Modelling Dimensional Change With Radiolytic Oxidation in AGR Moderator Graphite

Advanced Gas Cooled Reactors (AGRs) operated by British Energy Generation Ltd are graphite-moderated and cooled by CO₂. The moderator cores are constructed from graphite bricks which must allow unhindered movement of the control rods and fuel stringers. During the life of a reactor, the moderator core bricks are subject to irradiation both by neutrons and by ionising radiations. Neutron irradiation induces dimensional change and changes in mechanical, physical and electrical properties of the graphite; whereas the ionising radiation (principally γ) brings about oxidation of the graphite by the CO₂ coolant. Radiolytic oxidation results in weight loss from the graphite components and the development of internal porosity leading to a reduction in strength. A full understanding of radiolytic oxidation is difficult to achieve because it requires a good description of the size and shape of the open porosity and that of the porosity which may be opened to gas by oxidation. Information on the effects of radiolytic oxidation on graphite properties is currently limited to ~40% weight loss. However, higher local weight losses may develop in some peak-rated bricks towards the end of the anticipated core life.