Effect of radiation damage on the migration behaviour of Europium implanted into both single crystalline 6H-SiC and polycrystalline SiC

Abstract

The containment of fission products (FPs) in a nuclear fuel particle is the biggest concern regarding the safety of high temperature reactors (HRTs). In modern high temperature gas cooled nuclear reactors (HTGRs), safety is improved by coating the fuel particle with layers of chemical vapour deposited carbon and silicon carbide (SiC). In this Tri-structural Isotropic (TRISO) fuel particle, SiC is the main barrier layer of FPs. The release of radioactive FPs through the silicon carbide (SiC) containment layer presents an issue of radiological health concern to the environment. During normal operation, The TRISO particle contains most of the important FPs with the exception of other key FPs such as: silver (Ag), cesium (Cs), strontium (Sr) and europium (Eu). Most investigations have been performed on the migration behaviour of Ag and very limited investigations have been performed on the migration behaviour of Cs, Sr and Eu. In this study, the influence of radiation damage in the migration behaviour of Eu implanted into SiC was investigated. A model of ion implantation is known to be effective in simulating the neutron irradiation in a nuclear fuel particle, hence it was used in this study. Eu ions of 270 keV were implanted into polycrystalline SiC and single crystalline 6H-SiC to a fluence of 1 × 1016 cm2 at room temperature (RT), 350 oC and 600 oC. Some of the as-implanted samples were sequentially annealed at temperatures of 1000 oC to 1400 oC, in steps of 100 oC for 5 hours. The as-implanted and implanted then annealed samples were characterized by Raman spectroscopy, scanning electron microscopy (SEM), x-ray photoluminescence spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS). Implantation at RT resulted in amorphization of the implanted layer in SiC for both polycrystalline and single crystalline samples, while implantation at 350 oC and 600 oC retained a defective SiC with slightly more defects in the 350 oC implanted samples than in the 600 oC implanted samples. The radiation damage gradually annealed out with increasing annealing temperature in all implanted samples. However, the full re-crystallization was not achieved at the highest annealing temperature of 1400 oC reached in this investigation. The broadening of Eu depth profile, indicating some diffusion was taking place in the RT implanted polycrystalline SiC sample, was observed after annealing at 1000 oC up to 1300 oC. This broadening was accompanied by the formation of surface peak after annealing at 1000 oC, indicating the formation of europium oxalate compound on the surface. A larger amount of Eu loss was observed after annealing at 1100 oC. There was a steady loss of Eu from 1200 oC up to 1400 oC. The diffusion coefficients of: 0.015, 0.033 and 0.035 nm2/s were extracted at 1000 oC, 1100 oC and 1200 oC, respectively. Annealing the 350 oC implanted polycrystalline sample at 1000 oC resulted in the formation of rather small europium oxalate compound on the surface compared to the RT implanted polycrystalline SiC annealed at 1000 oC. Contrary to the RT implanted sample, no broadening was observed after annealing up to 1400 oC. However, the loss of Eu was observed after annealing at temperatures ≥ 1100 oC. Unlike the RT and 350 oC implanted samples, annealing the 600 oC implanted polycrystalline sample at 1000 oC did not result in a formation of europium oxalate surface peak. A slight broadening was observed after annealing the 600 oC implanted polycrystalline sample at temperatures of 1100 oC and higher. Due to the error limit of the RBS system used, no reliable diffusion coefficients could be extracted for this sample at these annealing temperatures. Almost all Eu was retained after annealing the 600 oC implanted samples at all temperatures. Annealing the RT implanted 6H-SiC at temperatures from 1000 oC to 1400 oC resulted in the behaviour similar to that observed in the RT implanted polycrystalline sample annealed in the same temperature range. Similar to the polycrystalline SiC, the highest loss of Eu was recorded after annealing at 1100 oC. Ultimately, the loss of Eu was steady in the samples annealed at 1200 oC up to 1400 oC. The diffusion coefficients of 0.017, 0.024 and 0.31 nm2/s were extracted at 1000 oC, 1100 oC and 1200 oC respectively. Neither diffusion nor loss of Eu was observed in the 350 oC implanted single crystalline 6H-SiC samples annealed at temperatures from 1000 oC to 1400 oC. Also, the Eu oxalate compound surface peak that was observed in the 350 oC implanted polycrystalline after annealing at 1000 oC, was not observed in the 350 oC implanted 6H-SiC sample. This difference is due to the fact that Eu is able to migrate via the grain boundaries and sit on the surface in the polycrystalline sample while this is not possible in the single crystalline 6H-SiC because of the absence of grain boundaries in the 6H-SiC sample.