Nuclear Fuel Cycle

 According to World Nuclear Association:  The nuclear fuel cycle is the series of industrial processes which involve the production of electricity from uranium in nuclear power reactors.  Uranium is a relatively common element that is found throughout the world. It is mined in a number of countries and must be processed before it can be used as fuel for a nuclear reactor.  Fuel removed from a reactor, after it has reached the end of its useful life, can be reprocessed so that most is recycled for new fuel.

 Mining and milling  Reactors  Fuel cycle facilities

 This is where you mine the raw material which will be used as fuel. The raw material which is mined is called Uranium.

 Uranium Hexafluoride (UF6) is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and unclear weapons.

 an apparatus or structure in which fissile material can be made to undergo a controlled, self-sustaining nuclear reaction with the consequent release of energy.  Reactors are used for generating electricity, moving aircraft carriers and submarines, producing medical isotopes for imaging and cancer treatment, and for conducting research.

 Nuclear fuel facilities collects uranium after is has been mind and converts, enrich and fabricates it into fuel for nuclear reactors

 At a conversion facility, the uranium oxide is first refined to uranium dioxide, which can be used as the fuel for those types of reactors that do not require enriched uranium. Most is then converted into uranium hexafluoride, ready for the enrichment plant. The uranium hexafluoride is then drained into 14- tonne cylinders where it solidifies. These strong metal containers are shipped to the enrichment plant.  The enrichment process separates gaseous uranium hexafluoride into two streams, one being enriched to the required level and known as low-enriched uranium; the other stream is progressively depleted in U-235 and is called 'tails', or simply depleted uranium.

 The main enrichment process in commercial plants uses centrifuges, with thousands of rapidly-spinning vertical tubes. As they spin, the physical properties of molecules, specifically the 1% mass difference between the two uranium isotopes, separates them. A laser enrichment process is in the final stage of development.  The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide. Up to this point the fuel material can be considered fungible (though enrichment levels vary), but fuel fabrication involves very specific design.

 Reactor fuel is generally in the form of ceramic pellets. These are formed from pressed uranium oxide (UO2) which is sintered (baked) at a high temperature (over 1400°C) a. The pellets are then encased in metal tubes to form fuel rods, which are arranged into a fuel assembly ready for introduction into a reactor. The dimensions of the fuel pellets and other components of the fuel assembly are precisely controlled to ensure consistency in the characteristics of the fuel. a  In a fuel fabrication plant great care is taken with the size and shape of processing vessels to avoid criticality (a limited chain reaction releasing radiation). With low-enriched fuel criticality is most unlikely, but in plants handling special fuels for research reactors this is a vital consideration.  Some 27 tonnes of fresh enriched fuel is required each year by a 1000 MWe reactor.

 Several hundred fuel assemblies make up the core of a reactor. For a reactor with an output of 1000 megawatts (MWe), the core would contain about 75 tonnes of low-enriched uranium. In the reactor core the U-235 isotope fissions or splits, producing a lot of heat in a continuous process called a chain reaction. The process depends on the presence of a moderator such as water or graphite, and is fully controlled. Some of the U-238 in the reactor core is turned into plutonium and about half of this is also fissioned, providing about one third of the reactor's energy output. As in fossil-fuel burning electricity generating plants, the heat is used to produce steam to drive a turbine and an electric generator, in this case providing over 7 billion kilowatt hours of electricity in one year.

 When removed from a reactor, the fuel will be emitting both radiation, principally from the fission fragments, and heat. It is unloaded into a storage pond immediately adjacent to the reactor to allow the radiation levels to decrease. In the ponds the water shields the radiation and absorbs the heat, which is removed by circulating the water to external heat exchangers. Used fuel is held in such pools for several months and sometimes many years. It may be transferred to naturally-ventilated dry storage on site after about five years.  Depending on policies in particular countries, some used fuel may be transferred to central storage facilities. Ultimately, used fuel must either be reprocessed in order to recycle most of it, or prepared for permanent disposal. The longer it is stored, the easier it is to handle, due to decay of radioactivity.