Neutron moderator

A moderator (Latin moderare, moderate ' ) is used to free neutrons, which usually relatively high in energy, so "fast" are at their release to brake. The braking is done by repeated elastic scattering on atomic nuclei, ie those lower nuclide mass number (see also Elastic Collision ). The energy emitted by the neutron is added as recoil from the taken atomic nucleus; this they are in other joints as heat to the surrounding matter.

The word moderator can, etc. denote the material used for this purpose or a finished component.

This " moderation " is to be distinguished conceptually from the slowing down of neutrons by inelastic scattering on medium-weight materials such as iron ( sometimes called " degradation "). This is used for neutrons with energies in the MeV range, for example, in shields frequently, often combined with subsequent moderation. In the inelastic scattering emitted by the neutron energy goes into excitation energy of the nucleus, which then emits them as gamma radiation again.

Materials

The average braking effect an elastic collision is strongest with equally large masses of the collision partners. In a centralized surge then a single collision would be sufficient to bring the neutron to a halt (see kinematics ( Teilchenprozesse ) ). Therefore, hydrogen is particularly to be most abundant isotope 1H, the core is a single proton in this respect, the most effective moderator. An advantage is that hydrogen in many materials (such as water, paraffin, many plastics ) represents approximately 2/3 of all atoms. Also possible to use deuterium as a component of heavy water, beryllium and carbon. Helium is as always gaseous substance practically unsuitable. Numerically, the advantage of light elements is expressed as a moderator by the mean logarithmic Energiedekrement.

The following table provides information on the average number of collisions necessary to decelerate a appear which nuclear fission neutron to thermal energy.

Applications

Nuclear reactors

The most important area of moderators are nuclear reactors, in which the produced during the nuclear fission of uranium -235, thorium or plutonium fast neutrons are slowed down to thermal energy. Fast neutrons call seldom produces a nuclear fission; a thermal neutron, however, triggers a much higher probability ( cross section ) of a new nuclear fission. Therefore, a moderated reactor required for self-sustaining fission chain reaction, a much lower amount of nuclear fuel (see also Critical Mass ) as a "faster", working without moderator reactor. Technically be used in nuclear power plants, hydrogen ( as a light ( ordinary ) water), deuterium (as heavy water) and carbon in the form of graphite.

In light water reactors ordinary water is used as moderator. A disadvantage is the absorption of neutrons by the water. This neutron loss is compensated by enriched uranium ( 235U ) was used and the amount of uranium is increased. For light water reactors is the fact that light water is inexpensive and nonflammable and in case of overheating of the reactor ( reactor accident) evaporated. Then no more moderation is present and the chain reaction goes out.

Pure graphite is relatively easy to manufacture and has a negligible neutron absorption. A graphite- core reactor can therefore be operated with non- enriched uranium ( natural uranium ). The first built in 1942 and functioning under the direction of Enrico Fermi in Chicago test reactor was constructed. In the Soviet Union later graphite-moderated water-cooled RBMK for power plants have been developed that are in operation today only in Russia. When Chernobyl reactor accident in 1986, the chain reaction of the superheated reactor could not be interrupted; the graphite retained its moderating properties, and thus the power generation was maintained up to core melt. The greatest damage was done but because of graphite ( pure carbon ) burned and the hot flue gases transported the radioactive particles to great heights. Another graphite- type reactor is the high temperature gas cooled reactor.

Reactor properties at various presenters

The choice of the moderator has effects on the properties of the reactor:

• Heavy water has only a slight tendency ( small cross -section) for neutron capture. Therefore moderated with heavy water reactors can be operated with natural uranium and built comparatively small, which is why they are preferred in mobile applications such as nuclear submarines.
• Lightweight water absorbed by the Neutroneneinfangsreaktion 1H (n, ) 2H neutrons. To compensate for this, these reactors must be operated with enriched uranium and have significantly more volume.
• Graphite, ie carbon, although only slightly absorbed, the neutron slows down but only after many collisions ( see table above) to the required low speed. Therefore, the graphite- core reactors are significantly greater than that of light water reactors.

Breeder reactors contain no moderator because their cleavage is desired by fast neutrons. The sodium here used for cooling (with its mass number 23) has a much smaller moderating effect than water.

Other Applications

Personalities are also used in shielding against neutrons, often in combination with an absorber of thermal neutrons, such as boron or a lithium compound.

In conjunction with a neutron source, a moderator is used when a neutron spectrum is to be provided with a large thermal component, for example, for neutron activation measurements.

In many neutron detectors, the neutrons are brought by a moderator to thermal energy and then use it to their evidence an absorption reaction such as 10B (n, alpha). An example is Long counter.

In all these cases, it is usually used as a moderator is hydrogen, often in the form of paraffin wax or plastics.