PBRs are intentionally operated above the 250 °C annealing temperature of graphite , so that Wigner energy is not accumulated . This solves a problem discovered in an infamous accident , the Windscale fire . One of the reactors at the Windscale site in England ( not a PBR ) caught fire because of the release of energy stored as crystalline dislocations ( Wigner energy ) in the graphite . The dislocations are caused by neutron passage through the graphite . Windscale had a program of regular annealing in place to release accumulated Wigner energy , but since the effect was not anticipated during the construction of the reactor , and since the reactor was cooled by ordinary air in an open cycle , the process could not be reliably controlled , and led to a fire . The second generation of UK gas-cooled reactors , the AGRs , also operate above the annealing temperature of graphite
Windscale fire
Windscale fire
Pebble-bed reactor