The Magnox reactor is illustrated schematically in Figure 2.4. The coolant is car­bon dioxide at a pressure of 20 bars (300 psia). The coolant is circulated through a core that consists of the moderator structure, which is built from

graphite bricks containing holes through which the coolant flows and in which the fuel elements are placed. Fuel elements consist of natural uranium bars clad in cans of a magnesium alloy known by the trade name Magnox (hence the name of the reactor). The alloy does not significantly absorb neutrons, so nat­ural uranium, rather than enriched uranium, can be used as a fuel. A typical Magnox core would be 14 m in diameter and 8 m high. The coolant gas leaves the core at 400 eC, flows to the steam generator, and from there flows back through the gas circulator to the reactor. In the earlier designs of Magnox reactors, the pressure vessel containing the core was made of steel. In later designs it was combined with the shielding in the form of a prestressed concrete pressure ves­sel, which also contained the heat exchangers (in the earlier designs these were external to the pressure vessel and the shielding as shown in Figure 2.4). Magnox reactors were constructed in the United ^ngdom, France, Italy, and Japan and have operated very successfully since their construction, which in some cases was around 35 years ago. The steam cycle efficiency of Magnox reactors is about

A Magnox fuel ^element

31%; this means that 69% of the nuclear heat is rejected to atmosphere via the cooling towers (Section 1.1.3).

A Magnox fuel element is shown in Figure 2.4b. The outside of the Magnox can is machined in a complex pattern of fins (“herringbone” pattern), which has been shown by detailed heat transfer experiments to be the optimal form. The swirl of the gas in the channel and the fins on the surface are an aid to heat trans­fer. The advantages and disadvantages of various coolants will be discussed in Chapter 3, where we shall also discuss some basic principles of heat transfer.

Although the Magnox reactor has been remarkably successful and reliable, it has disadvantages compared with some other reactor types. The principal one is its relatively low power output per unit volume of core. This leads to a large size for the core, a large investment in fuel, and high capital costs. Table 2.3 compares various reactors in terms of the average power generation rate per unit volume of the core (called the average volumetric power density). It also shows the rate of power generation per tonne of fuel (the averagefuel rating) and the power generation per unit length of fuel (the average linearfuel rat- infi). Compared with other reactors, the Magnox has a very low volumetric power density and a very low average fuel rating per unit mass of fuel. Both of these factors lead to high costs due to the high fuel inventory and large cores.