General Description
Magnox reactors are pressurised, carbon dioxide cooled, graphite moderated reactors using natural uranium (i.e., unenriched) as fuel and magnox alloy as fuel cladding. Boron-steel control rods were used. The design was continuously refined, and very few units are identical. Early reactors have steel pressure vessels, while later units (Oldbury and Wylfa) are of prestressed concrete; some are cylindrical in design, but most are spherical. Working pressure varies from 6.9 to 19.35 bar for the steel pressure vessels, and the two prestressed concrete designs operated at 24.8 and 27 bar. No British construction company at the time was large enough to build all the power stations, so various competing consortia were involved, adding to the differences between the stations; for example nearly every power station used a different design of Magnox fuel element.
On-load refuelling was considered to be an economically essential part of the design for the civilian Magnox power stations, to maximise power station availability by eliminating refuelling downtime. This was particularly important for Magnox as the unenriched fuel had a low burnup, requiring more frequent changes of fuel than enriched uranium reactors. However the complicated refuelling equipment proved to be less reliable than the reactor systems, and perhaps not advantageous overall.
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