The magnox reactor (Table 1.8 and Fig 1.31) uses a graphite moderator, carbon dioxide coolant and metal­lic natural uranium fuel clad in a magnesium alloy known as magnox. The core is cylindrical in shape and constructed of several hundred tonnes of graphite blocks which is penetrated by vertical 100 mm dia­meter channels in which the fuel elements, typically seven or eight in number, are stacked one on top of another. To achieve the required rate of heat transfer from fuel to coolant, the upward flowing CO2 is at pressure and the magnox cladding has an extended surface in the form of fins along its length. The fuel rod is about 25 mm diameter and 1 m in length.

Nine commercial twin-reactor magnox power sta­tions have been built in the UK. In the early design the core was enclosed in a spherical steel pressure vessel and connected by large diameter ducts to external boilers. However, as the design coolant pressure was progressively increased from 9 to 19 bar, a significant change was introduced for the last two of the type: the steel vessel was replaced by a cylindrical pre­stressed concrete pressure vessel. This enclosed the core, the pressure circuit, boilers and gas circulators. The concrete pressure vessel was also adopted for the AGRs and is referred to in the next Section 9.4.2.

The power density of a magnox core is low, 0.8 MW/m3, and thus the core is comparatively very large with several thousand channels on a half metre

pitch. Thermal output ranges from a few hundred MW(th) per reactor for the earlier stations to nearly 2 GW(th) for the latest. Refuelling, a fuel element at a time with average burn-up of 5500 MWd/t, is on-load and requires a sophisticated charge/discharge machine. Access to a channel is via a standpipe from the charge face above the core, each standpipe serv­ing a number of fuel channels. At full load three or four channels need to be refuelled daily.

Reactivity control is by means of mild steel and boron steel control rods. Also flux flattening absorb­ers may be loaded and removed by the refuelling machine.