In the pioneering nuclear technology of the 1950s, it took a lot of uranium to produce just a little plutonium.
The reactors 3,440 channels were packed with 72,000 fuel rods, each 300mm long and 25mm diameter. But only about 2% of this 180t mass of natural uranium was converted.
As uranium atoms naturally started disintegrating, they generated heat and expelled neutrons. These bombarded other uranium atoms and, if the neutrons were absorbed by them, plutonium was produced.
Success of the operation was governed by heat and the attack speed of the neutrons. The role of the surrounding graphite was to help control this speed.
Air flow through the core helped to regulate the heat, but constant neutron bombardment of the graphite itself could change both its shape and makeup. In what became known as the Wigner effect, energy was trapped and stored in the graphite, causing it to overheat.
This Wigner energy was dissipated by occasional controlled heating up of the graphite to a temperature sufficiently high to release the storey energy an operation called annealing. It was during such an anneal that the infamous 1957 fire occurred.