The pebble bed reactor (PBR) is generation IV reactor design that is highly efficient and passively safe. The core of a PBR consists of a silo full of graphite pebbles filled with tristructural-isotropic fuel. This fuel acts as both a moderator and radiation isotope containment preventing the release of fission biproducts. The fuel elements are first piled in the core until criticality is reached. As burnup occur, the fuel elements are recycled from the bottom of the core and replaced at the top of the core. With each pass the fuel is measured for burnup. If it is below the burnup limit, and has not received significant structural damage, it is recycled. If not, it will be placed directly in dry casks for storage. Because of its inert nature, there is no need for cutting or processing before storage. A fuel element is designed to stay in the core for roughly 3 years.

Both the fuel elements, and core structure material are composed mostly of graphite. This graphite will experience significant property changes based on both the thermal expansion, and the neutron irradiation. These changes include the Young’s modulus, Poisson’s ratio, density and swelling, and coefficient of thermal conductivity. The effects of irradiation on the core structure material, and the resulting core behavior, has been well studied. Previously the effect of dynamic properties of the graphite fuel pebbles due to irradiation has been assumed negligible.

With advances in computing power, it has become feasible to model graphite fuel element property changes and their effect on the core geometry and maximum pebble temperatures. This modelling can be accomplished using the discrete element method (DEM). In this paper the discrete element method is described. Previous research on graphite property changes based on temperature and irradiation dose is summarized. This previous research was used to augment the discrete element method with dynamic graphite pebble properties. The core was simulated over a three-year period with graphite pebble property changes. In this study the forces are examined. It was shown that the dynamic property changes lead to unsafe changes in pebble forces. These estimates are consistent with previous reactors and show that dynamic property changes could be better used to model the PBR core.