Development of Planar PyC/SiC Diffusion Couples to Investigate Irradiation Effects and Microstructural Variation on Fission Product Diffusion

The release of fission products from intact tristructural-isotropic (TRISO) coated particle fuel during operation is a metric of fuel performance and contributes to the source term. Historically, silver (Ag) has been observed to release from intact fuel; however, recent observations have also noted europium (Eu) and strontium (Sr) to be released. Accurate diffusion coefficients are sought to be able to predict fission product release and ensure safe and efficient reactor operation. Previous diffusion studies have shown a disconnect between diffusion coefficients obtained from in-pile release experiments and surrogate experiments. This is evident for the study of Ag diffusion, where orders of magnitude variation in diffusion coefficients are reported in the literature. The reason for the variation is not explicitly known but may be due to variation in silicon carbide (SiC) microstructures, diffusion systems, irradiation effects, or palladium (Pd)-assisted transport, among others. A systematic study of the effect of TRISO layer properties and neutron irradiation on the diffusion of relevant fission products (Ag, Eu, Sr) and fission product systems (Ag and Pd) in TRISO SiC has been initiated. Exploring these variables will provide insight on interpreting the observations from past diffusion studies and understanding the mechanisms for diffusion of each of the elements. The study also aims to provide high fidelity diffusion coefficients for the targeted fission products in TRISO SiC for fuel performance modeling. The first goal in the experiment is to develop planar diffusion couples with variable pyrolytic carbon (PyC) and SiC microstructures for neutron irradiation and high temperature thermal exposures. A planar geometry was targeted as it lends itself to advanced depth profiling techniques necessary for sensitive diffusion profile analysis. The PyC/SiC structures are produced using the same fluidized-bed chemical vapor deposition techniques used to produce TRISO fuel. This results in representative microstructures for diffusion analysis in similar TRISO layer systems. The approach for developing PyC/SiC diffusion couples is discussed. (author)