Every Magnox plant had a unique design of refuelling machine, reflecting the development of reactor technology over two decades. However, all had a number of common features.

In the early Windscale Piles, the fuel was inserted into horizontal channels, from which it could be pushed out into a cooling pool. The early French reactors also had horizontal fuel channels (Marcoule G1-G3), but these were followed by more conventional layout plants with vertical channels, as with the UK’s early commercial reactors. The first eight UK Magnox plutonium production reactors were designed and operated only with off-load refuelling but all subsequent reactors were designed for on-load refuelling to maximise time at power, although at the cost of considerable extra complexity both in plant and in safety case.

The large number of fuel channels in a Magnox reactor (several thousand) and the need to keep the number of pressure vessel penetrations to a minimum led to each refuelling standpipe servicing typically ~32 channels (with the potential in some cases to reach more remote channels). The fuelling machines are complex devices, with a number of functional requirements (in particular, for the majority of reactors in which on-load refuelling was undertaken):

• acting as a pressure boundary for the coolant circuit

• opening the standpipe closures

• loading and holding a reserve of new fuel

• having the means to locate any one of several tens of fuel channels from an individual standpipe

• lowering a fuel grab into each channel to pick up a fuel element

• storing used fuel elements in a carousel

• cooling discharged fuel

• shielding discharged fuel

• lowering new fuel into a channel

• discharging used fuel towards the fuel store

In addition, some fuel machines were designed to perform servicing activities on thermocouples, closures or control rods.