The effects of thermal annealing on the mechanical properties of PGA graphite

The two principal graphite ageing processes in a Magnox reactor core are fast neutron damage and radiolytic graphite oxidation. Graphite strength, Young's modulus and thermal conductivity are affected by both these processes. Relationships describing the separate effects of these two processes on PGA graphite have been derived from Materials Testing Reactor data. The combined effect of the two processes is assumed to be multiplicative in accordance with the so-called "Product Rule". At sufficiently low doses, property changes due to fast neutron irradiation are caused principally by dislocations in the graphite crystal lattice produced by the displacement of carbon atoms. These dislocations can be removed by thermal annealing, giving rise to recovery of graphite properties. The success of the Product Rule in describing the combined effects of irradiation damage and oxidation on strength and Young's modulus has been tested on PGA graphite by the use of thermal annealing. Results from early Materials Testing Reactors are briefly reviewed, and new data for material irradiated recently in the Idaho National Laboratories Advanced Test Reactor are presented together with baseline measurements made on virgin PGA graphite.