermogravimetry experiments. Compact and porous zirconia were prepared. The irradiation experiments were performed at the 4 MV Van de Graaff accelerator of IPN Lyon, using a 1.8 MeV proton beam at different fluxes, for different irradiation time and water vapour pressures. Oxygen and hydrogen distribution profiles have been determined by using respectively Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA).
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