Mineralogical reactions in the Tournemire argillite after in-situ interaction with steels

Abstract

Steels are possible materials for high level radioactive waste containers used in long term geological disposal in argillaceous environments. Under chemical conditions of a deep repository, the release of iron from these canisters could modify the properties of clay minerals. Thereby, many batch experiments have been realized on argillite under disposal conditions. The French Institute of Radioprotection and Nuclear Safety conducts an experimental program on steel/argillite interactions in underground natural conditions in its Tournemire experimental site (Aveyron, France). In order to determine these interaction processes, the present study focuses both on petrography and mineralogy characterization of modifications in the argillite in contact with steel samples. The accurate study of the argillite and steels after 6 years of contact indicates no significant changes for the argillite in contact with the stainless steels. For the argillite in contact with the carbon steels both numerous steel corrosion evidences and clear textural, petrographic, mineralogical and chemical modifications of the initial nature of the argillite are observed. The Fe°/argillite contacts indicate the development of a Fe-rich front within the argillite resulting from the iron diffusion from the corroded steel and which is connected with mineralogical changes of the initial argillite. This Fe-enrichment is accompanied by crystallization of significant amounts of goethite/lepidocrocite (FeOOH) and some traces of magnetite near the steel surface. The formation of Fe-rich phases within the argillite is associated with dissolution–crystallization processes. Thus, after bulk sample XRD and SEM data, these processes consist in dissolution of calcite, phyllosilicate (I/S) and pyrite and crystallization of some traces of gypsum and melanterite.