The Windscale nuclear reactor was built in the 1940s near Sellafield, a remote farm area of northern England, to supply nuclear power to the region. This early reactor was designed with a large graphite block in which cans containing the uranium fuel were embedded. The graphite served to slow down fast-moving neutrons produced during nuclear fission, allowing the reactor to operate more efficiently.

Graphite behaves in a somewhat unusual way when bombarded with neutrons. Water, its modern counterpart, becomes warmer inside the reactor and circulates to transfer heat away from the core. Graphite, on the other hand, increases in volume and begins to store energy. At some point above 572°F (300°C), it may then suddenly release that stored energy in the form of heat.

A safety system that allowed this stored energy to be released slowly was installed in the Windscale reactor...