Annealing studies of iodine implanted in pyrolytic carbon

Abstract

The behaviour of iodine, before and after annealing, in pyrolytic carbon (PyC) has been studied using the Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). PyC is used as a coating material in the design of the nuclear fuels such as the TRISO particles. TRISO particles are used to produce nuclear energy in nuclear reactors such as the PBMR. Iodine is one of the radioactive fission products produced during the production of nuclear energy by the nuclear fuels. The PyC layers in TRISO particles acts as a barrier for fission products. The main aim of this study was to investigate the effectiveness of PyC as a barrier of iodine diffusion. 360 keV iodine ions were implanted into the PyC to a fluence of 1×1015 iodine ions per cm2, at room temperature. After implantation the PyC samples were annealed (in vacuum) isochronally at 900 °C, 1000 °C, 1100 °C and 1200 °C; all for 9 hours. XRD measurements were performed using a cobalt (Co) XRD spectrometer on θ-2θ configuration; before and after the annealing temperatures. In-lens SEM images were also obtained for the PyC samples before and after heat treatment. RBS measurements were performed using a Van de Graaff accelerator and a 1.6 MeV He+ beam. XRD and SEM were used to study the structure of PyC before and after the annealing. Ion implantation and high temperature treatment affected the structure of PyC. Literature has shown that ion implantation caused structural damages in the implanted region of the PyC, reducing the degree of preferred orientation of the graphitic layers of the PyC in that region. The XRD results showed that high temperature annealing caused an increase in the degree of preferred orientation of the graphitic layers in the PyC used which is in agreement with other studies. Comparison of our SEM results to other reports has shown that the PyC used has a structure similar or close to the laminar structures of PyCs, with medium to high degree of preferred orientation. RBS was used to produce the iodine depth profiles, which provided insight on implantation depth of iodine in PyC as well as iodine behaviour after annealing. The results showed that iodine concentration in the implanted profile decreased with increasing annealing temperature. The full width at half maximum (FWHM) of the iodine profiles, obtained using the computer program Genplot, showed that after heat treatments of 900 °C, 1000 °C and 1100 °C more iodine initially diffused deeper into the PyC bulk than towards the PyC surface. At 1200 °C, the diffusion of iodine towards the PyC surface increased. It was proposed that this iodine behaviour was associated with the changes in the PyC structure due to ion implantation and high temperature treatment of the PyC. The obtained iodine profiles and the corresponding FWHM did not show evidence that the diffusion of iodine in both directions could be attributed to Fickian diffusion mechanism; hence no activation energy for the iodine diffusion in laminar PyC was determined. Copyright