Feature Report: Understanding Nuclear Energy

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The Nuclear Fuel Cycle

A gray, hard and very dense metal, natural uranium is widely found in the Earth's crust associated with other chemical compounds, both in granitic and sedimentary soils. But before it can be used in a nuclear reactor, it must undergo several forms of processing. This is the front-end of the fuel cycle. After the fuel is used in the reactor, it again goes through a series of steps known as the back-end of the cycle1.

To increase its uranium content, ore is processed into a yellow pasty concentrate called yellowcake, which is then used in the production of nuclear fuel. AREVA - TAILLAT JEAN-MARIE

Commercial-grade deposits, which can be easily identified by the radiation they emit, contain between 100 grams and 10 kilograms of uraniumGray, very dense radioactive metal that is relatively abundant in the Earth's crust and oceans in the form of UO2... per metric ton of ore. Extraction takes place either in underground or open-pit mines, or in situ, with injectionSecondary oil recovery method in which water or gas is injected into a deposit to increase pressure in the reservoir... into the soil of an acid or alkali solution that dissolves the uranium, which is then pumped out.

In conventional mining, ore slabs are crushed, ground and chemically processed. The resulting concentrate is called yellowcake (nuclear)Intermediate step in the fabrication of nuclear fuel from uranium ore..., due to its color and pasty texture. Yellowcake contains about 75% uranium.

In 2009, a joint study by the Organisation for Economic Co-operation and Development (organization for economic co-operation and development (oecd)Founded in 1960, the OECD promotes policies that will improve the economic and social well-being of people around the world...) for the Nuclear EnergyEnergy produced in nuclear power plants. The enormous amount of heat released during fission of uranium atom nuclei is transferred to water... Agency (NEA) and the International Atomic Energy Agency (IAEA)The IAEA was set up in 1957 within the United Nations family. It works for the safe, secure and peaceful uses of nuclear energy... estimated that known conventional resources, recoverable at an acceptable cost (less than $130 per kg), amounted to about 6.3 million metric tons. Between now and 2025, annual demand for uranium is expected to stand at between 80,000 and 100,000 metric tons. The potential therefore seems satisfactory, particularly given that the presence of uranium in phosphate deposits, even in sea water, and the prospects for more fuel-efficient Generation IV reactors will push back these limits considerably.

Enrichment

Natural uranium contains 99.3% uranium-238 (U-238) and 0.7% uranium-235 (U-235). However, most nuclear reactors use as fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... uranium containing between 3% and 5% uranium-235. The concentration of U235 in natural uranium therefore needs to be increased, or "enriched".

To do this, a gaseous phase is required to convert the uranium into uranium hexafluoride (UF6). Enrichment is achieved through two main industrial processes:

  • Gaseous diffusion, which consists in passing the gas through porous membranes. Being lighter, the U-235 molecules cross the barrier more easily than the U-238 molecules. Repeating the operation a great number of times yields uranium hexafluoride with the desired concentration of U-235. This highly energy-intensiveDescribes a building, mode of transportation or industrial process that uses large amounts of energy. method is currently being phased out.
  • Centrifugation, which is becoming increasingly widespread, uses centrifuges turning at extremely high speed. The heaviest molecules (U-238) are projected to the outer edges and the gas enriched with the lighter U-235 is recovered in the central part before being re-injected. A "cascade" of several hundred centrifuges is required to completely carry out the process.

Fuel production

Enriched uranium hexafluoride (UF6) is converted into uranium oxide in the form of a black powder. This is then compressed into pellets which are baked at 1,700°C to reach the required density. A pellet weights 7 grams and releases as much energy as a metric ton of coalCoal is ranked by its degree of transformation or maturity, increasing in carbon content from....

 

A pellet of 7 grams of nuclear fuel releases as much energy as a metric ton of coal.

The pellets are then inserted in metal tubes about four meters long. The two ends are plugged to create a fuel rod containing about 300 pellets. The rods are then grouped together in a metal structure to make a fuel assemblyFuel assemblies are installed in the core of nuclear reactors, where fission occurs and energy is released.... Each assembly has about 250 rods. At this stage, the fuel is ready to be brought to the heart of the nuclear reactorLocated in the core of a nuclear power plant, the reactor is a type of huge boiler. It has four main components....

 

Back-end of the fuel cycle

Once it has been consumed in the reactor for at least three years, the spent fuel contains various forms of uranium and plutonium (9 kg per metric ton). Through different reprocessing techniques, 96% of this spent fuel can be recycled. The resulting reprocessed uranium can then be re-enriched. The plutonium can also be mixed with the depleted uranium to produce a fuel called mixed oxide (mox) fuelNuclear fuel manufactured from spent reactor fuel. It consists 93% of depleted uranium and 7% plutonium...2.

80 %: the reduction in waste volume thanks to the reprocessing of spent fuel

 

These techniques also reduce the quantities to be stored. The remaining 4% waste is high- and intermediate-level radioactive waste with a long half-life. This final waste cannot yet be recycled (See Close-up: "Radioactive Waste Management").

Three countries have the expertise to implement this technology on an industrial scale: France, the United Kingdom and Japan, which built a plant that has now been shut down. The other countries with nuclear reactors either recycle or have recycled their spent fuel after reprocessing in France or the United Kingdom, or plan to store it like other radioactive waste.

 

Sources 

(1) Société française d’énergie nucléaire 

(2) Areva