The moderator

All Magnox reactors were graphite moderated and CO2 cooled. Graphite is a highly stable material under high temperatures, but it was realised in the very early days of nuclear power that radiation damage would cause atoms to occupy interstitial positions building up stresses in the crystalline structure. These stresses could be annealed out, with the release of Wigner energy. It was loss of control of an annealing operation to release Wigner energy that initiated the 1957 Windscale Pile 1 fire. Wigner energy considerations are important in determining the thermal operating design of the reactor to avoid the need for annealing operations and thermal cycling.

Generally speaking, each channel is centred on a stack of graphite bricks. Interstitial channels permitting the introduction of control rods, flux-flattening elements or instrumentation are located around each channel brick, and the core is held together with radial keying of the bricks. The keying is designed such that changes in graphite dimensions under irradiation and heating could be accommodated without undue distortion to the fuel and control channels.

Surrounding the core is a graphite reflector, to reduce neutron leakage from the core.

Although graphite is highly corrosion resistant to CO2 , irradiation of coolant gives rise to free radicals, which are corrosive. Two undesirable effects may come into play: oxidation of graphite leading to loss of strength and moderating capacity, and deposition of carbonaceous material on fuel or boiler surfaces leading to reduction of heat transfer capacity. Avoidance of steel corrosion is also an important consideration in coolant chemistry.