High-energy neutrons damage and degrade materials over time ; bombardment of materials with neutrons creates collision cascades that can produce point defects and dislocations in the material , the creation of which is the primary driver behind microstructural changes occurring over time in materials exposed to radiation . At high neutron fluences this can lead to embrittlement of metals and other materials , and to swelling in some of them . This poses a problem for nuclear reactor vessels and significantly limits their lifetime ( which can be somewhat prolonged by controlled annealing of the vessel , reducing the number of the built-up dislocations ) . Graphite moderator blocks are especially susceptible to this effect , known as Wigner effect , and must be annealed periodically . The Windscale fire was caused by a mishap during such an annealing operation
Windscale fire
Windscale fire
Neutron radiation