The presence of a moderator; is essential for operating reactors whose fuel is poor or slightly enriched in fissile elements. The purpose of this moderator medium is to slow the neutrons to promote fission.
Fission neutrons are slowed down by successive collisions on the moderator nuclei. The aim is to avoid as least as possible neutron capture during this series of collisions: the moderator must be "transparent." The slowdown should be fast to avoid captures by other nuclei. To thermalize a 2 MeV neutron it takes on average 26 collisions in an hydrogenated environment, 31 in deuterium, 120 in carbon and 2202 in uranium.
The rapidity of the slowdown with hydrogen is due to the fact that hydrogen is composed of protons, which have almost the same mass as neutrons. Hydrogen is the fastest medium for slowdown, but has the disadvantage of capturing some neutrons to form deuterium, the heavy isotope of hydrogen.
The deuterium, in the form of heavy water, is a better moderator. It is a little less "fast", but being already composed of a proton and a neutron, it captures fewer neutrons than hydrogen. It is an ideal retarder, whose handicap is to be expensive (one finds one deuterium atom for 6500 atoms of hydrogen in Nature). Moderated with heavy water, a reactor is able to produce energy with natural uranium. For instance, the CANDU Canadian and Indian reactors use heavy water as moderator
Graphite is also a very effective moderator. Carbon is a very stable nucleus reluctant to accept a new neutron. The first nuclear reactor designed by Enrico Fermi in 1942, which functioned during the Second World War with natural uranium was "moderated" using graphite. Graphite is abundant, but as it must be cleansed of impurities, it is expensive.
The first plants using natural uranium required moderators such as deuterium (heavy water) and graphite to operate. When enriched uranium became available as fuel, ordinary water became suitable as a moderator and a coolant. Most existing reactors usually operate now with pressurized or boiling water.
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