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After 12-24 months use in a reactor, nuclear fuel is removed because of the build up of fission products and transuranics that absorb neutrons that would otherwise take part in the nuclear reaction.
Used nuclear fuel is stored and may ultimately disposed be of. It can also be reprocessed to separate the uranium and plutonium from the waste products so that the uranium and plutonium can be recycled to make new fuel.
Why fuel becomes "used fuel"
When uranium and other fissile atoms in nuclear fuel split - or fission - they form new, lighter elements, known as fission fragments or fission products. With time, the concentration of fission fragments and transuranics formed in fuel will increase to the point where it is no longer practical to continue to use the fuel. So after 12-24 months the used fuel is removed from the reactor.
Typically, more than 36 million kilowatt-hours of electricity are produced from one tonne of natural uranium. The production of this amount of electrical power from fossil fuels would require the burning of over 20,000 tonnes of black coal or 8.5 million cubic metres of gas.
Used fuel storage
When removed from a reactor, a fuel bundle will be emitting both radiation, principally from the fission fragments, and heat. Used fuel 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.
Used fuel can be stored safely in these ponds for long periods. It can also be dry stored in engineered facilities, cooled by air. However, both kinds of storage are intended only as an interim step before the used fuel is either reprocessed or sent to final disposal. The longer it is stored, the easier it is to handle, due to decay of radioactivity.
Depending on policies in particular countries, some used fuel may be transferred to central storage facilities.
Options for Used Fuel
There are two alternatives for spent fuel:
Reprocessing
Reprocessing separates uranium and plutonium from waste products (and from the fuel assembly cladding) by chopping up the fuel rods and dissolving them in acid to separate the various materials.
Used fuel is about 95% U-238, 1% U-235 that has not fissioned, about 1% plutonium and 3% fission products, which are highly radioactive, with other transuranic elements formed in the reactor.
Reprocessing enables recycling of the uranium and plutonium into fresh fuel, and produces a significantly reduced amount of waste (compared with treating all used fuel as waste).
The remaining 3% of high-level radioactive wastes (some 750 kg per year from a 1000 MWe reactor) can be stored in liquid form and subsequently solidified.
Reprocessing of spent fuel occurs at facilities in Europe, Japan and Russia with capacity over 5000 tonnes per year and cumulative civilian experience of 90,000 tonnes over almost 40 years.
Uranium and Plutonium Recycling
The uranium recovered from reprocessing can be used in MOX fuel or it can be returned to the conversion plant for conversion to uranium hexafluoride and subsequent re-enrichment. It can then be used in fresh uranium oxide fuel, although it does need special handling because it has a different isotopic mix than uranium oxide made from fresh uranium.
The plutonium (which is reactor-grade plutonium and not suitable for weapons) can be blended with uranium to produce a mixed oxide (MOX) fuel, in a fuel fabrication plant.
MOX fuel fabrication occurs at facilities in Belgium, France, Germany, UK, Russia and Japan, with more plants under construction. Across Europe about 30 reactors are licensed to load 20-50% of their cores with MOX fuel and Japan planh to have one third of its 54 reactors using MOX by 2010.
In reactors that use MOX fuel, plutonium substitutes for the U-235 in normal uranium oxide fuel. Typically reactors will use a combination of MOX fuel assemblies and uranium oxide fuel assemblies, although it is possibly to fuel a reactor using just MOX fuel.
Final Disposal of Used Fuel
If used fuel is not reprocessed it can be disposed of as waste. At the present time used fuel not destined for reprocessing is stored in interim storage facilities. There are currently no final disposal facilities (as opposed to interim storage facilities) in operation in which used fuel can be placed. There is currently no pressing technical need to establish such facilities, as the total volume of such wastes is relatively small, although technical issues related to disposal have been addressed.
The longer used fuel is in interim storage the easier it is to handle, due to the progressive diminution of radioactivity. There is also a reluctance to dispose of used fuel because it represents a significant energy resource which could be reprocessed at a later date to allow recycling of the uranium and plutonium.
A number of countries are carrying out studies to determine the optimum approach to the disposal of used fuel, as well as wastes from reprocessing. The waste forms envisaged for disposal are vitrified high-level wastes sealed into stainless steel canisters, or used fuel rods encapsulated in corrosion-resistant metals such as copper or stainless steel. The general consensus favours its placement into deep geological repositories, initially recoverable. Many geological formations such as granite, volcanic tuff, salt or shale will be suitable. The first permanent disposal is expected to occur about 2010.
Further ReadingProcessing of Used Nuclear Fuel for RecycleMixed Oxide Fuel (MOX)PlutoniumThe Nuclear Fuel CycleWaste Management in the Nuclear Fuel CycleJapanese Waste and MOX Shipments From Europe