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Measurements are reported of the principal strains and changes in principal thermal expansion coefficients of various grades of pyrolytic graphite after fast-neutron bombardment at constant temperatures in the range 150 to 650 °C. The pyrolytic graphites used in these studies possess properties approaching those of a monocrystal and the irradiation effects observed thus closely represent those expected in a crystal. The observations are compared with previous inferences of the behaviour of crystals in less well oriented polycrystalline materials under the same irradiation conditions and are discussed in terms of recent theories of the accumulation of irradiation damage in fast-neutron irradiated graphite. The existence of a new type of vacancy configuration is postulated to explain the crystal dimensional changes at high doses and high temperatures. A qualitative explanation of the changes in crystal thermal expansion coefficients is proposed.

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The changes in dimensions and linear thermal expansion coefficients of several polycrystalline graphites irradiated at 170 and 200 °C are presented. The results are compared with the changes in these parameters which were observed in very highly oriented pyrolytic polycrystalline graphite and were discussed in the preceding paper. The relation between dimensional changes occurring in polycrystalline aggregates and those in a monocrystal is discussed in terms of structural parameters Ax. The variation of these parameters with crystal strain is derived for the different graphites which have been investigated. The analysis shows that, at low crystal strains, c axis crystal growth is absorbed by porosity so that the bulk volume changes in the polycrystalline materials are less than those in a single crystal. However, as crystal strain closes the porosity, the rate of bulk volume change approaches that of the single crystal and eventually equals it. At still higher crystal strains nett porosity generation can occur so that the bulk expands at a faster rate than the single crystal. The results also show that the relationship between linear growth rate and the linear thermal expansion coefficient, which has been well established at low crystal strains, breaks down at high strains. However, the results have been used to develop a new model. This enables the crystal dimensional changes to be determined as a function of fast-neutron dose at any irradiation temperature, using bulk dimensional change data from experiments on pile grade A graphite. The crystal strains determined from the new model can be used to predict the dimensional changes of other graphites at other irradiation temperatures from their behaviour at 200 °C.