Modeling of the oxic stage in a HLW disposal cell in an argillaceous host rock
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Modeling of the oxic stage in a HLW disposal cell in an argillaceous host rock François Marsal1, Laurent De Windt2 and Delphine Pellegrini1 1
IRSN, DSU/SSIAD/BERIS, B.P. 17, 92262 Fontenay-aux-Roses Cedex, France
2
Mines-ParisTech, Centre de GĂ©osciences, 77305 Fontainebleau cedex, France
ABSTRACT Determining the redox conditions in the near field of deep underground radioactive waste disposal cells is a key question regarding the performance of metallic components (e.g. waste overpack), which may undergo drastic corrosion processes in oxic conditions. This oxic transient is supposed to be short due notably to the oxygen consumption by corrosion and pyrite oxidation. However, the observed precipitation of Fe(III)-minerals as well as localized corrosion patterns on steel coupons placed during 6 years in a borehole drilled in the Toarcian argillite of Tournemire (France) may suggest that in-situ oxic conditions lasted several years, which is not consistent with reactive transport simulations performed with usual hypotheses (perfect contact between materials, high pyrite accessibility, water saturated conditions). Multicomponent reactive transport simulations considering gas diffusion were performed with the code HYTEC and reproduce correctly the observations made in Tournemire while considering imperfect interfaces and resaturation processes. The model was then applied to a disposal cell for highlevel waste (HLW) representative of the design developed in France, putting into evidence the possibility of a redox contrast between the front and back of a disposal cell in an argillaceous medium, as well as a duration of the oxic stage within the cell as long as the ventilation of handling drifts is maintained. INTRODUCTION Metallic materials are likely to be involved in deep geological disposal of radioactive waste. Depending on the designs, their watertightness and mechanical stability will be required in order to ensure their containment functions over periods up to a few thousand years and must thus be assessed. Yet, the environment of radioactive waste repositories is supposed to be initially aerated, due to the oxygen introduced into the disposal cell during its excavation and waste emplacement. The ventilation of handling drifts may then renew oxygen at the front of the disposal cell, which is as a result entrapped in the porosity of materials and in the residual voids between components. So far, the reducing conditions prevailing generally in deep geological environments are supposed to be restored after the closure of facilities, due to the consumption of the oxygen by corrosion processes, pyrite oxidation and, to a probably lesser extent, microbial activities [1-2]. However, the duration of this oxic stage is of first importance since corrosion processes under oxic conditions are known to be particularly intense [3] and may thus lead to a premature loss of the watertightness of metallic components, as well as an alteration of their mechanical integrity. The objectives of the present work are to assess the poten
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