Thermal behavior of deuterium implanted into nuclear graphite studied by NRA

Abstract

This paper focuses on the thermal behavior of deuterium, simulating tritium, implanted into virgin nuclear graphite of French gas-cooled reactors, which are being decommissioned. Deuterium ions D+ were implanted into graphite (around 3at.% at the projected range Rp) at two different depths (around 670nm and 2.8μm) and annealed up to about 300h in a temperature range from 200°C to 1200°C under vacuum or argon flow. Before and after heat treatments, D distribution profiles in the samples were followed using the nuclear reaction D(3He,p)4He, with a millimetric beam at the 4 MV Van de Graaff accelerator of IPNL (Institut de Physique Nucléaire de Lyon, France). The results show that the deuterium release becomes significant at temperatures higher than 600°C and is almost totally completed at 1200°C. The comparison of the results, obtained for both implantation depths, points out the role of the porosity with respect to deuterium permeation. The release follows two stages: a rapid step where it occurs within a few hours, followed by a much slower step during which the release of deuterium saturates. The initial stage is characterized by an activation energy of 1.3eV and might correspond to detrapping of D located at crystallite edges and its diffusion at the crystallite surfaces. We assume that the second stage kinetics corresponds to a very slow diffusion of D located inside the crystallites and chemisorbed to carbon atoms through sp2 or sp3 bonds.