Fuel assembly
The fuel elements are essential components of a nuclear reactor and, together with the other internals, form the reactor core . They contain the nuclear fuel .
In most power reactors , i.e. systems for energy generation, the individual fuel element is a bundle of many thin fuel rods around which the coolant, usually water, is washed. This arrangement results in a sufficiently large area for the heat transfer. The rods contain the nuclear fuel, mostly uranium oxide in the form of cylindrical pellets . However, there are other forms of fuel elements in high-temperature reactors (see below).
Various other fuel element shapes are also possible in research reactors with their relatively low thermal output.
Types of fuel assemblies
Depending on the type of reactor, the fuel elements have different shapes and compositions. In reactors with liquid coolant, the fuel elements of which are arranged in a common reactor pressure vessel - these include the pressurized water reactors , boiling water reactors , but also breeder reactors mentioned below - the fuel element has a square or hexagonal cross-section, since the cross-sectional area of the reactor can be completely filled with such a shape . The fuel element of pressure tube reactors , on the other hand, has a circular cross-section, corresponding to the shape of the tube (see e.g. CANDU ). There are other types of fuel assemblies in gas-cooled reactors.
Pressurized water reactors
Fuel assemblies for pressurized water reactors consist of a bundle of individual fuel rods , between which there are also guide tubes for the control rods introduced from above . Such a fuel assembly can for example consist of 236 fuel rods and 20 control rod guide tubes in a 16 × 16 arrangement. The control rods of the control element are held together by a spider and are distributed over the cross section of the fuel assembly. The cladding tubes of the fuel rods are made of the zirconium alloy Zirkalloy -4. The enrichment of the fuel is around 3–4%.
Boiling water reactors
Fuel elements for boiling water reactors also consist of a fuel rod bundle, but the number of fuel rods per fuel element is significantly lower (e.g. 64 fuel rods and a so-called water rod in an 8x8 arrangement). The water rods are omitted in the case of newer fuel elements. A case made of zircalloy enclosing the fuel element forms the cooling channel. The boxes of several fuel assemblies each form the guide channel for a control rod coming from below with a cruciform cross section. The cladding tubes of the fuel rods are made of Zircalloy-2. The enrichment of the fuel corresponds roughly to that of pressurized water reactors.
Examples:
- In the Biblis nuclear power plant (pressurized water reactor) there are 193 fuel elements in a reactor, each of which consists of 236 fuel rods. Each fuel assembly has a length of 4905 mm, a cross section of 230 mm by 230 mm and a weight of 830 kg.
- In the reactor of the Krümmel nuclear power plant (boiling water reactor) there are 840 fuel elements.
High temperature reactors
The fuel elements of the high-temperature reactor of the type pebble bed reactor according to Farrington Daniels consist of graphite spheres with a diameter of about 6 cm, in which the fuel is embedded in the form of many small spheres made of uranium thorium oxide, around 15,000 spheres per spherical fuel element with a capacity of around 0.2 Watts per bead. Each bead is covered with a ceramic barrier (so-called coated particles , see Pac beads ). According to Rudolf Schulten , the ceramic envelopes of the spheres act like mini containments in which fissile material and fission products are packed in mini quantities. This covering of the fuel globules is very resistant to thermal stress and retains heavy metals and noble gases even at high temperatures. The decay heat is removed from the spherical fuel elements by radiation and conduction. The robust spherical fuel elements and their needs-based arrangement are important components of the safety architecture of the pebble bed reactor. From the mid-1980s onwards, Schulten referred to the potential of the pebble bed reactor, the nuclear power burned spherical fuel elements without dismantling, i.e. H. to be disposed of in their entirety in order to avoid the safety risks of the physico-chemical treatment of large radioactive quantities in large reprocessing plants . Schulten took the view that the graphite spheres with their ceramic-coated fuel particles, because of their strength and impermeability, allow final storage at great geological depths of several 1000 meters without treatment. The size of the graphite spheres is not only determined by their function as a fuel carrier, but also by their role as a moderator . This combination of fuel carrier and moderator leads to volumes which, for reasons of economy, can be disadvantageous for direct disposal. State institutions and companies from Great Britain, Sweden, the Netherlands, Belgium, France, Italy, Switzerland and Germany were involved in the development of the fuel elements, with substantial support from Euratom . The fuel elements were used in the pebble bed reactors AVR in Jülich and THTR in Hamm-Uentrop , both of which have been decommissioned since 1988. Rainer Moormann pointed out various problems : These include the release of some highly toxic fission products such as silver and cesium through diffusion and the formation of dust through abrasion of the graphite balls. The flammability of graphite spheres and their high reactivity with water vapor with the formation of flammable gases must be seen as further safety problems. In Europe, fuel element development for HTR was largely ended around 1990. Development in South Africa was also discontinued in 2010. A limited further development is taking place in China, taking into account the experiences made at AVR and THTR in terms of solving the problems identified there.
Most of the fuel in the plants implemented up to now was highly enriched. The uranium is the fissile material in these fuel elements (enrichment to approx. 93%), and thorium is the breeding material from which 233 U is produced as another fissile material in operation . These fuel elements are not directly weapons-grade, as they only consist of approx. 10% uranium and 90% thorium. However, the highly weapons-grade uranium can be separated chemically. Because of this risk of proliferation, it was necessary to gradually switch to low-enriched fuel from around 1980.
In other high-temperature reactors (in England and USA), fuel elements shaped as angular (prismatic) rods were used. The fuel material also partly contained thorium and was similarly composed of graphite and coated particles .
Spent fuel assemblies
The operation of all types of nuclear power plants generates spent fuel . These are the fuel assemblies that have reached their intended burn- up and must therefore be replaced.
The reactor core of the Grafenrheinfeld nuclear power plant z. B. contains 193 fuel elements with a total fuel weight of 103 t, which initially contain 4% by weight of 235 U ( enrichment ).
Every year 40 fuel assemblies are replaced during the overhaul: They are initially stored in the nuclear reactor's decay basin until their radioactivity and heat production have subsided sufficiently to be able to continue processing them.
In the past, spent fuel elements were often taken abroad for reprocessing . There, the waste materials are separated from the reusable nuclear fuels uranium and plutonium still contained in the fuel element with the help of chemical processes .
In Germany, transports to reprocessing plants are no longer permitted by law since July 1, 2005.
Another disposal method that has only been pursued since then is direct disposal . The spent fuel elements are suitably packaged and taken to a repository without prior reprocessing . According to official statements by the German Federal Government, the commissioning of such a repository is targeted for around 2030.
Spent fuel assemblies are often also referred to as spent or spent fuel assemblies. The collective term irradiated fuel assemblies must be distinguished from this. In addition to the spent fuel assemblies, this also includes fuel assemblies that were only temporarily unloaded and are to be re-used because they have not yet reached their intended burnup.
Fuel tax
The German government announced in June 2010 that it was planning a fuel element tax (as part of a large austerity package) . The new tax should bring in 2.3 billion euros annually. The aim is for the operators of nuclear power plants in Germany ( RWE , E.ON , EnBW and Vattenfall )
- on the one hand to be involved in the rehabilitation of ailing repository - and
- on the other hand, one wants to skim off part of the additional income to be expected with a term extension.
The Nuclear Fuel Tax Act of December 8, 2010 came into force on January 1, 2011. On June 7, 2017, the Federal Constitutional Court declared the nuclear fuel tax to be unconstitutional. This means that the federal government has to repay the nuclear companies more than 6 billion euros.
See also
Web links
Individual evidence
- ^ A b Oskar Höfling : Physics . 13th edition. tape 2 , part 3, quanta and atoms. Dümmler, Bonn 1986, ISBN 3-427-41163-X , p. 931 (Dümmlerbuch, 4116).
- ^ Rudolf Schulten, Heinrich Bonnenberg: Fuel element and protection goals. VDI Society for Energy Technology, Yearbook 91, 1991, p. 175.
- ^ A b Rainer Moormann : AVR prototype pebble bed reactor: a safety re-evaluation of its operation and consequences for future reactors , 2009.
- ↑ KKW Technical Data ( Memento of the original from January 9th, 2009 in the Internet Archive ) 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.
- ↑ Nuclear tax shocks utility . In: ZeitOnline , June 9, 2010
- ↑ Constitutional Court: Nuclear fuel tax is illegal . In: The time . June 7, 2017, ISSN 0044-2070 ( zeit.de [accessed July 23, 2017]).
