The poor performance of the Kelly and Burchell model (eqn [25]) at predicting the high temperature (900 °C) and high dose 6 MPa tensile creep data suggests that the model requires further revision.50,7i H-451 graphite irradiated at 900 °C goes through dimensional change turn-around in the dose range 1.3—1.5 x i022ncm~2 [E>50] (—8.8-10.2 dpa). This behavior is understood to be associated with the

generation of new porosity due to the increasing mismatch of crystal strains. The Kelly-Burchell model accounts for this new porosity only to the extent to which it affects the CTE of the graphite, through changes in the aligned porosity.

Gray70 observed that at 550 °C the creep rate was approximately linear. However, at 800 °C he reported a marked nonlinearity in the creep rate and the changes in CTE were significant. Indeed, for the two high density graphites (H-327 and AXF-8Q) Gray reports that the 900 °C creep strain rate reverses. Gray postulated a creep strain limit to explain this behavior, such that a back stress would develop and cause the creep rate to reduce. Other workers have shown that a back stress does not develop.62 However, Gray further argued that a creep strain limit is improbable as this cannot explain the observed reversal of creep strain rate. Note that a reversal of the creep rate is clearly seen in the 900 °C tensile creep strain data reported here for H-451 (Figure 23). Also, a creep strain limit would require that tensile stress would modify the onset of pore generation behavior in the same way as compressive stress, because the direction of the external stress should be immaterial.70 More recent data52 and the behavior reported by Burchell71 show that this is not the case. Gray70 suggests that a more plausible expla­nation of his creep data is the onset of rapid expansion accelerated by creep strain; that is, net pore generation begins earlier under the influence of a tensile applied stress. Indeed, it has been observed52 that compressive creep appears to delay the turnaround behavior and tensile creep accelerates the turnaround behavior (Figure 18).

In discussing possible explanations for his creep strain and CTE observations, Gray70 noted that changes in the graphite pore structure that mani­fested themselves in changes in CTE did not appear to influence the creep strain at higher doses. The classical explanation of the changes in CTE invokes the closure of aligned porosity in the graphite crys­tallites. Further crystallite strain can be accommo­dated only by fracture. A result of this fracture is net generation of porosity resulting in a bulk expansion of the graphite. A requirement of this model is that the CTE should increase monotonically from the start of irradiation. A more marked increase in CTE would be seen when the graphite enters the expan­sion phase (i. e., all accommodating porosity filled). The observed CTE behavior, reported previously50 and in Gray’s70 work, does not display this second increase in CTE; thus, the depletion of (aligned) accommodation porosity is not responsible for the early beginning of expansion behavior.

The observation by Gray70 and Kennedy63 that creep occurs at near constant volume (up to moder­ate fluence) indicates that creep is not accompanied by a net reduction ofporosity compared to unstressed graphite, but this does not preclude that stress may decrease pore dimensions in the direction of the applied stress and increase them in the other, that is, a reorientation of the pore structure. Pore reorienta­tion could effectively occur as the result of a mecha­nism of pore generation where an increasing fraction of the new pores are not well-aligned with the crystal­lites basal planes (and thus they would not manifest themselves in the CTE behavior) or accompanied with the closure of pores aligned with the basal planes.

Kelly and Foreman53 report that their proposed creep mechanism would be expected to break down at high doses and temperatures, and thus deviations from the linear creep law (eqn [12]) are expected. They suggest that this is due to (1) incompatibility of crystal strains causing additional internal stress and an increasing crystal creep rate, (2) destruction of interstitial pins by diffusion of vacancies (thermal annealing of vacancies in addition to irradiation an­nealing), and (3) pore generation due to incompati­bility of crystal strains.

It is likely that pore generation can manifest itself in two ways: (1) changes in CTE with creep strain — thus, pores aligned parallel to the crystallite basal planes are affected by creep strain — and (2) at high doses, pore generation or perhaps pore reorientation, under the influence of applied and internal stress that must be accounted for in the prediction of high neutron dose creep behavior.

Brocklehurst and Brown62 report on the annealing behavior of specimens that had been subjected to irradiation under constant stress and sustained up to 1% creep strain. They observed that the increase in creep strain with dose was identical in compression and tension up to 1% creep strain, and that the CTE was significantly affected in opposite directions by compressive and tensile creep strains. Irradiation annealing of the crept specimens caused only a small recovery in the creep strain, and therefore provided no evidence for a back stress in the creep process, which has implications for the in-crystal creep mech­anism. Thermal annealing also produced a small recovery of the creep strain at temperatures below 1600 °C, presumably because of the thermal removal of the irradiation-induced defects responsible for dislocation pinning. Higher temperature annealing produced a further substantial recovery of creep strain. Most significantly, Brocklehurst and Brown62 reported the complete annealing of the creep induced changes in CTE, in contrast to the total creep strain, where a large fraction of the total creep strain is irrecoverable and has no effect on the thermal expan­sion coefficient. Brocklehurst and Brown62 discuss two interpretations of their results, but report that neither is satisfactory. One interpretation requires a distinction between changes in porosity that affect the CTE and changes in porosity affecting the elastic deformation under external loads, that is, two distinct modes ofpore structure changes due to creep in broad agreement with the mechanism discussed earlier.

The modified Simmons model29,30,67 for dimen­sional changes (eqn [18]) and that for dimensional changes of a crept specimen (eqn [20]) both have pore generation terms which are currently neglected. It now appears necessary to modify the current Kelly-Burchell creep model (eqn [25]) to account for this effect of creep strain on this phenomena; that is, we need to evaluate and take account of the terms Fx and F’x as well as include the term (F’x — Fx) in eqn [25]. Such a term should account for pore gener­ation and/or reorientation caused by fracture when incompatibilities in crystallite strains become exces — sive.71 Clearly, further work is needed in the area of irradiation-induced creep of graphite.