Considerations pertaining to the achievement of high burn-ups in HTR fuel

The satisfactory irradiation performance of coated fuel particles up to burn-ups of, say 10%, has been demonstrated in the past. However, it is shown that some of the most important physical properties of their constituents, which determine this good performance, are only poorly known. This ignorance is likely to become more serious when particles are required to attain higher burn-up values. For example, neutron dose limits above which the anisotropy of the pyrocarbon layers become unacceptably high need to be established. Again, the particle must be designed with adequate voidage to accommodate the gas released by fission, and kernel-coating mechanical interaction must be avoided. Experiments are proposed, in addition to the irradiation and post irradiation examination of particles, which should address these issues. The way this new information can be used in fuel performance codes is discussed, thereby providing a scientific understanding of the behaviour of coated particles—as of interest to researchers, by industry, by utilities and, perhaps most importantly, by regulators.