DAMAGE EFFECTS TO GRAPHITE IRRADIATED UP TO 1000 C

Abstract

Irradiation effects at 500 deg C in experiemtal graphites with varied density, crystallinity, surface area, and pore distribution are discussed. Changes in macroscopic properties are dependent upon the initial crystallite structure; however, the mechanism by which the increase of interlayer spacing and decrease of apparent crystallite size effect these changes is not well understood. Macroscopic properties are also dependent upon the arrangement of crystallites and whole coke particles within the over-all structure which raniation may change slightly. Graphite irradiations in the Materials Testing Reactor extend exposure temperatures from 800 to 1000 deg C. The temperature coefficient of property damage decreases with temperature, aad only slightly less damage occurs at 1000 deg C than at 500 deg C. Thermal conductivity decreases by a factor of 50 with 30 deg C irradiation, a factor of 3 with 500 deg C irradiation, and a factor of 2 with 750 deg C irradiation. Within the irradiation temperature range 500 to 1000 deg C, the contraction rate after the first 1500 Mwd/t is measurably the same: C/ sub 0/ spacing expands slightly: and apparent crystallite size decreases by one- half. The total stored energy content is decreased with increased irradiation temtemperature. The way in which damage effects in irradiated graphite are distributed may be measured by the ease with which they may be thermally annealed. By estimating an activation energy for a given temperaturetime annealing and measuring the amount of property annealed, at repeated increasing temperatures, it has been possible to characterize the damage with a damage distribution curve. Thermal annealing experiments on graphites with varied exposures and temperatures of irradiation have included property measurements of dimensions, thermal conductivity, interlayer spacing and crystallite size, A damage mechanism is discussed which attempts tp correlate and explain the changes in properties resulting from graphite irradiations at high crystallite does not contract (C/sub 0/ expands slightly) while the density of the graphite increases, reactor radiation of thigh temperature must result in a more efficient packing of the crystallites. Damage distribution curves of samples irradiated at a variety of temperatures can be understood in terms of a process whereby energetic carbon atoms transfer a large amount of energy to the lattice close to a crystal ddefect. In this way, the activation energy for annealing which is in excess of equilibrium lattice temperature is supplied to the damaged lattice. (auth)