Effect of heat treatment on the migration behaviour of selenium implanted into polycrystalline SiC

Abstract

The success of a very high temperature gas cooled reactors (VHTGRs) like a pebble bed modular reactor (PBMR) depends greatly on the retainment of fission products. In PBMR the retainment is achieved by coating the fuel kernel with layers of chemical vapour deposited carbon and silicon carbide (SiC). Of these coating layers, SiC is the main barrier of fission products. Hence, the migration behaviour of radioactive fission products in SiC is vital. The migration behaviour of fission products such as strontium, iodine, cesium and silver in SiC at temperatures above 1000 oC have been studied extensively. There is no reported information on the migration behaviour of selenium in SiC, which needs to be studied due to the risks posed by its emission and presence in the environment. In this study, Se ions of 200 keV were implanted into polycrystalline SiC wafers to a fluence of 1×1016 cm-2 at room temperature (RT), 350 oC and 600 oC. RT implanted samples were sequential annealed at temperatures ranging from 1000 to 1500 oC in steps of 100 oC for 10 h and at 1300 oC, 1350 oC and 1450 oC for 20 h while the hot implanted samples were only sequential annealed at temperatures ranging from 1000 to 1500 oC in steps of 100 oC for 10 h. Another set of implanted samples were isothermal annealed at 1300 oC, 1350 oC and 1400 oC for 10 h cycles of up to 80 h. The migration of implanted Se was monitored by Rutherford backscattering spectrometry (RBS) while structural and morphological changes were monitored by Raman spectroscopy and scanning electron microscopy (SEM). Implantation of Se at room temperature amorphized the near-surface region of the SiC substrate while implantation at 350 oC and at 600 oC retained the crystallinity with defects. More defects were observed in the 350 oC implanted samples compared to 600 °C. Annealing the RT implanted samples at 1000 oC resulted in the recrystallization of the amorphous SiC. No diffusion was observed for annealing up to 1200 oC. Slight peak broadening indicating diffusion was observed after annealing at 1300 oC. This broadening increased with increase in annealing temperature. Annealing at temperatures above 1200 oC resulted in the Se profile shifting towards the surface resulting in the loss of about 10 %, 20 % and 40 % of Se at 1300 oC, 1400 °C and 1500 oC, respectively. The diffusion coefficients at 1300 and 1350 °C were estimated to be 6.3×10-21 and 2.0×10-20 m2 s-1, respectively. Isothermal annealing the RT implanted samples at 1300 oC, 1350 oC and 1400 oC for 10 h cycles of up to 80 h caused the broadening of selenium profile during the first and second annealing cycle and no further a broadening was observed in subsequent annealing cycles up to 80 h. These indicated the migration of Se in damaged SiC region. The diffusion coefficients at 1300 oC, 1350 oC and 1400 oC were estimated to be 1.4 ×10-20 m2s-1, 2 ×10-20 m2s-1 and 2.5 ×10-20 m2s-1, respectively which yielded to an activation and pre-exponential factor were found to be 2 ×10-22 J and 1.7 ×10-16 m2s-1, respectively. Both sequential annealing and isothermal annealing of hot (350 oC and 600 oC) resulted in no measurable diffusion of implanted Se further pointing to radiation enhanced migration of Se as suggested by RT implanted results.