Stages needed to obtain nuclear fuel from uranium

What is uranium?

Uranium is a gray metallic element from the actinides series, discovered in 1789 by the German physicist M. H. Klaproth, and named after the planet Uranium, which had been discovered eight years before.

Its chemical symbol is ‘U’ and its atomic number is 92. It has the greatest atomic number among all the elements found in nature, and is approximately 70% denser than lead.

It is hardly ever used in its pure state. It is most common to work with its oxides, the most stable of them being U₃O₈.

Uranium in its natural state is a mix of three isotopes: U-234 (0.02%, trace levels), U-235 (0.7%) and U-238 (99.28%) and is slightly radioactive, which facilitates its mining, transformation and manufacturing as nuclear fuel. Uranium is mainly located in the Earth’s crust, it is 500 times more abundant than gold and has no use other than as nuclear fuel.

Stages for obtaining nuclear fuel

First stage of the cycle: from nature to the reactor

Stage 1: Exploration and mining by lixiviation “on the spot”, opencast or underground, to extract the uranium that is usually processed to reduce the material to a uniform particle size, and then proceed to the grinding. This produces a dry dust formed by natural uranium, called “yellow cake”, and sold in the uranium market as U₃O₈.

Uranium is abundant in nature, however it is found in very small proportions. These rocks are crushed and ground to facilitate the chemical treatments that are applied afterwards (lixiviation, clearing and refining) in order to extract the uranium that they contain in the form of a yellow solid, yellow cake. It is mostly composed of U₃O₈ and is dried to continue the process.

Step 2: First conversion stage (U₃O₈ a UF₆). This stage consists in that the uranium concentrate, U₃O₈, must be converted to uranium hexafluoride, UF₆, normally found in the gaseous state. This is the state required by most uranium enrichment plants as well as a necessary requisite to use uranium as nuclear fuel.

Stage 3: Enrichment. The concentration of the fissionable isotope U-235 (0.71% in natural uranium) is inferior to that required to maintain a nuclear chain reaction in light water reactors. Natural UF₆, thus, must be enriched in the fissionable isotope in order to be used as nuclear fuel. The different enrichment levels depend on the reactor, but a light water reactor is usually enriched up to nearly 5% of U-235. However, enriched uranium is also necessary at lower concentrations. Generally the enrichment is achieved using gaseous diffusion or centrifuged gas.

Step 4: Second conversion stage (UF₆ to UO₂). For its use as nuclear fuel, enriched UF₆ is turned into uranium dioxide powder (UO₂), and then compacted into cylindrical ceramic pills of approximately 1-cm diameter and a 1-cm height, and with stable characteristics at high temperatures like the ones they will withstand inside the reactor.

Step 5: Manufacture of fuel elements. This stage is the only one carried out in Spain, at the fuel element plant in Juzbado (Salamanca, Spain), property of Enusa. It consists of: 

• Fuel bar manufacturing: this is the first safety barrier at the nuclear plant. The uranium pills are placed inside, and all the fission products liberated during the fuel burning will be stored.
• Fuel element: once the fuel bars are prepared, they are grouped into special assemblies formed by the fuel elements. Their main function is to maintain the bars at an appropriate distance so that the coolant will circulate among them, getting the generated heat.

Second stage of the cycle: Fuel inside and outside the reactor

As they generate energy inside the reactor, fuel elements lose effectivity due to the reduction of the fisionable element and the accumulation of fission products. For this reason it is necessary to replace part of these elements with new fuel. This operation is known as recharge.

The second part of the cycle includes all the operations undergone by the fuel inside the reactor up until the time it is definitely stored and isolated. When the used fuel is removed from the reactor, only 5% of the initially contained energy has been used. The used fuel, thus, still maintains a great remanent energy capacity which can be re-used in other reactors.

Once the fuel ends its useful life in the reactors, after operating for two or three cycles – around three to five years – it still retains 95% of the uranium, which is enriched up to a level depending on its final burning. 1% is plutonium, and the rest are lower actinides, long-life and short-life products and stable fission products.

Nuclear fuel cycles

• Open cycle. If it is decided not to reuse the energy resources contained in the used fuel, then it is handled as high activity radioactive waste, since fission products are confined within it. This fuel’s final destination is its storage in Centralized Storage Facilities (ATC), retrievable for a reprocessing or Deep Geological Repository (AGP) , which is its definitive storage.
• Closed cycle or reprocessing. Should the reuse of non burnt U-235 and generated Pu-239 be considered, the fuel must be recycled to be used in other nuclear plants, since it retains 95% of its initial energy capacity.

The recycled fuel is known as Mixed Oxide (MOX), and is composed by a mix of natural uranium oxide, reprocessed uranium and plutonium oxide. With this operation, carried out at reprocessing plants in France, China, India, Russia and the United Kingdom, these two elements are separated from the fission products, which constitute the high activity waste.