Wigner energy in irradiated graphite and post-closure safety

Wigner energy is formed in graphite as a result of irradiation with neutrons in research, production and power reactors. If sufficient activation energy is supplied subsequently, the Wigner energy can be released, potentially in an uncontrolled way. This is thought to have been connected with the fire of 1957 in one of the Windscale Piles. The temperatures in commercial power reactors are high enough to release a significant fraction of the Wigner energy during reactor operation. There are, however, a number of research and production reactors in the UK that operated at low temperatures. Most of these facilities are at various stages of decommissioning, and this step will generate graphite wastes containing Wigner energy that may be released at relatively low temperatures, with the solid potentially becoming self-heating. Current sources of such graphite at Sellafield include Pile 1 and a mixed waste silo in which Magnox, graphite and aluminium are the main components. The overall aim of the study reported in this paper was to provide the Environment Agency with a basis for developing guidance to waste producers on the management of irradiated graphite wastes. Current policy notes that the preferred route for managing the wastes in the short term has to accommodate the requirement for safety in the long term, that is, the route should be sustainable. Important themes associated with this include not foreclosing options if the current policy of geological disposal changes as a result of the current review and the ease of reworking the waste packages under these circumstances