Multi-Scale Modelling of Nuclear Reactor Core Graphite

Magnox and Advanced Gas Cooled reactors operated in the UK are cooled by carbon dioxide gas. The graphite moderator bricks in the reactor core lose mass and become more porous during service due to radiolytic oxidation caused by energy deposition, mainly gamma radiation. The microstructure of these graphites comprises filler particles embedded in a graphitised matrix which contains porosity arising from the manufacturing process and subsequent radiolytic oxidation. Computer models have been developed to be representative of the graphite microstructure and these results are used as an input to multi-scale finite element models. These idealised models provide descriptions of the key features that contribute to an understanding of the relationship between the role of filler particles and the amount of porosity on both load-displacement (stress-strain) and fracture characteristics. Two models are considered to span the length-scales: (i) up to millimetre dimensions and (ii) centimetre dimensions. The results are considered with respect to (i) the model length-scale, (ii) the role of the volume percentage of porosity and (iii) the size and distribution of filler particles.