| Themes > Science > Chemistry > Nuclear Chemistry > Nuclear Reactions > Transmutation > Breeding and transmutation > Official reprocessing policy |
An official reprocessing policy has taken shape, especially in France and Japan. Plutonium obtained after reprocessing of standard PWR fuel is used for fabrication of MOX fuel, a mix of Plutonium and depleted Uranium oxides.. The MOX fuel elements are used as substitutes of normal enriched Uranium fuel elements in PWR. Note that the presence of 238 Uranium in the MOX fuel is needed for safety arguments: keep a negative temperature coefficient and partially breed the nuclear fuel so as to prevent a too fast decrease of the reactivity. While being irradiated the plutonium mixture is depleted of the fissile 239 isotope and enriched in the 240 isotope which is not fissile by thermal neutrons but has a large capture cross-section(it is a neutron poison), as well as in transplutonic nuclei (minor actinides) (especially americium) which are not or weakly fissile. In the course of the successive reprocessing it is, therefore, necessary to increase the total concentration of plutonium with respect to that of uranium. Such an increase is, however, not possible ad infinitum, since while 240Pu is a poison for thermal neutrons, it is not for fast neutrons. Thus a fuel too enriched in plutonium might yield to a criticality of the reactor for fast neutrons, and to a divergence in case of partial or total loss of coolant. After two or three reprocessing cycles in PWR, a ''dirty'' plutonium is left with an increased quantity of minor actinides. It is proposed to incinerate this mix in fast, sodium cooled , reactors. These would incinerate more plutonium than they would produce, contrary to breeder reactors. However, due to safety considerations, the fuel would include a minimum amount of 238U which would limit the net consumption of plutonium. Fast reactors should, also, be able to incinerate minor actinides efficiently. Incineration of dirty plutonium and minor actinides would require 1 fast reactor for 4 to 5 PWR. |
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