Migration behaviour of Europium implanted into single crystalline 6H-SiC

Abstract

The release of radioactive fission products during nuclear energy reactor operation imposes a serious danger to personnel’s health and the environment. In a modern Pebble Bed Modular Reactor (PBMR) TristructuralIsotropic (TRISO) fuel, silicon carbide (SiC) is used as the main barrier to fission products (FPs). TRISO particles retain most of the FPs well with the exception of silver (Ag), strontium (Sr) and Europium (Eu) during operation. The migration behavior of Ag in SiC has been thoroughly investigated over decades with fewer investigations done on the migration behavior of Sr and Eu. In this study the migration behavior of Europium (Eu) implanted into 6H-SiC was investigated using Rutherford backscattering spectrometry (RBS), RBS in a channeling mode (RBS-C) and scanning electron microscopy (SEM). Eu ions of 360 keV were implanted into 6H-SiC at 600°C to a fluence of 1×1016 cm-2. This high temperature was chosen because PBMR reactors are designed to operate at high temperatures. The implanted samples were sequentially annealed at temperatures ranging from 1000 to 1400 °C, in steps of 100 °C for 5 hours. RBS-C showed that implantation of Eu into 6H-SiC at 600 °C retained crystallinity with some radiation damage. Reduction of the radiation damage retained after implantation, already took place after annealing at 1000 °C. This annealing of radiation damage progressed with increasing annealing temperature up to 1400 °C. A shift of Eu towards the surface took place after annealing at 1000 °C. This shift became more pronounced and was accompanied by loss of Eu from the surface at annealing temperatures >1000 °C. This shift was accompanied by broadening of Eu peak, indicating Fickian diffusion occurring after annealing at temperatures > 1100 °C. The migration of Eu occurring concurrently with the annealing of radiation damage was explained by trapping and de-trapping of Eu by radiation damage. The diffusion coefficients were found to be 1.56× 10- 18 m2s-1 and 2.98× 10-18m2s-1at 1200 °C and 1300 °C respectively.