Abstract:
In modern nuclear reactors safety is improved by cladding the fuel kernel with chemical vapor deposited layers of pyrolytic carbon and silicon carbide. Amid these cladding layers SiC is the primary diffusion barrier of radioactive fission products. During normal operations at about 900 ℃ to 1250 ℃, the coated fuel particle retains almost all the radioactive fission products excluding silver (Ag), strontium (Sr) and europium (Eu). Substantial research has been studied on the migration behavior of Ag in SiC while limited work has been done on the migration behavior of Sr and Eu in SiC. Moreover, less is known on the migration behavior of Ag in the presence of other important radioactive fission products in SiC.
In this study, the effects of Ag and Sr co-implanted into polycrystalline SiC were studied. 360 keV of Ag and Sr ions were individually implanted into polycrystalline SiC, both to a fluence of 2×〖10〗^16 〖cm〗^(-2) at 600 ℃ (Ag-SiC and Sr-SiC). Some of the Ag-SiC samples were then implanted with 280 keV Sr ions to a fluence of 2×〖10〗^16 〖 cm〗^(-2) at 600 ℃ (Ag&Sr-SiC). This high temperature of implantation was chosen to maintain the crystallinity of the substrate and also the modern nuclear reactors are premeditated to function at elevated temperatures.
The implanted samples were isochronally annealed at temperatures varying from 1000 ℃ to 1400 ℃ in steps of 100 ℃ for 5 hours. Structural and surface morphological evaluations were monitored by Raman spectroscopy and scanning electron microscopy (SEM) while the migration behavior of implanted species were monitored by Rutherford backscattered spectrometry (RBS).
Both individual and co-implantations retained crystallinity of polycrystalline-SiC with some defects. More defects were observed in the co-implanted samples. Annealing the samples caused progressive annealing of defects and appearance of SiC crystallites on the surface. These crystallites grew with annealing temperature and their growth was enhanced in the samples implanted with Ag. Neither migration nor loss of Sr were observed in Sr-SiC samples throughout the annealing steps. While annealing the Ag-SiC and Ag&Sr-SiC samples affected implants to shift towards the surface accompanied by loss at temperatures above 1400 ℃. Hence the presence of Ag has role in the migration and the loss of the implanted species.
