The effect of heat treatment on the microstructure and diffusion of silver in pyrolytic carbon coatings

It is well accepted that TRISO (tristructural isotropic) coated nuclear fuel particles are capable of retaining fission products up to 1600 °C, however above this temperature fission products can diffuse through the pyrolytic carbon (PyC) and silicon carbide coatings that act as the containment barriers in this fuel. Despite decades of research and development, little is known on the origin of this fuel temperature limit. In order to understand the origin of this fuel temperature PyC coatings produced by fluidized bed chemical vapor deposition were heat treated at 1000 °C, 1400 °C and 1700 °C for 200 h in an innert atmosphere. We have observed that above 1400 °C the anisotropy, domain size and level of graphitization increases to twice its original value. Furthermore, at 1700 °C some samples exhibited the formation of nano-pores, which could be the origin of the maximum fuel temperature limit or at least contribute to it. The increased diffusivity of elements due to microstructural changes was corroborated by silver diffusion experiments. Furthermore, we have observed that not all the samples suffer the same level of graphitization, thus suggesting that some PyC coatings can maintain their capability to retain fission products even after temperature excursions above 1600 °C.