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Abstract: The cores of the fourteen Advanced Gas-cooled nuclear Reactors (AGRs) within the UK comprise Gilsocarbon graphite, a manufactured material surrounded predominantly by CO2 at high pressure and temperature to provide heat exchange. The intense ionising radiation within the reactors causes radiolytic oxidation, and the resulting mass loss is a primary factor in determining reactor lifetime. The void structure of the porous Gilsocarbon graphite affects the permeability and diffusion of the carbon dioxide, and the sites of oxidation. To model this void structure, the porosities and densities of ten virgin Gilsocarbon graphite samples have been measured by powder and helium pycnometry. For comparison, results are also presented for highly ordered pyrolytic graphite (HOPG), and a fine-grained Ringsdorff graphite. Samples have been examined at a range of magnifications by electron microscopy. Total porosities and percolation characteristics have been measured by standard and cyclic mercury porosimetry up to an applied mercury pressure of 400MPa. Inverse modelling of the cyclic intrusion curves produces simulated void structures with characteristics which closely match those of experiment. Void size distributions of the structures are presented, together with much Supplementary Information. The simulated void networks provide the bases for future simulations of the radiolytic oxidation process itself. [Copyright &y& Elsevier]

An algorithm has been developed for the a priori calculation of tortuosity in a simulated void network, assuming a Markovian random walk process, with paths identified using the algorithms of Yen and Dijkstra, and path searching extended by successive pruning of the network graph. The void network is derived from the inverse modelling of percolation characteristics derived from mercury intrusion porosimetry. Experimental tortuosities have been determined for two porous limestones and one porous sandstone, by measuring the electric conductivity of inter-pore brine relative to that of the same quantity of bulk brine. A close match between simulation and experiment is obtained ( R 2 =0.95). Tortuosities are also calculated for larger charged particles and for viscous transport. Further validation is provided in the form of a sensitivity analysis of tortuosity with respect to network connectivity. The new approach is particularly useful for the many materials that can be characterised by mercury porosimetry or porometry, but for which tortuosity cannot be measured directly. It has applications in a wide range of areas of current interest, such as oil and gas engineering, nuclear reactor core modelling, filtration, catalysis, ceramics, membranes and soil science. [ABSTRACT FROM AUTHOR]

This work analyses the applicability of the Lucas–Washburn equation to experimental observations of imbibition into real network structures. The experimental pore structures used in this study are constructed from tablets of two finely ground calcium carbonates, with defined differences in particle size distribution. These are compressed under a range of different applied pressures to achieve a controlled series of porosities while maintaining the surface chemical, particulate and morphological pore characteristics constant. The porosities are determined by mercury intrusion porosimetry applying corrections for mercury compression and penetrometer expansion together with a correction for sample skeletal compression (Gane et al., J. Am. Chem. Soc., 35 (1996)). Imbibition studies are made by bringing each porous sample into contact with a supersource of liquid and the dynamic imbibition is recorded gravimetrically. Results follow a long timescale macroscopic absorption rate depending on the square root of time but show a failure to scale according to pore size in the Lucas–Washburn equation even though the constants of surface energy, contact angle and fluid viscosity have been maintained. Furthermore, values of average measured pore radius are shown to be finer than the Lucas–Washburn predicted equivalent hydraulic capillary radius. The predominance of a relevant pore size within a given pore size distribution forming a selective pathway filling based on inertial retardation of larger pores and short-term linear time wetting in finer pores is argued to account for the departure from simple pore size scaling. [Copyright &y& Elsevier]