1.1 Events Leading to the Accident

In the early hours of the morning on the 10th of October 1957 a reactor (Number 1 of the two Windscale ‘‘Piles’’), developed to produce plutonium and tritium for the UK’s atomic bomb programme, caught fire. The fire occurred during a procedure to release so-called ‘‘Wigner’’ energy from the reactor core.

Wigner energy is chemical potential energy stored in the lattice structure of the graphite moderator during operation of this type of nuclear reactor. The operators at Windscale routinely released this stored energy (to prevent an uncontrolled release) by an annealing process in which the core temperature was temporarily raised. This procedure normally released the Wigner energy in the graphite resulting in a temporary heating, then cooling of the core. On the day of the accident, however, the annealing process caused the temperature of some parts of the core to rise substantially. This, possibly coupled with rupture of a fuel element, caused the reactor to set on fire. After failed attempts to remove the overheated fuel elements and to put out the fire by carbon dioxide, the reactor was flooded with water on the following day (11th of October). The operators believed at the time that use of water carried the risk of a hydrogen explosion as it contacted red-hot metal, but it was felt that the risk of breach of containment by the burning reactor core was greater. By the evening of the 11th, the fire was fully extinguished.

The fire resulted in the release of ‘‘some of the fission products and activation products contained in a few percent of the core’’.1 The releases from the Windscale fire have recently been re-evaluated,1 giving estimates for a wide range of radionuclides including 1.8 PBq of 131I; 0.18 PBq of 137Cs and 0.042 PBq of 210Po. As with TMI and Chernobyl, large quantities of noble gases (including 26 PBq of 133Xe) were released, though these were less radiologically significant.