The sorption and diffusion of caesium in two nuclear graphites at a concentration range of 5 x 10-2 to 10-3 atom % and at temperatures in the range 500 to 1400 has been studied using a microgravimetric desorption technique. Concentration profiles have been obtained at various stages in sin adsorption, heat treatment, desorption programme by electron probe microanalysis. The desorption curves consisted of an initial fast period controlled by evaporation of caesium from the surface, followed by a slow rate, controlled by diffusion from the bulk of the graphite. The desorption curves were analysed to obtain diffusion coefficients by several techniques and the validity of each method is considered. The effects of a non-uniform concentration of caesium and a surface evaporation step on the transport of caesium in the graphite were also considered. The concentration profiles for caesium in graphite, found by electron probe microanalysis, showed evidence of a complex mechanism for transport of caesium in graphite with apparent violations of Fick's Law. This behaviour was qualitatively explained in terms of a two part diffusion mechanism combined with surface evaporation. Slow diffusion in gilsonite graphite was described by log(D) = -5.1 - 137000/2.303 RT. Desorption from the fine grain graphite was controlled by mixed diffusion. Evidence for sensitivity of diffusion coefficients to graphite structure and thermal history was found, although no evidence for the dependence of diffusion coefficients upon the concentration of caesium was found. The construction of a Knudsen double cell-mass spectrometer apparatus designed to measure caesium-graphite equilibrium vapour pressures is also described.
|Date of Award||1974|