Diffusion of fission products and radiation damage in SiC

A major problem with most of the present nuclear reactors is their safety in terms of \nthe release of radioactivity into the environment during accidents. In some of the \nfuture nuclear reactor designs, i.e. Generation IV reactors, the fuel is in the form of \ncoated spherical particles, i.e. TRISO (acronym for Triple Coated Isotropic) particles. \nThe main function of these coating layers is to act as diffusion barriers for radioactive \nfission products, thereby keeping these fission products within the fuel particles, even \nunder accident conditions. The most important coating layer is composed of \npolycrystalline 3C-SiC. This paper reviews the diffusion of the important fission \nproducts (silver, caesium, iodine and strontium) in SiC. Because radiation damage \ncan induce and enhance diffusion, the paper also briefly reviews damage created by \nenergetic neutrons and ions at elevated temperatures, i.e. the temperatures at which \nthe modern reactors will operate, and the annealing of the damage. The interaction \nbetween SiC and some fission products (such as Pd and I) is also briefly discussed. \nAs shown, one of the key advantages of SiC is its radiation hardness at elevated \ntemperatures, i.e. SiC is not amorphized by neutron or bombardment at substrate \ntemperatures above 350°C. Based on the diffusion coefficients of the fission products \nconsidered, the review shows that at the normal operating temperatures of these new \nreactors (i.e. less than 950°C) the SiC coating layer is a good diffusion barrier for \nthese fission products. However, at higher temperatures the design of the coated \nparticles needs to be adapted, possibly by adding a thin layer of ZrC.