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