Investigation of silver and iodine transport through silicon carbide layers prepared for nuclear fuel element cladding

Transport of silver and iodine through polycrystalline SiC layers produced by PBMR (Pty) Ltd. for cladding of TRISO fuel kernels was investigated using Rutherford backscattering analysis and electron microscopy. Fluences of 2 x 10(16) Ag+ cm(-2) and 1 x 10(16) I+ cm(-2) were implanted at room temperature, 350 degrees C and 600 degrees C with an energy of 360 key, producing an atomic density of approximately 1.5% at the projected ranges of about 100 nm. The broadening of the implantation profiles and the loss of diffusors through the front surface during vacuum annealing at temperatures up to 1400 degrees C was determined. The results for room temperature implantations point to completely different transport mechanisms for silver and iodine in highly disordered silicon carbide. However, similar results are obtained for high temperature implantations, although iodine transport is much stronger influenced by lattice defects than is the case for silver. For both diffusors transport in well annealed samples can be described by Fickian grain boundary diffusion with no abnormal loss through the surface as would be expected from the presence of nanopores and/or micro-cracks. At 1100 degrees C diffusion coefficients for silver and iodine are below our detection limit of 10(-21) m(2) s(-1), while they increase into the 10(-20) m(2) s(-1) range at 1300 degrees C. (C) 2011 Elsevier B.V. All rights reserved.