Surface area and porosity of unmodified graphitic matrices A3-27 and A3-3 (1950) and oxidized matrix A3-3 (1950)

Abstract

Graphitic matrices consist of graphite grains and ungraphitized binder carbon and are used as structural material in the spherical fuel elements of high-temperature, helium-cooled nuclear reactors (HTR). Like conventional nuclear graphites, matrix materials are porous, with an open and closed porosity of 11 vol% and 14 vol%. The generation of new pores and the opening of existing pores due to corrosion by gaseous coolant impurities alters the diffusion kinetics of the fission product cesium and the retention of tritium. Therefore, the oxidation-induced changes of the porosity of two matrix varieties were studied. The pore volumes accessible to xylene at 300 K and the nitrogen adsorption and desorption isotherms at 77 K were measured. The surface area was determined by the BET (Brunauer-Emmett-Teller) procedure and the α s -method. As standards for the α s -plots, a recently proposed isotherm for ungraphitized carbon and the isotherm for the original bulk matrix was used. The α s -plots are indicative of the appearance of micropores and a large increase in the surface area in mesopores due to oxidation. At low burnoff, the enlargement of the accessible pore volume is mainly caused by the opening of closed pores. Assuming a cylindrical pore shape, the mean radius of pores <0.1 μm was estimated and was found to be independent of the degree of oxidation. Based on the apparent BET surface area and the surface area in mesopores, r̄ BET = 16 Å and r ̄ α = 26 A ̊ was obtained.