The discharge irradiation of Magnox fuel was determined by burn-up of the fissile U-235 component, and also by the potential for swelling or deformation of the uranium fuel.

Changes in the crystalline structure of the fuel under irradiation conditions lead to anisotropic changes in dimension: irradiation growth/creep. The effect is particularly important under conditions of low temperature and high stress, such as those found in the bottom of the fuel stack. Irradiation creep can be minimised by careful control of the crystalline structure of the fuel, which is influenced by the minor alloying components and by the manufacturing process.

As irradiation proceeds, fission gas builds up within the fuel. Diffusional processes result in microscopic bubbles forming within the fuel matrix, leading to swelling and possibly deformation of fuel elements. Swelling occurs primarily within a narrow band of temperatures centred around about 400 °C. This results in a typically annular region of porosity and swelling within a fuel element, which swells increasingly as irradiation (and so fission gas inventory) proceeds. The uranium metal was alloyed with small amounts of (principally) aluminium and iron to improve its resistance to swelling.

The results of irradiation growth and fuel swelling can lead to breaching of the fuel cladding. Bowing of fuel under irradiation also occurs and could lead to the fuel becoming hard to withdraw from the channel, given the typical clearance of a few millimetres from fin tip to channel wall. Bowing of the fuel was exacerbated by each fuel element supporting the weight of all of the elements above it.

Cladding ductility is also an issue for Magnox fuel. In general, a ductile clad is good as it will accommodate changes in fuel dimensions. Fine grain structures aid ductility, but are weaker at high temperatures. As a result of this two types of fuel element, known as HT and LT variants, were produced for many reactors. HT (or HTA) fuel was annealed at a high temperature giving a coarse-grained structure suitable for high-temperature operations. LT fuel was annealed at a lower temperature giving a fine-grain structure. As maximum fuel deformation, driven by dimensional changes in the uranium bar, occurs at relatively low operating temperatures, LT fuel gave better performance for elements at the bottom region of the fuel channel.

Considerable research was undertaken by the main UK operator of Magnox reactors, the Central Electricity Generating Board (CEGB) during its lifetime from 1957 to 1990, on optimisation of fuel irradiation conditions and of fuel element design. The resulting improvement in fuel performance led to channel average irradiations doubling from 3.6 GWd/Te(U) for early operations to eventually 7.2 GWd / Te(U) and to peak element irradiations of 9 GWd/Te(U). At the later levels irradiation was limited by burn-up of the fuel leading to loss of reactivity. As permissible irradiations increased, ‘double dwelling’ of fuel became standard practice, in which lower irradiation elements from top and bottom of the channel stack were returned for a second irradiation period