Typical Magnox coolant compositions are 1-1.5% CO v/v, 25-45 volume parts per million (vpm) H2, 10 vpm CH4 and 1 vpm H2O, the balance being CO2. In very simple terms, the impurities protect the graphite against corrosion, but act as a source of carbonaceous deposition.

Coolant control is effected by drier units (which remove water, including a large amount of tritiated water) and recombination units, which remove hydrogen, methane and carbon monoxide by the addition of oxygen over a catalyst bed. Oldbury Power Station also has a liquefaction plant for coolant clean-up, which removes impurities by distillation (including rare gases in the coolant, which are subsequently released to the stack).

Air ingress during outages is inevitable, and removed by purging prior to return to power.

Graphite corrosion has been a problem on some reactors, leading to loss of strength and reduction in the moderating power. As a consequence, Oldbury Power Station has moved to using slightly enriched fuel, and at one time was considering the use of Magnox fuel: a small assembly of AGR-type fuel pins with steel cladding and enriched oxide pellets.