A study of the reactions of graphite with carbon dioxide with reference to gas‐cooled nuclear reactors

Abstract

The reaction of graphite and carbon dioxide is discussed in relation to the performance and safety of gas‐cooled graphite‐moderated nuclear reactors. Reaction is promoted by reactor radiation but this radiation‐induced process is unlikely to be temperature dependent. The limiting temperature for operating the system graphite‐carbon dioxide is, therefore, probably set by the thermal reaction. A wide variety of experimental means has been used to examine the thermal reaction and it is concluded that gasification of carbon by carbon dioxide is unlikely below temperatures of 600°. This conclusion appears valid even if applied to graphite which has undergone heavy neutron damage. Accelerated life tests covering several thousand hours have been used to confirm these observations. These same tests show that gasification of graphite occurs at temperatures above 600° but is strongly inhibited by the presence of carbon monoxide. This provides an additional factor of safety from the reactor point of view. The most pessimistic estimate for the rate of gasification of graphite in carbon dioxide by purely thermal means at 500° is calculated to be 0.1% (by weight) in 20 years. The carbon monoxide observed after exposing graphite to carbon dioxide at temperatures below 600° arises from the formation of a stable oxide on the carbon surface. Carbon monoxide can reduce this surface oxide at 500° and the clean surface produced will adsorb carbon monoxide. Carbon monoxide in contact with ungraphitised carbon may decompose and deposit carbon. This deposition reaction is not self‐propagating and leads to an inert carbon surface.