The development of transport and non-transport porosity in radiolytic graphite oxidation

In graphite moderated, carbon dioxide cooled, reactors, radiolytic graphite oxidation arises from the production of short lived energetic species and is confined to the internal porous structure. Exponential weight loss is possible as the volume of internal porosity, absorbing radiation, increases with time. Inhibitors are added to the coolant to minimise weight and hence strength loss. Carbon monoxide and methane are the principal gas phase inhibitors, competing with the graphite for the oxidising species. Methane has the further effect of producing a sacrificial layer at the pore wall. It follows from the mechanisms of inhibition that oxidation is reduced in the larger pores. In the small pores, the probability is high that oxidising species will reach the pore wall and exponential weight loss can occur until such time as the pores become sufficiently large for the inhibitors to operate. The results from a high weight loss experiment confirm this behaviour and allow predictions to be made with some confidence for other coolants - for which initial oxidation rate data are available. In this paper the results from an earlier weight loss experiment in an 'uninhibited', nominally pure, carbon dioxide coolant are assessed. Particular attention is drawn to the information which can be obtained from a study of transport properties as they develop with graphite weight loss. The objectives in understanding more exactly the process occurring inside the complex pore structure are to allow extensions in planned life, or greater flexibility in coolant plant operation and compatibility with fuel clad. (author)