Since there are about 235 nucleons involved in each fission and each is bound by an extra 1.0 MeV (8.5 - 7.5 MeV) after the fission, the energy released must be in the order of 235 MeV (as binding energy refers to the energy released by the nucleus).
The reverse is also true, when the power level is to be decreased or stopped, control rods are inserted and more neutrons are absorbed to bring the average to below one per fission until the new power level is attained.
Advantages and Disadvantages of Nuclear Fission Power
After initial start up costs, the power is relatively cheap in large-scale production. The energy extracted is much greater than for the same amount of fossil fuels.
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
Using relatively small special-purpose nuclear reactors it has become possible to make a wide range of radioactive materials (radioisotopes) at low cost. For this reason the use of artificially produced radioisotopes has become widespread since the early 1950s, and there are now some 270 "research" reactors in 59 countries producing them.
About 25 tonnes of spent fuel is taken each year from a nuclear reactor.
Either all waste (in USA and Canada),
Reprocessed (as in Europe).
Whichever option is chosen, the spent fuel is first stored for several years under water in large cooling ponds at the reactor site. The concrete ponds and the water in them provide radiation protection, while removing the heat generated during radioactive decay.
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.
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.
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.
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.
Radioisotopes can be produced in a reactor by either irradiating a target isotope with the neutrons present in the reactor or by extracting the required radioisotope if present amongst the fission fragments of the reaction.
The fusion reaction is more difficult to achieve because the reacting nuclei have to be brought together close enough for the strong nuclear force to act which requires overcoming their mutual repulsion.
3. The problems associated with fission power plant mishaps are almost non-existent. Although operating at very high temperatures, the energy of the plasma is very low and so the reactor temperature will not rise more than a few degrees.