Thermal migration of deuterium implanted in graphite: Influence of free surface proximity and structure

This paper is a contribution to the study of the behavior of activation products produced in irradiated nuclear graphite, graphite being the moderator of the first French generation of CO2 cooled nuclear fission reactors. This paper is focused on the thermal release of Tritium, a major contributor to the initial activity, taking into account the role of the free surfaces (open pores and graphite surface). Two kinds of graphite were compared. On one hand, Highly Oriented Pyrolitic Graphite (HOPG), a model well graphitized graphite, and on the other hand, SLA2, a porous less graphitized nuclear graphite. Deuterium ion implantation at three different energies 70, 200 and 390 keV allows simulating the presence of Tritium at three different depths, corresponding respectively to projected ranges R-p of 0.75, 1.7 and 3.2 mu m. The D isotopic tracing is performed thanks to the D(He-3,p)He-4 nuclear reaction. The graphite structure is studied by Raman microspectrometry. Thermal annealing is performed in the temperature range 200-1200 degrees C up to 300 h annealing time. As observed in a previous study, the results show that the D release occurs according to three kinetic regimes: a rapid permeation through open pores, a transient regime corresponding to detrapping and diffusion of D located at low energy sites correlated to the edges of crystallites and finally a saturation regime attributed to detrapping of interstitial D located at high energy sites inside the crystallites. Below 600 degrees C, D release is negligible whatever the implantation depth and the graphite type. The present paper clearly puts forward that above 600 degrees C, the D release decreases at deeper implantation depths and strongly depends on the graphite structure. In HOPG where high energy sites are more abundant, the D release is less dependent on the surface proximity compared to SLA2. In SLA2, in which the low energy sites prevail, the D release curves are clearly shifted towards lower temperatures when D is located close to free surfaces. Extrapolating our data to Tritium mobility in irradiated graphite, we show that thermal selective extraction of T would be all the more so efficient as the graphite structure is more disordered, which means in the most irradiated and damaged graphite zones in the reactor. (C) 2015 Elsevier B.V. All rights reserved.