Magnox reactors employ pile grade A (PGA) graphite as moderator. Reactor cores are typically constructed of twelve to thirteen layers of interlocking graphite bricks. Their temperature varies from 250°C at the bottom to 360°C at the top of the core. It serves a dual role as both the moderator and encasing the fuel in the channels. These are through the middle of the bricks and continuous through the core. The bricks are either octagonal or square in shape. A unit of eight brick of equal numbers of each shape has a single interstitial channel at the point where the corners of two octagonal and two square bricks meet. The interstitial channels are used for control rods, absorbers and canisters of graphite samples installed to replicate the service exposure of reactor bricks and to be used for measurements. The graphite loses mass during service due to radiolytic oxidation, by CO_2 caused by energy deposition, mainly y radiation. Neutron irradiation brings about hardening and dimensional change which decrease with the increasing distance from the bore to the outer surface of the brick. The gradient in the dimensional changes as well as thermal transients generate internal strains and in turn stresses. This paper reviews changes of some physical and mechanical properties of graphite during service and describes the cracking and fracture behavior of graphite. Statistical analysis of density showed that it decreases during the service with increasing neutron dose and decreasing reactor core height/temperature. Crack initiation involves a prior formation of a process zone.
展开▼
