ANALYSIS OF THE DIMENSIONAL CHANGES AND STRUCTURAL-CHANGES IN POLYCRYSTALLINE GRAPHITE UNDER FAST-NEUTRON IRRADIATION

The dimensional changes of two very different graphites under fast neutron irradiation are analysed in detail. One of the graphites is anisotropic, manufactured from needle coke (Pile Grade A graphite), while the other is isotropic, manufactured from Gilsonite pitch coke(AGR moderator graphite). Samples of both graphites were irradiated in the temperature range 400-600-degrees-C in the PLUTO Materials Testing Reactor at AEA Technology Harwell in their normal form and several variants. The latter included a series of Pile Grade A base stock (i.e., unimpregnated), heat treated to various final temperatures and both graphites doped substitutionally with boron eleven. It is found that the dimensional changes, pore volume changes and various structure factors relating macroscopic to microscopic properties are controlled by the parameter X(T), the difference in crystallite strains parallel and perpendicular to the hexagonal axis. A number of correlations between the structure factors connecting various properties are proposed and a qualitative model described which, with appropriate parameters, should apply to all graphites under neutron irradiation.