Abstract:
In modern high temperature nuclear reactors, silicon carbide (SiC) is used as the main
diffusion barrier for the fission products in coated fuel spheres called TRISO particles.
In the TRISO particle, pyrolytic carbon and SiC layers retain most of the important
fission products like xenon, krypton and cesium effectively at temperatures up to
1000 oC. Previous studies have shown that 400 oC to 600 oC implantation of heavy
ions into single crystal 6H-SiC causes the SiC to remain crystalline with many point
defects and dislocation loops (damage). The release of Xe at annealing temperatures
above 1400 oC is governed by the normal volume diffusion without any hindrance of
trapping effects.
In this study two phenomena in single crystal 6H-SiC implanted by 360 keV Xenon
ions were studied using Rutherford Backscattering Spectroscopy (RBS) and
channeling. Radiation damage and its annealing behavior at annealing temperatures
ranging from 1000 oC to 1500 oC, and the diffusion of xenon in 6H-SiC at these
annealing temperatures were investigated.
360keV xenon ions were implanted into a single crystalline wafer (6H-SiC) at 600 oC
with a fluence of 1 × 1016 cm-2. The sample was vacuum annealed in a computer
control Webb 77 graphite furnace. Depth profiles were obtained by Rutherford
backscattering spectrometry (RBS). The same set-up was used to investigate
radiation damage of the 6H-SiC sample by channeling spectroscopy.
Isochronal annealing was performed at temperatures ranging from 1000 to 1500 °C in
steps of 100 oC for 5 hours. Channeling revealed that the 6H-SiC sample retained
most of its crystal structure when xenon was implanted at 600 °C. Annealing of the radiation damage took place when the sample was heat treated at temperatures
ranging from 1000 oC to 1500 oC. The damage peak almost disappears at 1500 oC but
the virgin spectrum was not achieved. This happened because of dechanneling due to
extended defects like dislocations remaining in the implanted region. RBS profiles
showed that no diffusion of the Xe occurred when the sample was annealed at
temperatures from 1000 oC to 1400 oC. A slight shift of the xenon peak position
towards the surface after annealing at 1400 °C was observed for 600 oC implantation.
After annealing at 1500o C, a shift toward the surface accompanied by a broadening of
the Xe peak indicating that diffusion took place. This diffusion was not accompanied
by a loss of xenon from the SiC surface. The shift towards the surface is due to
thermal etching of the SiC at 1400-1500 °C.
Modern high temperature gas-cooled reactors operate at temperatures above 600 oC in
the range of 750 oC to 950 oC. Consequently, our results indicate that the volume
diffusion of Xenon in SiC is not significant in SiC coated fuel particles.
