The reactivity of graphite to air is important in the magnox steel pressure vessel stations since a loss of coolant pressure with consequent air ingress is con­sidered a ‘credible’ accident. Fault study calculations are regularly carried out using appropriate graphite data, since the oxidation reaction is exothermic and the resulting heat release could raise temperatures in the core and so further increase the rate of the oxi­dation. It is thus important to demonstrate that the release of heat from graphite oxidation, and from other sources such as the oxidation of amorphous carbon deposits and from the release of stored ‘Wigner’ ener­gy can be contained by natural circulation until pony — motor or full flow blower power cooling flow can be reinstated.

The chemical reactivity of the graphite increases with irradiation, both through neutron damage af­fecting the reactivity of surface sites and also through the opening of closed pore structure by the radiolytic oxidation during normal operation. Further catalytic material, e. g., metal oxides, may accumulate raising the potential oxidation rate in oxygen. The quantity of the more reactive deposits also increase throughout’ reactor life. Therefore extensive monitoring of these related graphite chemical properties is carried out and used in conjunction with established models to predict the core state for future operational periods.

The computer program ‘RHASD’ (Reactor Heating After Shut Down) takes account of these and other relevant core data in calculating fuel and graphite temperatures throughout this hypothetical transient. Pessimised data is employed to ensure that both graph­ite and fuel temperatures peak at a safe value and then show a subsequent fall if subsequent operation of the reactor were permitted.

AGRs (air)

In AGRs, the maximum credible accident does not result in air ingress into the graphite core and hence the thermal reactivity of graphite-to-air is not of ma­jor concern. However, there are two instances where a knowledge of air reactivity can influence reactor operation. Firstly, major reactor overhauls are carried out in an air environment and it is necessary to deter­mine the maximum acceptable temperature during the subsequent raise power sequence when the air has to be purged and a CO2 atmosphere established. Second­ly, if a fuel pin deposit burn-off-coolant, e. g., 1-10 vpm О;/CO2 has to be adopted at any time. In addition the fuel stringer will see an air environment subsequent to its removal from the reactor and it is necessary to define acceptable temperature limits for the graphite sleeves. Experiments have therefore been carried out to measure the reactivity between modera­tor and sleeve graphite, both virgin and irradiated.

AGR reactors (carbon dioxide)

During normal AGR operation the temperature of the moderator is so low that thermal oxidation by CO2 is negligible. This is also the case for the graphite fuel sleeves but in this latter case, due to the higher temperature, it has been necessary to confirm this by experiment with both virgin and irradiated graphite samples, although no effect of irradiation has been observed. In some designs of AGR, graphite bearings have been used as boiler supports and experiments have been carried out to confirm their design life, although in this case the radiation levels are low and no irradiated samples were tested.