It is not a secret that nuclear power plants run on Uranium. But few are aware that before Uranium gets loaded into the core of a nuclear reactor, it has a long journey to make.
As all other chemical elements, Uranium is found in Earth’s crust as a component of various ores. About a hundred Uranium ores are currently known, but only a dozen out of them represents practical interest; the Uranium content ranges from 0.03 per cent (low-grade ores) to one per cent or higher (high-grade ores). Australia has the biggest Uranium reserves, followed by (in receding order) Kazakhstan, Russia and Canada. Uranium is found in ores in the form of oxides and salts.
Presently, Uranium ores are usually available quite deep into the Earth Crust (1km or even deeper).
Uranium is extracted from the Earth crust by applying the shaft method. It means that a deep shaft is drilled and ore is extracted from the ground in the form of big chunks with the use of drilling-and-blasting operations.
But there is a more environment- friendly way of Uranium ore extraction- underground (in-situ) leaching. A drill-hole is bored in to the center of the ore body; several additional drill-holes are bored on its periphery. Sulphuric Acid solution is pumped under pressure into side drill-holes. The solution goes through the ore deposits and leaches out (washes out) Uranium; Uranium solution is pumped out through the central drill-hole. This method allows to avoid accumulation of huge waste piles on the surface, as is typical for shaft extraction; besides, Sulphuric Acid at large depths is harmless to ground water ,since the latter flows at much higher levels.
When the shaft method is used, big lumps of Uranium ore are found, containing Uranium compounds as well as dead rock. The last has to be separated and removed. The ore is crushed and treated by solutions of Sulphuric or Nitric Acid, or Sodium Carbonate. Uranium is dissolved and dead rock goes to sediment. After that, the target metal is precipitated by chemical reagents, yielding Uranium compounds in the form of powder.
Enrichment of Uranium
The obtained powder is to be dissolved again in order to separate Uranium from the impurities whose presence in a reactor is impermissible. This operation is carried out with use of special chemical methods. The process is known as ‘refinement’ and is a necessary step of nuclear fuel production. Refinement transforms Uranium into oxides.
The obtained pure oxides of Uranium are fluorinated (this operation is known as a ‘conversion’) in order to obtain Uranium Hexafluoride (UF6) – volatile crystalline substance. This compound is one of the most important compounds in the nuclear power industry, as it is used for enrichment of Uranium (i.e. increase content of fissionable isotope U-235). The enrichment to a certain level is essential to make the fuel work effectively in the nuclear reactor . The fact is that natural Uranium contains only 0.71 per cent of this isotope, while the main isotope Uranium-238 technically cannot be used by the majority of nuclear power plants (NPP). Uranium hexafluoride is a chemical compound that converts into gaseous state at the temperature as low as 56 degrees (C). If the gas is loaded into a rapidly rotating centrifuge, Uranium-238 as the heavier isotope would be ‘pushed’ to the wall whereas the lighter isotope would concentrate in the center. But the degree of enrichment in a single unit is very low; therefore one centrifuge is not enough: there are tens or even hundreds of thousands of centrifuges at a modern enrichment plant.
Enriched Uranium (which contains several per cent of U-235) is transformed into Uranium Dioxide (UO2).
Fuel assembly manufacturing
Enriched Uranium in the form of UO2 is compressed into small cylinders (pellets) by means of powder metallurgy techniques. Afterwards, pellets are placed into thin-wall tubes of zirconium, the metal that is very resistant to high temperatures and corrosive environment. This tube, called ‘fuel rod’, is several metres long. The amount of fuel in one nuclear fuel rod is about 1.5 kg. Several hundreds of fuel rods are combined into a fuel assembly.
Fuel in a reactor
Finally fuel is loaded into a reactor, and Uranium ‘works’ there for several years. During this period, nuclear fission reaction takes place and a great amount of heat is produced, which is used for electricity generation.
While fuel is ‘working’, Plutonium and fission products are accumulated in fuel rods. Plutonium and other generated isotopes are valuable materials. Afterwards, the fuel is removed from the reactor as Spent Nuclear Fuel (SNF). The unloaded spent fuel is stored in a spent fuel pool, where SNF is gradually cooled down for 3-5 years. However, this spent fuel can return into a reactor after reprocessing to ‘work’ again.
Reprocessing of SNF
SNF contains Uranium, Plutonium and isotopes of other elements, which might be used in medicine, science and industry. During reprocessing of SNF Uranium and Plutonium are separated; other valuable isotopes are extracted for further usage. Uranium can be returned to the fuel cycle and used for manufacturing new nuclear fuel.
After reprocessing Uranium is purified from the waste radioactive products and becomes safe to handle. Hence, it can again undergo enrichment processes and be used for manufacturing fresh fuel that can again be loaded into a reactor.
Now only Uranium-235 is used for energy generation. But if we learn how to use Uranium-238, we will have enough fuel for ten thousand years to come. Research and development projects worldwide have been studying this possibility for quite a while, but only Russia possesses operational ‘Fast Breeder’ Reactors (Units BN-600 and BN-800 at Beloyarsk NPP), which are capable of generating fresh nuclear fuel from Uranium-238.
The ‘Fast’ Reactors convert the ‘useless’ Uranium-238 into Plutonium-239. The Plutonium isotope can also be used as nuclear fuel in a mixture of Uranium and Plutonium oxides. This type of nuclear fuel is known as MOX-fuel( ‘Mixed-oxides’) is already used in commercial power generating reactors.
(Andrey Akatov and Yuriy Koryakovskiy are senior lecturers at St. Petersburg State Institute of Technology in Russia).