Nuclear Power Plant Isar, Germany

Spent nuclear fuel pool

Turbine of a nuclear plant

Neutron moderator definition

Neutron moderator is a component part of the nuclear reactors. It is located in the reactor core. The function of the moderator is to reduce the speed of the neutrons in a nuclear fission reaction.

During nuclear reactions fission neutrons collide with fissile atoms (uranium and plutonium) present in the nuclear fuel causing their nuclear fission. With each fission reaction are released one or two high-speed neutrons.

The aim to maintain a fission chain reaction is that these neutrons collide with other fissile atoms but so fast it becomes very difficult. The aim of the moderator is to reduce this speed and, thus, get a better performance of the reactor.

Physical functioning of the neutron moderator

Funcioamiento scheme of neutron moderator

Neutrons, because of their speed, have a high kinetic energy. When meeting atoms of the moderator material, these neutrons collide with this atoms transmitting part of its kinetic energy to the nuclei of the atoms of the moderator. Neutrons, losing kinetic energy, lose speed.

Materials suitable for the role of moderator are those with a low atomic mass to maximize the energy transferred in each collision. They are usually hydrogen, deuterium (present in the heavy water) or carbon.

It is important that the moderator does not absorb neutrons as we only want to slow down them. To avoid this it is important that the moderators materials have a low capture cross section.

At one point we do need to capture neutrons to control the chain reaction. To do this the control rods are used.

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Last review: February 1, 2016