Microstructural Deformation and Modulus Changes of Nuclear Graphite due to Hydrostatic Pressure Loading

Abstract

For the first time, the influence of pore closure on elastic modulus in the UK's Magnox and Advanced Gas Cooled Reactors (AGRs) graphite was determined by subjecting the graphite samples to high hydrostatic pressure and dynamic loading. Two grades of nuclear graphite were tested; anisotropic Pile Grade A (PGA) which was used in the Magnox reactors, and semi-isotropic Gilsocarbon as used in the AGRs [1] . The volumetric strain was measured in two directions during compression and separately the velocity of sound through the samples was also determined as a function of confinement pressure. Under hydrostatic loading, the stiffness of PGA graphite was reduced after a few percent of volume strain before increasing again after about ∼20% volumetric strain. Gilsocarbon showed similar behaviour to PGA at lower volumetric strain (∼10 to 13%) however due to Gilsocarbon having a higher density and lower porosity compared to PGA, the response was generally stiffer than PGA. During unloading, significant hysteresis was observed in both graphite grades, with the sample volume almost fully recovering when the pressure was completely removed. Micromechanical models are used to explain the poro-elastic response of graphite during hydrostatic compaction, and to make comparisons and extend data on the influence of pore change due to oxidation on modulus.