Effect of high-flux fast-neutron irradiation on the physical properties of graphite

Abstract

The changes in the physical properties of British reactor grade graphite induced by high-flux fast-neutron irradiation, under closely controlled conditions of temperature and flux, have been studied to higher doses than those previously published. Irradiation-induced changes in thermal conductivity, thermal expansion coefficients, Young's modulus, electrical resistivity, and dimensions have been observed for irradiation doses of up to 3 × 10 21 n. cm −2, as measured by the reaction 58 Ni ( n, p) 58Co, at irradiation temperatures in the range 225–650°C. The results are discussed briefly in terms of the most recent theories of irradiation damage in graphite. In particular, it is shown that the observed dimensional changes can be entirely accounted for by crystallite growth in the c-axis direction and shrinkage in the a-axis direction. Some preliminary data on the changes in dimensions and coefficients of thermal expansion in highly orientated pyrolytic graphite are also described and these are shown to be in reasonable agreement with the crystallite changes calculated from the observed behaviour of the polycrystalline reactor grade material.