Effect of helium, strontium and silver implanted into silicon carbide on the structural changes and migration of the implants
Containment of radioactive fission products (FPS) is a cornerstone in the revival of nuclear reactors as a clean energy source. In modern generation IV nuclear reactors, safety is enhanced by coating the fuel kernel with four layers of chemical vapour deposited carbon and silicon carbide (SiC). The coated fuel is known as the tri-structural isotropic (TRISO) particle. In this coated particle, silicon carbide (SiC) is the main barrier for radioactive fission products. TRISO particle retains quite well majority of the fission products with exception of some radioactive FPs such as silver (Ag) and strontium (Sr). The release of radioactive FPs poses significant danger to humans and the environment. Several investigations have been conducted on the migration behaviour of Ag and Sr ions in SiC at high temperatures mimicking the operating and accident conditions. Even though, FPs co-exist in the presence of helium (He), the role of He ions in the migration of different FPs in SiC has not been thoroughly investigated. He ions are known to form bubbles in SiC, which can greatly affect the effectiveness of SiC as the main barrier of FPs. Hence, its role in the migration of important fission products needs to be investigated. In this study, the effect of helium (He), strontium (Sr) and silver (Ag) ions implantation into silicon carbide (SiC) on the structural changes and migration of the implants was investigated. Ag ions of 360 keV were implanted into polycrystalline SiC to a fluence of 2×1016 cm-2 at 600 oC. On the same samples, Sr ions of 280 keV were also implanted to a fluence of 2×1016 cm-2 at 600 oC (Ag&Sr-SiC). Some of the co-implanted samples were then implanted with 17 keV He ions to a fluence of 1×1017 cm-2 at 350 oC (Ag&Sr&He-SiC). The as-implanted samples were then isochronally annealed at temperatures ranging from 1000 oC to 1300 oC in steps of 100 oC for 5 hours. The as-implanted and annealed samples were characterized by Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy (RBS). In both the Ag&Sr-SiC and Ag&Sr&He-SiC samples, implantations retained some defects without amorphization of SiC structure. Annealing caused progressive removal of defects in both implanted samples. Co-implantation of He ions resulted in the formation of blisters and holes on the surface. The formation blisters and holes were the results of He bubbles in the implanted region and high He ions implantation temperature which resulted in the some out diffusion of He leaving holes or cavities in the implanted region. The exfoliation of the surface increased with increasing annealing temperature resulting in more holes on the surface. iv Annealing the Ag&Sr-SiC and Ag&Sr&He-SiC samples at 1000 oC caused the migration of implanted Ag ions and Sr ions towards the bulk and the surface in the annealed Ag&Sr&He SiC samples indicating some trapping in cavities. Since the migration of implanted species was not seen in the Ag&Sr-SiC samples annealed at 1000 oC, He bubbles enhance some migration while cavities trap the implanted species.
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