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Nuclear fuel
Nuclear fuel refers to any substance, typically fissile material, which is used by nuclear power stations or other nuclear devices to generate energy.
For fission reactors, the fuel (typically based on uranium) is usually based on the metal oxide; the oxides are used rather than the metals themselves because the oxide melting point is much higher than that of the metal and because it cannot burn, being already in the oxidized state.
Uranium dioxide is a black semiconducting solid. It can be made by heating uranyl nitrate to form UO
2.
This is then converted by heating with hydrogen to form UO2. It can be made from enriched uranium hexafluoride by reacting with ammonia to form a solid called ammonium diuranate, (NH4)2U2O7. This is then heated (calcined) to form UO
3 and U3O8 which is then converted by heating with hydrogen or ammonia to form UO2. The UO2 is mixed with an organic binder and pressed into pellets. The pellets are then fired at a much higher temperature (in hydrogen or argon) to sinter the solid. The aim is to form a dense solid which has few pores.
The thermal conductivity of uranium dioxide is very low compared with that of zirconium metal, and it goes down as the temperature goes up. Corrosion of uranium dioxide in water is controlled by similar electrochemical processes to the galvanic corrosion of a metal surface.
While exposed to the neutron flux during normal operation in the core environment, a small percentage of the 238U in the fuel absorbs excess neutrons and is transmuted into 239U. 239U rapidly decays into 239Np which in turn rapidly decays into 239Pu. The small percentage of 239Pu has a higher neutron cross section than 235U. As the 239Pu accumulates the chain reaction shifts from pure 235U at initiation of the fuel use to a ratio of about 70% 235U and 30% 239Pu at the end of the 18 to 24 month fuel exposure period.
Mixed oxide, or MOX fuel, is a blend of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.
Some concern has been expressed that used MOX cores will introduce new disposal challenges, though MOX is a means to dispose of surplus plutonium by transmutation. Reprocessing of commercial nuclear fuel to make MOX was done in the Sellafield MOX Plant (England). As of 2015, MOX fuel is made in France at the Marcoule Nuclear Site, and to a lesser extent in Russia at the Mining and Chemical Combine, India and Japan. China plans to develop fast breeder reactors and reprocessing.
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Nuclear fuel
Nuclear fuel refers to any substance, typically fissile material, which is used by nuclear power stations or other nuclear devices to generate energy.
For fission reactors, the fuel (typically based on uranium) is usually based on the metal oxide; the oxides are used rather than the metals themselves because the oxide melting point is much higher than that of the metal and because it cannot burn, being already in the oxidized state.
Uranium dioxide is a black semiconducting solid. It can be made by heating uranyl nitrate to form UO
2.
This is then converted by heating with hydrogen to form UO2. It can be made from enriched uranium hexafluoride by reacting with ammonia to form a solid called ammonium diuranate, (NH4)2U2O7. This is then heated (calcined) to form UO
3 and U3O8 which is then converted by heating with hydrogen or ammonia to form UO2. The UO2 is mixed with an organic binder and pressed into pellets. The pellets are then fired at a much higher temperature (in hydrogen or argon) to sinter the solid. The aim is to form a dense solid which has few pores.
The thermal conductivity of uranium dioxide is very low compared with that of zirconium metal, and it goes down as the temperature goes up. Corrosion of uranium dioxide in water is controlled by similar electrochemical processes to the galvanic corrosion of a metal surface.
While exposed to the neutron flux during normal operation in the core environment, a small percentage of the 238U in the fuel absorbs excess neutrons and is transmuted into 239U. 239U rapidly decays into 239Np which in turn rapidly decays into 239Pu. The small percentage of 239Pu has a higher neutron cross section than 235U. As the 239Pu accumulates the chain reaction shifts from pure 235U at initiation of the fuel use to a ratio of about 70% 235U and 30% 239Pu at the end of the 18 to 24 month fuel exposure period.
Mixed oxide, or MOX fuel, is a blend of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.
Some concern has been expressed that used MOX cores will introduce new disposal challenges, though MOX is a means to dispose of surplus plutonium by transmutation. Reprocessing of commercial nuclear fuel to make MOX was done in the Sellafield MOX Plant (England). As of 2015, MOX fuel is made in France at the Marcoule Nuclear Site, and to a lesser extent in Russia at the Mining and Chemical Combine, India and Japan. China plans to develop fast breeder reactors and reprocessing.