Jean-Marc Delaye
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Jean-Marc Delaye was graduated as an engineer of the “Ecole Supérieure d’Electricité” (Gif/Yvette, France) in 1987. He received a doctorate in Material Science field (University Paris 6) after a PhD at CEA Saclay dedicated to the  diffusion mechanisms in amorphous materials using molecular simulations. In 1995, he was engaged at CEA Saclay to study radiation effects in nuclear glasses and migrated to CEA Marcoule in 1997. He obtained accreditation to direct research (HDR) at University of Montpellier in 2000 and was appointed as Senior Expert then Research Director.

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Molecular simulations applied to simplified nuclear glass alteration
Frédéric Angeli1, Axelle Baroni1, Frédéric Bouyer1, Marie Collin1, Jean-Marc Delaye*1, Amreen Jan1,2, Stéphane Gin1, Sebastien N. Kerisit2
EA DEN DE2D SEVT, Laboratoire d’étude du Comportement à Long Terme, 30207, Bagnols-sur-Céze, France

2Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA

In France, the nuclear glasses containing the non-recoverable radioactive wastes from spent fuel processing are aimed to be stored in a deep geological repository. To guarantee the long term stability of the storage, it is necessary to assess its durability under aqueous corrosion. Glass alteration is a complex process with a succession of stages (Figure 1) involving specific elementary mechanisms: hydration, interdiffusion, hydrolysis, condensation, precipitation …
Figure 1: Phenomenological scheme of the alteration mechanisms at a glass – solution interface

At CEA Marcoule (France), and in collaboration with PNNL (USA), complementary molecular simulations at different scales have been developed to improve insight into these elementary mechanisms. This presentation will propose an overview of these investigations.

Using a kinetic Monte Carlo algorithm, the alteration layer formation has been simulated at a timescale not far from the laboratory one. The principle is to accumulate local hydrolysis and recondensation processes in order to reproduce the dissolution of the soluble species (B, Na) and the formation of a passivating layer at the glass surface. By this technique, it is possible to clarify the relations between the chemical composition and the alteration behavior. In particular the impact of the ZrO2 concentration on the alteration layer properties has been assessed [1].

At a smaller scale, classical molecular dynamics simulations have been performed using the CLAYFF force field to investigate the water diffusion into porous silicate systems and to compare to the experimental orders of magnitude. The coupling between experiments and simulations shed a new light on the gel layer porosity (Figure 2). It appears that two different porosities can be distinguished, an open one easily accessible to the water molecule, and a close one more hardly accessible.

Figure 2: Schematic representation of the model considering to porous networks with different diffusion coefficients

At an even smaller scale, ab initio calculations have been performed to simulate borosilicate and alumino silicate glass hydrolysis. Correlations between the local structures around the bonds and the reaction energies have been evidenced [2].

Finally, the hydration influence on the radiation effects in a simplified alteration layer (SiO2-Al2O3-CaO + H2O) has been studied using classical molecular dynamics with force fields entirely fitted on ab initio data. By comparing dry and hydrated systems, it has been shown that initial depolymerization induced by the water molecules leads to a better resistance to ballistic effects.

[1] Cailleteau C., Devreux F., Spalla O., Angéli F., Gin S., J. Phys. Chem. C, 115 (2011) 5846.
[2] Bouyer F., Geneste G., Ispas S., Kob W., Ganster P., J. Solid State Chem., 183. (2010) 2786.