63. APPLICATION - NUCLEAR REACTORS AND POWER Energy Conversion: Typical Heat Values of Various Fuels (MJ = Megajoules), * natural U 500,000 MJ/kg Uranium* - in light water reactor 45-46 MJ/kg Crude Oil 39 MJ/m 3 Natural Gas 24-30 MJ/kg Black coal 13-20 MJ/kg Black coal (low quality) 9 MJ/kg Brown coal 16 MJ/kg Firewood
79. Nuclear Waste The 3% of the spent fuel which is separated high-level wastes amounts to 700 kg per year and it needs to be isolated from the environment for a very long time. These liquid wastes are stored in stainless steel tanks inside concrete cells until they are solidified. The vitrified waste for one year would fill about twelve canisters, each 1.3m high and 0.4m diameter and holding 400 kg of glass.
80. Nuclear Waste The ultimate disposal of vitrified wastes, or of spent fuel assemblies without reprocessing, requires their isolation from the environment for long periods. The most favoured method is burial in dry, stable geological formations some 500 metres deep. Several countries are investigating sites that would be technically and publicly acceptable. The USA is pushing ahead with a repository site in Nevada (Yucca Mountain) for all the nation’s spent fuel.
81. Nuclear Waste Layers of protection The principal barriers are: Immobilise waste in an insoluble matrix, eg glass, Synroc (or leave them as uranium oxide fuel pellets - a ceramic) Seal inside a corrosion-resistant container In wet rock: surround containers with bentonite clay to inhibit groundwater movement Locate deep underground in a stable rock structure Site the repository in a remote location.
82. Nuclear Waste A more sophisticated method of immobilising high-level radioactive wastes has been developed in Australia. Called 'SYNROC' (synthetic rock), the radioactive wastes are incorporated in the crystal lattices of the naturally-stable minerals in a synthetic rock. In other words, copying what happens in nature.