The calculation of methane profiles in graphite structures

The role of methane as an inhibitor of the radiolytic oxidation of the graphite moderator of an Advanced Gas Cooled Reactor (AGR) by the carbon dioxide coolant has been the subject of considerable study over the past decade. Although the exact mechanism of this inhibition process is not completely understood it is obvious that regions of the porous graphite structure remote from the surfaces directly accessible to the coolant can only be protected if in-depth penetration by the methane is possible. These in-pore mass transfer effects have been studied mathematically by equating the methane destruction rate with the rate of transport to and from the reaction site by means of both permeable flow and diffusion. More recently a numerical approach to the solution of the methane diffusion problem based on the FACSIMILE program for carrying out the relevant integrations has been adopted. This program has been written particularly to solve the mixed chemical, flow and diffusion problems which are typical of the kind occurring in the interpretation of AGR coolant chemistry. The advantage of a numerical technique such as this is that the solution method is quite general and may be applied to any type of radiolytic destruction term. Additionally, it is possible to solve for situations where parameters such as dose rate are a function of distance into the graphite phase and account may also be taken of the effect on the rate of the methane destruction process of changes in the water and carbon monoxide concentration produced within the structure as a result of this process. The model has recently been extended to cover this latter point and the resulting code is referred to as the MEDIC (MEthane DIffusion Calculation) program. The purpose of this paper is to describe briefly the assumptions made in the calculation and to highlight the more important conclusions. (author)