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.

Microstructural characterisation of nuclear grade graphite

“…The irradiated induced changes to the graphite crystal structure within nuclear graphite have been shown to lead to disruption of the bonding across the basal plane [9] and closure of Mrozowski cracks [10,11], giving similar graphitic structures to that seen before graphitization. It is also known that the elastic modulus and coefficient of thermal expansion of graphite irradiated to a moderate fast neutron dose (40 Â 10 20 n/cm 2 EDND) are returned to values similar to that of carbon block stock.…”

Microcracks in nuclear graphite and highly oriented pyrolytic graphite (HOPG)

“…Intensive efforts have been made to measure the irradiation induced dimensional changes of graphite and to understand the mechanism of these changes [1][2][3]. Because naturally occurring single crystals of graphite are too small for use in the measurements of irradiation induced dimensional change, highly oriented pyrolytic graphite (HOPG) has been used as an approximant of single crystal graphite [1,4].…”

Irradiation induced creep behavior of H-451 graphite

“…The modified Simmons model [20,21] for dimensional changes (Eq. (8)) and for dimensional changes of a crept specimen (Eq.…”

“…The term F x is a pore generation term that becomes significant at intermediate doses when incompatibilities of irradiation induced crystal strains cause cracking of the bulk graphite [21]. For the purposes of this analysis the term F x is ignored.…”