Irradiation behavior of isotropic nuclear graphites

Various types of graphite are utilized as high temperature reactor materials, such as the neutron moderator, reflector, structural member and fuel matrix for high temperature gas-cooled reactors. The unique properties of graphite are its excellent moderating capability, adequate strength up to above 2000 deg C, good thermal properties, excellent machinability, and relatively low cost. The dimensional and property changes of reactor materials due to the damage by high energy neutron irradiation are dependent on the operational temperature, neutron flux and the microstructure. The isotropic graphite with relatively high density and high strength if most stable. The behavior of two types of commercial isotropic graphite with high density and high strength under irradiation was studied. The irradiation of 2.0 x 1021 n/cm2 was carried out in the Japan Material Testing Reactor at 550 deg C, and the changes of density, porosity, volume, microstructure, lattice constants, crystal size, elastic modulus and thermal conductivity were investigated. The specimens were DFP-1 graphite made by Poco Graphite Co. and Angloform graphite made by Anglo Great Lakes Co. The experimental method and the results are reported. Only the qualitative interpretation is possible at present because of the complexity of reactor graphite structures. (Kako, I.)