Delphine Pellegrini

Coupled modeling of cement/claystone interactions and radionuclide migration.

The interactions between cement and a clayey host-rock of an underground repository for intermediate-level radioactive waste are studied with the reactive transport code HYTEC for supporting performance assessment. Care is taken in using relevant time scales (100,000 years) and dimensions. Based on a literature review, three hypotheses are considered with respect to the mineralogical composition of the claystone and the neo-formed phases. In the long term, the pH is buffered for all hypotheses and important mineral transformations occur both in cement and the host-rock. The destruction of the primary minerals is localized close to the cement/claystone interface and is characterized by the precipitation of secondary phases with retention properties (illite, zeolite). However, beyond the zone of intense mineral transformations, the pore water chemistry is also disturbed over a dozen meters due to an attenuated but continuous flux of hydroxyl, potassium and calcium ions. Four interdependent mechanisms control the profile in the whole system: diffusion of the alkaline plume, mineralogical buffering, ion exchange and clogging of the pore space at the cement/claystone interface. The migration of a selected group of radionuclides (Cs, Ra, Tc and U) is explicitly integrated in the simulations of the strongly coupled system. Theoretical profiles of distribution coefficient (Kd) and solubility limit values are derived from the simulations, and their sensitivity with respect to the system evolution is estimated.

Reactive transport modelling of a spent fuel repository in a stiff clay formation considering excavation damaged zones

Summary The near-field evolution of a spent fuel disposal in a deep stiff clay formation is studied with the coupled chemistry-transport code HYTEC. The study gives an example that such models can be currently used for geometries (2D and 3D) and time scales (100000 y) relevant for performance assessment. The repository consists of short tunnels with MX80 bentonite barriers and cementitious materials for mechanical support. Cesium, iodine and uranium are released from the waste packages considering instant release fractions and congruent dissolution of the fuel pellets. The calculations are carried out with special focus on the excavation damaged zone (EDZ) comparing diffusion process and different advection scenarios in this zone. Cement represents a source of alkaline perturbations but, under the pure diffusion scenario, the alteration of the multi-barrier system remains limited. The presence of the EDZ does not significantly modify radionuclide migration in the pure diffusion case. The advection scenarios, even with very slow flow velocities, illustrate the possibility of preferential pathways through the EDZ for iodine but show almost no effect on the alkaline plume and cesium migration.

Fluctuation of redox conditions in radioactive waste disposal cell: characterisation of corrosion layers formed on archaeological analogues

Abstract Oxygen trapped during the operational phase in disposal cells of an underground radioactive waste repository is often considered to be quickly consumed, notably by corrosion of metallic materials or reducing microorganisms. This would lead to anoxic conditions in most of each disposal cell. In addition to this, a shift from anoxic to oxic conditions could not be excluded locally in disposal cells after their closure due to the ventilation in handling drifts that could contribute to the regeneration of oxygen at the head of each disposal cell. The impact of these transient phases on corrosion processes may affect the confinement properties of metallic components and should thus be assessed. To this end, ferrous archaeological analogues are studied. The present paper focuses on the characterisation of nails that have undergone such transient phases. First, a sample exposed for several hundred years to an aerated environment and then to an anoxic carbonated environment in laboratory for 3 years has been analysed. Corrosion layers after the oxic phase contain mostly oxihydroxides (such as goethite) and some layers perpendicular or parallel to the interface, which is in good agreement with an aerated corrosion. After 3 years under anoxic conditions, the main phase of the corrosion layer is siderite. Second, a sample exposed to an anoxic environment for several hundred years has been immersed in an aerated solution for 5 weeks. The main phases of the corrosion layer after the anoxic phase are iron carbonates and a goethite layer formed on the outer part of the corrosion layer after the immersion in an aerated solution. In both cases, the formation of new phases in the corrosion layer is in good agreement with predictions of thermodynamic modelling based on the respective environmental conditions.

Microbial Diversity at Iron-Clay Interfaces after 10 Years of Interaction Inside a Deep Argillite Geological Formation (Tournemire, France)

In the context of a geological disposal of radioactive waste in clayey formations, the consequences of microbial activity are of concern regarding the corrosion of metallic components. The purpose of the present work was to characterize the microbial diversity that may have impacted corrosion processes at the interface between re-compacted argillite and steel coupons after 10 years of interaction inside the Toarcian argillite layer in Tournemire (France) under in situ conditions. Several types of steel coupons were introduced in 1999 in two boreholes (so-called CR6 and CR8) drilled in the geological formation and filled with re-compacted argillite. CR6 borehole was drilled horizontally from a century-old railway tunnel and coupons were placed in the undisturbed argillite. CR8 borehole was drilled vertically under the tunnel, in conditions influenced by the draining of the Cernon fault water. CR6 and CR8 boreholes were overcored 10 years later and steel coupons as well as re-compacted argillite samples were analyzed separately. The characterization of their microbial diversity was carried out using culture media and molecular methods using 16S rRNA genes cloning. Data resulting from both approaches were complementary. Isolates and clone sequences were affiliated to only 3 bacterial phyla: Firmicutes, Actinobacteria and Proteobacteria. The biodiversities differed depending on the steel type and the borehole considered, indicating the influence of both iron-clay interactions and in situ environmental conditions. This analysis has highlighted the presence of sulphate-reducing bacteria, iron-reducing bacteria and isolates capable to develop at high temperatures. These microorganisms can grow at the interfaces between materials in a very short period of time compared with planned durations of disposal. Thus, these results should be considered to assess the consequences of microbial activities on the evolution of the metallic components like overpacks.

Assessment of Geochemical Containment Properties in the Near-field of a Deep Underground Repository

For safety evaluation of deep repositories, the evolution of chemical containment properties of clayey barriers in spent fuel disposal tunnels are assessed using reactive transport modelling. The disturbances related to cement components are more particularly studied for relevant time scales (100,000 years) and dimensions. Theoretical distribution coefficients (Kd) and maximum concentrations are derived for Cs, Tc and U and their sensitivity to the system evolution estimated. Mineralogical transformations and ion sorption are shown to be interdependent mechanisms controlling the intensity and spatial expansion of the alkaline plume. Simulations for a normal diffusive scenario and an altered one involving an advective flow lead to limited perturbations of the mineralogy and containment properties of the multi-barriers system, but emphasize the possibility of a migration pathway through the excavation damaged zone.