Fuel rod
A fuel rod is a tube filled with nuclear fuel that is used in nuclear reactors . The nuclear fuel usually consists of sintered tablets (pellets) made of uranium dioxide or a mixed oxide of uranium dioxide and plutonium dioxide .
Fuel rods are not used individually, but are always bundled into fuel elements .
"Burn"
The term “burning” in connection with nuclear energy (“fuel rod”, “ fuel element ” etc.) is only to be understood in a figurative sense. It is not actually a matter of combustion, i.e. oxidation .
After some time in operation, a fuel element has " burned out ", i. H. The chain reaction converts such a proportion of the fuel into fission products that the element can no longer be used effectively to generate energy. It is then exchanged for a fresh element. The spent, that spent fuel will be stored for later once the final disposal to be fed, the question is still open for a repository today (2020). Some spent fuel elements are reprocessed . Germany and Switzerland no longer send fuel assemblies for reprocessing.
Dimensions and arrangement
In the Brokdorf pressurized water reactor , for example, a single fuel rod has a height of 4.8 m and a diameter of 11 mm.
Many individual fuel rods are bundled with spacers to form fuel elements or fuel assemblies . Depending on the type of reactor, these can be round, rectangular, polygonal or plate-shaped.
Modern fuel element designs also contain shortened fuel rods ( part length fuel rods ) in order to optimize the power distribution in the reactor.
Fuel rod cladding
The fuel rod cladding, the cladding tube, is a metal tube that encloses the nuclear fuel. Depending on the type of fuel element, it has a wall thickness of around 0.6-0.8 mm. In order to achieve good heat transfer in the gap between the nuclear fuel and the cladding tube, the helium gas is injected there and the fuel rod cladding is then welded in a gastight manner .
In thermal (e.g. water-cooled) reactors , zirconium alloy is used as the material for the cladding tubes because the effective cross-section of zirconium for neutron capture is small and the material has good strength and corrosion properties . However, zircalloy becomes very brittle after overheating. A certain degree of corrosion during reactor operation is also inevitable with Zircalloy. The thickness of the oxide layer that forms increases steadily over time, depending on the nature of the material, the cladding tube temperature and the chemical composition of the surrounding cooling water. In addition to radiation damage, corrosion is one of the processes that limit the service life of the fuel elements in a reactor to around three to five years.
In high-speed reactors , mostly titanium-stabilized austenitic stainless steels have been used for the cladding tubes .
The fuel rod cladding separates the nuclear fuel from the coolant of the reactor and prevents fuel and fission products from getting into the coolant. It is thus one of the nested barriers to hold back the radioactive substances. However, structural changes in the alloy occur even during normal operation due to corrosion and radiation damage. Therefore, a small part of the cladding tube develops cracks through which gaseous fission products can escape. They are mostly radionuclides with medium half-lives , mainly isotopes of iodine , xenon and krypton .
Meltdown accident
Fuel rods can melt if they are not cooled sufficiently during operation. Other parts of the reactor core also melt in the process; this is known as a core meltdown . This risk exists due to the resulting decay heat even when the reactor is switched off. Even after the fuel rods have been removed from the reactor core during their storage in the cooling pool , constant cooling is still necessary for a few years in order to avoid overheating.
Security criteria
In the western world, a prerequisite for the issue of an operating license is that the following conditions set by the NRC are guaranteed to be met during operation:
- The temperature must not exceed 1200 ° C.
- The thickness of the oxidation layer of the cladding tube must not exceed 0.17 times its unoxidized wall thickness at any point.
- The hydrogen release may not exceed 0.01 times the amount that would result from complete shell tube oxidation.
- The geometry of the cladding tubes must not change in such a way that cooling can no longer be guaranteed.
- The decay heat must be able to be dissipated in the long term.
Individual evidence
- ↑ FUEL REVIEW: Fuel design data ( Memento from June 17, 2012 in the Internet Archive ), In: Nuclear Engineering International, September 2004.
- ↑ Jan Kopitz, Wolfgang Polifke: Heat transfer: Fundamentals, analytical and numerical methods . Pearson Deutschland GmbH, 2009, ISBN 978-3-8273-7349-6 , p. 72 ( limited preview in Google Book search).
- ^ Günter Kessler: Sustainable and safe nuclear fission energy. Technology and safety of fast and thermal nuclear reactors . Springer 2012, ISBN 978-3-642-11989-7 , page 77.
- ^ Günter Kessler: Sustainable and safe nuclear fission energy. Technology and safety of fast and thermal nuclear reactors . Springer 2012, ISBN 978-3-642-11989-7 , page 158.
- ↑ NRC: 10 CFR 50.46 Acceptance criteria for emergency core cooling systems for light-water nuclear power reactors