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However, in fast reactors a moderator is not needed, and the neutrons within it move much more quickly. Currently, almost all operating reactors are thermal and thus require a moderator to slow down fast neutrons to the thermal level so that nuclear fission can continue. Nuclear reactors can be either thermal or fast. Reactors that use graphite moderator include the RBMK, pebble bed reactors, and the magnox reactor. Another potential issue with using graphite as a moderator is its ability to oxidize in the presence of air, and its low strength and density which could cause it to change dimensions in the reactor. However, at high temperatures the graphite can react with oxygen and carbon dioxide in the reactor and this decreases its effectiveness. In addition, graphite is a good moderator as it is thermally stable and conducts heat well. One benefit of graphite is that even at the high purity that is necessary for graphite to perform well, it is available at a fairly low price. Graphite can be made artificially using boron electrodes, however, since boron is a very good neutron absorber- a small amount of contamination will make the graphite an ineffective moderator. Graphite has been a popular moderator in the past, however, one drawback is that it needs to be extremely pure to be effective.

Reactors that use heavy water include the CANDU designs and the pressurized heavy water reactor. The main disadvantage to the use of heavy water is its high cost of production, as it is made using the Girder-Sulfide process. Heavy water is used in reactors because its benefits are similar to light water, but since it contains deuterium atoms, its neutron absorption cross section is much lower. Reactors that use light water are known as light water reactors and include the pressurized water reactor (PWR), the boiling water reactor (BWR), and the supercritical water cooled reactor (SCWR). Thus light water can only be used as a moderator along with enriched fuels. One drawback is that hydrogen has a relatively high neutron absorption cross-section because of its ability to form deuterium. In addition, light water is abundant and fairly inexpensive. This means that one collision will significantly reduce the speed of the neutron because of the laws of conservation of energy and momentum. Hydrogen works well as a neutron moderator because its mass is almost identical to that of a neutron. Light water (no different than regular water) is used in many reactors because it contains large amounts of hydrogen. The moderators vary in terms of their moderating abilities, as well as in their costs. The relative properties of these materials are compared below. Typically-used moderator materials include heavy water, light water, and graphite. There are several different types of moderating materials, and each have places where they are used more effectively. When this process is repeated the fissile events are doubled each time. The moderator will then slow these fast neutrons, and produce more slow neutrons to continue the nuclear chain reaction. When a slow neutron collides with a fissile material like Uranium-235, it produces fast neutrons. The table below shows that common moderators have a low neutron absorption cross-section but a comparatively large neutron scattering cross-section.įigure 1. For this reason, lighter elements tend to be more efficient moderators. If the collisions between neutrons and nuclei are elastic collisions, it implies that the closer in size the nucleus of an atom is to a neutron, the more the neutron will be slowed. This neutron scattering is a measure of how likely a neutron will interact with an atom of the moderator. Thus, in an ideal moderator the neutron scattering cross-section is high. However, the moderator should be able to slow down neutrons to an acceptable speed. This means that the moderator should have a low neutron absorption cross-section. First, a moderator cannot absorb neutrons itself. Materials used for moderation need to a very specific set of properties. This slowing or moderation of the neutrons allows them to be more easily absorbed by fissile nuclei, creating more fission events (see Figure 1). Neutron moderators are a type of material in a nuclear reactor that work to slow down the fast neutrons (produced by splitting atoms in fissile compounds like uranium-235), to make them more effective in the fission chain reaction.