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Stages involved in nuclear fuel cycle

EBR Staff Writer Published 16 March 2018

A nuclear fuel cycle is a series of processes involved in the generation of electricity from nuclear reactors. While the first stage in the nuclear fuel cycle is the mining of uranium, the final stage is the disposal of nuclear waste. Using a nuclear fission reaction, the power plants produce high amount of electricity with uranium as fuel.

Nuclear power is regarded as one of the prominent sources of energy with a huge potential. Nuclear power units can produce power continuously for several months without any interruption. However, the construction of nuclear power plants usually takes several years to complete as they require large infrastructure. The difficulties in producing power from a nuclear plant in a short period of time makes them less favourable compared to building electricity plants that run on traditional sources.

Nuclear energy sources have higher density than fossil fuels and release massive amounts of energy. Due to this, nuclear power plants require low quantities of fuel but produce enormous amounts of power.  While transportation of nuclear fuel to a power plant can cause pollution, the process involved in mining and refining uranium is also a concern.

Electricity produced from nuclear power plant emits fewer greenhouse gas emissions. Currently, nuclear power plants are operated in 31 countries, with a significant number of them located in Europe, Northern America, East Asia and South Asia.  The US, France, China, Russia, South Korea, Canada, and Ukraine are the major nuclear power producing countries. As of October 2017, the US had 99 nuclear reactors in 30 states, which produced nearly 20% of the country’s total electrical output, according to World Nuclear Association (WNA).

Here are the various stages in nuclear fuel cycle:

Uranium mining:  The first stage in nuclear fuel cycle is uranium mining. Open pit and underground mining techniques are deployed to extract uranium. A method called in-situ leaching that involves no surface or underground digging is mainly used in the extraction of uranium. In this type of mining, ore is left in the ground, but minerals are recovered from it by dissolving them. The solvent solution is brought to the surface to obtain the minerals. Similar to fossil fuels, uranium reserves are limited and present in few nations. The processes carried out to mine and refine uranium involve huge costs.

 

 

 

 

 

 

 

 

 

 

 

Image: Mi Vida uranium mine (109°15'35.77"W 38°11'25.07"N) near Moab. Photo courtesy of Matt Affolter at English Wikipedia.

Uranium milling:  It is the next stage in nuclear fuel cycle that involves extraction of uranium from the ore. Milling activities are generally conducted near a uranium mine. Milling involves separation of uranium by crushing of ore.  The milling process produces a uranium oxide concentrate, which is referred as 'yellowcake'.  The concentrate, which is ready for transport from the mill, contains over 80% of uranium.  The waste byproducts that are formed after the extraction of uranium from the ore are uranium tailings. They are radioactive sludge that remains after the uranium-bearing substances are removed from the ore.

Uranium conversion and enrichment:  A uranium oxide concentrate produced from milling needs additional processing before it is used as fuel in the nuclear power reactor. At this stage of nuclear fuel cycle, uranium oxide is converted to uranium hexafluoride. In the conversion process, uranium oxide is first refined to uranium dioxide and then converted into uranium hexafluoride for enrichment. At the enrichment stage, gaseous uranium hexafluoride is enriched to desired level, which is called as low-enriched uranium.

Fabrication:  To use enriched uranium hexafluoride as nuclear fuel, it is converted into uranium dioxide (UO2) powder. Later, the powder is processed into pellet form, which is burnt in a high temperature sintering furnace to create hard, ceramic pellets of enriched uranium. Fuel rods are prepared by encasing ceramic pellets in metal tubes. The rods are grouped into a fuel assembly, which will be ready for use in a nuclear power reactor.

 

 

 

 

 

 

 

 

 

 

Image: A nuclear fuel rod assembly bundle being inspected before entering a reactor. Photo courtesy of Ruslan Krivobok /Wikipedia.

Fuel burn-up: A nuclear reactor core contains few hundreds of fuel assemblies. During the fission reaction, fuel is consumed to produce heat, which will be used to produce electricity. Used fuel rods are replaced periodically with new ones for efficient functioning of nuclear reactors. As fuel consumption occurs at varying rates in a nuclear reactor, only few fuel assemblies are removed for replacement.

Interim storage of used fuel: After the completion of a nuclear reactor’s operating cycle, it is shut down to remove the fuel. The discharged fuel is stored at the nuclear plant site or any other place away from it. The spent fuel rods are stored in water or boric acid, which offer them cooling and prevent the radiation from entering into the atmosphere.                    

Reprocessing: Spent fuel rods discharged from nuclear reactors contain significant amount of fissile and other radioactive materials. These materials can be separated and recovered from the spent fuel. Uranium and plutonium recovered from the used fuel can be recycled to a nuclear fuel source. Reprocessing of spent fuel rods helps in producing fresh fuels and generates signifcantly low levels of nuclear waste.

 

 

 

 

 

 

 

 

 

 

Image: Modern medium to high level transport container for nuclear waste. Photo courtesy of Bill Ebbesen/Wikipedia.

Waste disposal: Though nuclear energy is regarded as one of the prominent sources of energy with a huge potential, waste generated from nuclear activity can pose a significant risk to the environment if it is not properly handled. The vast amount of nuclear waste created by power plants can lead to high radiation and raise temperature levels. The various types of nuclear waste include uranium tailings, transuranic (TRU) waste, low-level nuclear waste, intermediate-level waste, high-level waste and spent fuel rods.