Dimensional changes in pyrolytic graphite under fast-neutron irradiation

Abstract
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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