Graphite bricks are both the moderator and structural components at the heart of the Advanced Gas-cooled Reactor (AGR) cores. Internal stress arises from the moderation role of the graphite components, which leads to uneven material ageing across the component. These stresses are greatly increased by changes to the coefficient of thermal expansion (CTE), which is a function of irradiation and stress. These stresses can themselves be relaxed over time by the irradiation-induced creep. The need for reliable experimental data on these phenomena has driven this proposal. We propose the direct and in situ measurement of the crystalline CTE of irradiated Gilsocarbon nuclear graphite, over a test matrix comprising load, irradiation history, and temperature. The results will be incorporated into existing models for the stress state of polygranular graphite, improving model reliability.