Magnox (magnesium non-oxidising) reactors are pressurised, carbon dioxide–cooled, graphite-moderated reactors using natural uranium (i.e. unenriched) as fuel and Magnox alloy as fuel cladding. Working pressure varies from   for the steel pressure vessels, and the two reinforced concrete designs operated at . Magnox alloy consists mainly of magnesium with small amounts of aluminium and other metals&mdash;used in cladding unenriched uranium metal fuel with a non-oxidising covering to contain fission products. This material has the advantage of a low neutron capture cross-section, but has two major disadvantages:
* It limits the maximum temperature, and hence the thermal efficiency, of the plant.
* It reacts with water, preventing long-term storage of spent fuel under water - such as in a spent fuel pool.
Magnox fuel incorporated cooling fins to provide maximum heat transfer despite low operating temperatures, making it expensive to produce. While the use of uranium metal rather than oxide made nuclear reprocessing more straightforward and therefore cheaper, the need to reprocess fuel a short time after removal from the reactor meant that the fission product hazard was severe. Expensive remote handling facilities were required to address this issue.