The mechanisms of irradiation creep in graphite

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Abstract
The motion of edge and screw dislocations in a graphite polycrystal subjected to external forces and fast neutron irradiation is considered. A transient creep strain, proportional to the external stress and proportional to the ratio of the dimensions of the component crystallites in the a and c directions, is deduced. The calculated and experimentally observed magnitudes agree. From the time constant of this transient a value for the number of defect pairs created by a neutron during its moderation is calculated. There is fair agreement with accepted estimates. Graphite crystals show irradiation growth and a polycrystal must therefore either disintegrate or show yielding creep. The equations of yielding creep predict a creep rate greater than that observed, but they do not take into account that graphite is not a continuous solid. The discrepancy may thus be resolved. The possibility of an increase in the number of modes of deformation during neutron irradiation is discussed.

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