From uranium to fuel

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09/13/2010

Most nuclear power plants do not use natural uranium directly to produce electricity. The nuclear fuel cycle involves a number of processes: ore extraction, uranium conversion, and enrichment.

Extraction and Processing

Ore can be extracted in three different ways, depending on the deposit's depth and features :

   • Underground mining: ore is accessed via a system of wells and galleries.

   • Open pit mining: the rocky layer covering the ore is stripped back.

   • In-situ extraction: an acid or alkaline solution is injected underground to dissolve the ore and the solution is then pumped out.

The final stage in ore processing, yellow cake, contains about 75% uranium.

In conventional mines, blocks of ore are broken up, finely crushed, and subjected to chemical processing. After separation, washing, filtering, and drying, a concentrate called yellow cake is obtained (so called because of its color and cakey texture). It contains about 75% uranium, i.e. 750 kg per ton¹. With in-situ extraction, the uranium is separated once it is brought back to the surface. The pulp obtained is dehydrated and converted into yellow cake².

Uranium is found in abundance on earth.

True. Natural uranium, a hard, very dense grey metal is found all over the earth's crust, bound to other chemicals in the form of ore. It is common in both granite and sedimentary terrain, with average content of 3g per ton. Fields that are currently being worked contain between 100g and 10kg of uranium per ton of extracted ore.

In 2009, a joint study by the OECD's Nuclear Energy Agency (NEA) and the International Atomic Energy Agency (IAEA) estimated that total conventional resources that could be extracted at less than $130 US per kg amounted to about 6.3 million tons. Uranium requirements are expected to rise to an annual amount of 80,000-100,000 tons by 2025. Therefore, the resources currently known about are sufficient³.

Enrichment - From One Form of Uranium to Another

Natural uranium contains 99.3% of uranium 238 (U₂₃₈) and 0.7% of uranium 235 (U₂₃₅). For fuel, most nuclear reactors (90% of reactors in operation worldwide) use uranium containing 3-5% of uranium 235. Therefore, natural uranium's concentration of U₂₃₅ needs to be increased. To do this, it is first turned into gas - uranium hexafluoride or UF₆ - this is conversion. Next, the proportion of U₂₃₅ is increased - this is enrichment. To carry out this operation, there are two main industrial processes- gaseous diffusion and centrifugation. Both of these processes exploit the small difference in mass between molecules of uranium hexafluoride 235 and molecules of uranium hexafluoride 238
(U₂₃₅ is lighter than U₂₃₈).

Gaseous diffusion involves forcing the gas through porous membranes. The lighter molecules (U₂₃₅) pass through faster than the U₂₃₈ molecules. The operation is repeated over 1000 times to obtain uranium hexafluoride with the right concentration of U₂₃₅.

Centrifugation uses centrifuges rotating at extremely high speed, into which UF₆ is introduced. The heavier molecules (U₂₃₈) are projected toward the outside of the centrifuge while the lighter molecules (U₂₃₅) migrate towards the center. The gas enriched in the lighter U₂₃₅ rises and is recovered at the very top of the centrifuge⁴.

Manufacturing Fuel

After enrichment, uranium hexafluoride (UF₆) is converted into uranium oxide, a black powder.

A pellet of uranium oxide weighs 7 grams and releases as much energy as a ton of coal.

This is then compressed into little pellets that are baked in a furnace at 1700°C to obtain the right density. A pellet of uranium oxide weighs 7 grams and releases as much energy as a ton of coal.

The pellets are then placed in 4m-long metal tubes. The tubes are sealed at both ends to obtain a fuel rod containing about 300 pellets. The rods are grouped into a metal square array to make fuel assemblies. Each assembly contains about 250 rods⁵. The fuel is finally ready to be placed inside the reactor core.

Managing and Recycling Radioactive Waste

Most radioactive waste comes from producing electricity in nuclear power plants (85% in France⁶). There are several categories of waste, depending on the intensity of the radiation they emit and their life span (length of time during which they remain radioactive).

Short-lived Low-activity Radioactive Waste

Given the speed of radioactive decay (i.e. the half-life), this waste will take 300 years to reach the level of natural radioactivity. Most of this waste is operational waste related to the maintenance of nuclear power plants; such as tools or cloths used in the nuclear zone to carry out maintenance tasks, or replaced parts, such as valves and filters.

In France, the volume of waste has been considerably reduced (by a factor of three in ten years) and it now represents about 100m³ per reactor per year.

This waste is bundled together and encased in steel or concrete. The waste is encased in a matrix, a material whose purpose is to render it inert and contain radioactivity. The containers isolate waste from the environment.

Long-lived Intermediate- and High-activity Waste

This is waste from spent nuclear fuel, which represents 10% of nuclear waste. The spent fuel is highly radioactive and will remain so for a very long time. It can be reprocessed to recycle its energy material. To do this, the various components are separated.
   • The metal tube containing the fuel

   • The energy material (uranium and plutonium) i.e. 95% of the spent fuel

   • The waste generated by fission: fission products and minor actinides (elements formed through the absorption of neutrons by uranium nuclei)

Reprocessing involves recovering material that still contains energy to reduce the volume of waste to be stored by a factor of three. The final high-activity waste is vitrified, i.e. poured into glass, a material that is heat- and radiation-resistant. This glass is packed into stainless steel tanks and stored in concrete wells that are ventilated to keep them cool.

This temporary storage solution can last another few decades pending a definitive solution. Because of reprocessing, volumes are low. Finland and Sweden use deep geological storage - a site is currently under construction in Finland (scheduled to open in 2020) and another is in the study phase in Sweden. The United States has had a storage center for military waste since 1996. Spent civil fuel is currently stored in nuclear power plants.

Not all countries recycle their spent fuel.

True. Some countries have not opted for reprocessing, like Sweden, Canada, and Finland. In this case, fuel spent in its entirety is considered as waste and processed and stored as such. On the other hand, France, the United Kingdom and, Japan have chosen to build reprocessing plants to recover energy material. Finally, some states, such as Germany, Switzerland, and Belgium send their waste abroad for recycling. They then recover all the final waste as they are responsible for managing it.