The fuel for magnox reactors is natural uranium clad in a magnesium/aluminium alloy (magnox) supported in a graphite core which is also the moderator and cooled by pressurised ССЬ. The early magnox reactors are contained w’ithin a mild steel pressure vessel and the later ones in a prestressed concrete pressure vessel.

Magnox melts at about 640°C and its structural strength becomes significantly reduced some 10-20°C below this. At temperatures above 640°C in dry air or CO: there is an increasing risk that magnox will ignite with a large energy release. In wet air or CO;, ignition may occur at temperatures some 10-20°C below 640°C.

Natural uranium oxidises in CO: at a very signifi­cant rate at temperatures aboe 400°C. with a large increase in volume releasing particulate uranium oxide and fission products into the circuit. In air, this re­action occurs at temperatures as low as 250°C.

Graphite oxidises exothermallv in air, producing significant heat at temperatures above 500°C. The oxidation rate, and hence heat release rate, increases rapidly with temperature. It is against these proper­ties that the fault studies for magnox reactors must be carried out.