A mechanism for the irradiation-induced creep of graphite was proposed by Kelly and Foreman53 which involves irradiation-induced basal plane dis­location pinning/unpinning in the graphite crystals. Pinning sites are created and destroyed by neutron irradiation (radiation annealing). Under neutron irradiation, dislocation lines in the basal planes may be completely or partially pinned depending upon the dose and temperature of irradiation. The pinning points were speculated to be interstitial atom clus­ters 4 ± 2 atoms in size,54,55 that is, the same defects clusters assumed to contribute to the reduction in thermal conductivity. The interstitial clusters are temporary barriers as they are annealed (destroyed) by further irradiation. Thus, irradiation can release dislocation lines from their original pinning site and the crystal can flow as a result of basal plane slip at a rate determined by the rate of pinning and unpinning of dislocations. Kelly and Foreman’s the­ory assumes that polycrystalline graphite consists of a single phase of true density p0 and apparent density p. The material may be divided into elementary regions in which the stress may be considered uniform and which may be identified as monocrystalline graphite. Significantly, the model excludes porosity. It is further assumed that the only deformation mode is basal plane slip for which the strain rate is determined by

exz k(sxz)f [9]

and

£yZ = k(syz)f [10]

where f is the fast neutron flux; k, the steady-state creep coefficient, and a is the stress in the given direc­tion. The microscopic deformation assumes the usual relationship between the basal plane shear strain rate (є) and the mobile dislocation density (O), and is given by

e = Obn = kaf [11]

where b is the Burger’s vector and n is the dislocation velocity as a function of the pinning point concentra­tion in the basal plane as the pins are created and destroyed by neutron flux. The dislocation line flow model used the flexible line approach where the dislo­cation line is pinned/unpinned and the dislocation line bowing is a function of the line tension and pin spacing. The concentration of pinning sites increases under irradiation from the initial value (from intrinsic defects) to a steady-state concentration. The initial creep rate is high and decreases to a steady-state value as the pinning concentration saturates at a level controlled by the neutron flux and temperature. This saturation would be expected to occur over the same dose scale as the reduction of thermal conductivity to its saturation limit (see Section 4.10.5.2).

Thus, a two stage model can be envisioned where the primary creep rate is initially high and falls to a secondary or ‘steady-state’ creep rate. The steady-state creep term should be the dominant term when the dose has reached values at which physical property changes due to dislocation pinning have saturated (see Section 4.10.5.2). Kelly and Foreman state that at higher tem­peratures the steady-state (secondary) creep rate (k)