Nuclear fuel is the material in which the fission reaction of a nuclear reactor takes place. There are different nuclear fuels for the different types of reactors. Every nuclear fuel contains at least one fissile nuclide , usually 235 U ; even the single fissile nuclide is sometimes referred to as nuclear fuel. In most reactors, uranium is not used as natural uranium , but in enriched form. During operation, further fissile nuclides, for example plutonium isotopes, are created by the capture of neutrons in the nuclear fuel . Transuranium waste could be used as nuclear fuel in the future, but only in special types of reactors (see transmutation ) that are still the subject of research today (2016).
Nuclear fuels can be differentiated according to their chemical nature or their technical application. The change in the composition and other properties over the service life is referred to as burn -up.
A distinction must be made between nuclear fuels and breeding materials from which new fuel is produced in the reactor. The breeding material is sometimes referred to as weak nuclear fuel .
Oxidic nuclear fuel
As of 2016, the vast majority of the nuclear fuels used are oxidic , i.e. UO 2 or PuO 2 . They are primarily used in light water reactors , but also in other systems. The advantages are the thermal and chemical stability up to relatively high temperature ranges. The disadvantages include the low thermal conductivity.
Metallic nuclear fuel
Metallic uranium was used in the now disused Magnox reactors and the early fast breeders EBR-1 , EBR-2 and the very first reactor ever . The simple production, the high thermal conductivity and the high density were decisive for this. Due to the reactivity with water, spontaneous density changes at certain temperatures and the swelling during operation, metallic nuclear fuel is no longer used. Exceptions such as research and training reactors can still be found (e.g. CROCUS at EPFL Lausanne).
Other solid nuclear fuel
In the course of the further development of reactor systems (fourth generation) there are concepts for carbidic and nitridic nuclear fuels. The focus is on the advantages of ceramic materials. Some of these were already tested in the 1950s and 1960s, but were not pursued further in favor of the oxides. The advantages lie in the higher density, comparably high melting temperature and roughly ten times higher thermal conductivity compared to oxide.
Liquid nuclear fuel
Another development is molten salts , in which the fuel is dissolved. One example is FLiNaK. The liquid phase results in completely different technological possibilities and challenges for the reactor design. Advantages include the possibility of continuous cleaning of fission products, the high possible temperature range and the elimination of fuel element production. A major disadvantage is the corrosiveness of the salts. Together with aqueous uranium solutions, these concepts were also examined earlier, but were not pursued any further. As part of the fourth generation, they too are receiving new attention.
Technological subdivision of solid nuclear fuels
Fuel rods are by far the most widely used form of nuclear fuel. Typically, a gas-tight jacket tube several meters long surrounds a stack of ceramic fuel pellets. Ceramic fuel can also be used in the form of a granulate fill (see Pac-pellets ). The cladding tube for light water and heavy water reactors is made of zircalloy , for breeder reactors of stainless steel.
The fuel rods are not used individually, but are combined into bundles ( fuel elements ) in all reactor types .
Fuel assemblies for high temperature reactors
High-temperature reactors use nuclear fuel, which - in the form of small UO 2 grains - is embedded in graphite . In some designs, these fuel elements are balls the size of a tennis ball, while in others they are vertical columns with a prismatic cross-section.
Fuel assemblies for research reactors
In some research and training reactors , special nuclear fuels have been and are used: in the Siemens teaching reactor, panels made of polyethylene containing uranium oxide (U 3 O 8 ) powder; a combination of uranium, zirconium and hydrogen in the TRIGA reactor ; Specially shaped plates made of uranium silicide-aluminum dispersion fuel in the Munich research reactor FRM II .
Fuel is referred to as ' burned out ' when it can no longer make a significant contribution to the production of heat in the reactor. This applies in particular to all fuels or fuel assemblies that have been replaced in the reactor for this reason.
- University of Munich: lecture manuscript section nuclear fuels . After the lecture Fundamentals of Energy Supply. Resch Verlag 1990
- DOE :  "Carbide and Nitride Fuels for Advanced Burner Reactor", many numbers on metal / oxide / nitride / carbide
- z. B. in R. Zahoransky (Ed.): Energietechnik . 7th edition, Springer 2015, ISBN 978-3-658-07453-1 , page 103
- Oak Ridge National Laboratory: Thermophysical Properties of MOX and UO 2 Fuels including the Effects of Irradiation. (No longer available online.) Oak Ridge National Laboratory , September 2000, archived from the original on July 2, 2015 ; Retrieved April 19, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- SE Jensen, E. Nonbol: Description of the Magnox Type of Gas Cooled Reactor (MAGNOX). IAEA , November 1998, p. 12 , accessed April 21, 2016 .
- CROCUS website: Archived copy ( memento of the original from August 25, 2013 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- IAEA : High Temperature Gas Cooled Reactor Fuels and Materials. March 2010, p. 5 , accessed April 21, 2016 .