Neutron absorber
In reactor physics, a neutron absorber is a material that extracts free thermal neutrons from the fission chain reaction by means of another suitable nuclear reaction . With neutron absorbers, the number of neutrons in the reactor core can therefore be controlled, i.e. the course of the chain reaction can be regulated or completely prevented.
Neutron absorbers are not only used in reactors, but also, for example, in shielding against neutrons.
Reactions used
The nuclear reactions used are mostly, but not always, of the type (n, γ), i.e. neutron capture . The capture reaction produces gamma radiation , while other absorption reactions release charged particles. Examples:
A sheet of cadmium 1 mm thick reduces the thermal neutron flux passing through by about 10 5 times, i.e. to 1 hundred thousandth.
Boron is mostly used in its natural isotopic composition (80.9% 11 B and 19.1% 10 B) as boron carbide (B 4 C) in granulate form. At a density of e.g. B. 1.76 g / cm 3 (70% of the density of the solid material) a 1 mm thick layer of this material reduces the thermal neutron flux passing through to 6.9%, i.e. by more than ten times.
Borax is used as an aggregate in concrete shielding walls.
Reactor control and shutdown
Reactor control rods contain elements with a large effective cross section for an absorption reaction, for example cadmium, gadolinium or boron.
In light water reactors, in addition to control rods, water-soluble boric acid is often used to regulate the reactor output and for emergency shutdown (emergency boration) of the reactor in the event of a malfunction of the control rods. For an emergency shutdown, the reactor pressure vessel is flooded with water that contains a lot of boric acid in order to very quickly bring the chain reaction to a standstill. The water-soluble gadolinium (III) nitrate can also be used for this purpose .
Neutron poisons
Some neutron absorbers are created in the reactor as inevitable by-products. Their effect must be compensated for by removing other absorbers while the reactor is in operation. One of these so-called neutron poisons is the xenon isotope 135 Xe, which, due to the so-called xenon poisoning, makes restarting impossible for a certain time after the reactor has been shut down.
Neutron-absorbing nuclides in general
In addition to the materials already mentioned above, z. B. silver , indium , cobalt , hafnium and many rare earth elements are relatively strong neutron absorbers.
The picture on the right shows a nuclide map with the color coding of the cross section for neutron capture ( i.e. not for every type of thermal neutron absorption). The “ magic ” proton and neutron numbers are highlighted by double lines ; one recognizes that this cross-section of such magic atomic nuclei is usually small, but far from magic numbers it is large.
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- ↑ A. Ziegler, H.-J. Allelein (Hrsg.): Reaktortechnik : Physical-technical basics . 2nd edition, Springer-Vieweg 2013, ISBN 978-3-642-33845-8 , page 277
- ^ Emergency shutdown of the Kozloduy NPP in March 2006.
- ↑ researchgate.net: Effect of Gadolinium nitrate concentration on the Corrosion Compatibility of Structural Materials in a Proposed Indian Tube Type Boiling Reactor (PDF Download Available) , accessed on January 22, 2017