During fission nuclear reactions, neutrons collide with fissile atoms (uranium and plutonium) present in the nuclear fuel causing fission. With each fission reaction, one or two neutrons are released at high speed.
The objective to maintain a chain fission reaction is that these neutrons hit other fissile atoms but at such a speed it is very difficult. The objective of the moderator is to reduce this speed and, in this way, obtain a better performance of the reactor.
Physical functioning of the moderator
Neutrons, due to their speed, have a high kinetic energy. When encountering atoms of the moderator material, the neutrons collide with these atoms transmitting part of their kinetic energy to the nuclei of the moderator atoms. Neutrons, losing kinetic energy, lose speed.
The materials suitable to perform the function of moderator are those that have a low atomic mass to maximize the energy transferred in each shock. Generally it is hydrogen, deuterium (present in heavy water) or carbon.
It is important that the moderator does not absorb the neutrons since we only want to reduce the speed. To avoid this it is important that the moderator materials have a low capture effective section.
At a certain moment, it is necessary to be able to capture the neutrons to control the chain reaction. For this, the control bars are used.
In a thermal neutron reactor, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable and then split nuclear fission) into two smaller atoms. The fission process for the 235 U cores produces two fission products, two or three free neutrons that move rapidly, plus an amount of energy that manifests mainly in the kinetic energy of the retreat fission products. Free neutrons are emitted with a kinetic energy of 2 MeV each. Because there are more free neutrons. They are released from a uranium fission event that thermal neutrons are needed to initiate the event, the reaction can become self-sufficient, a chain reaction, under controlled conditions, thus releasing one).
The fission cross-section is a function of the energy (called the excitation function) of the neutron that collides with a nucleus of uranium-235. The probability of fission decreases as the kinetic energy of the neutrons (and velocity) increases. This explains why most reactors powered by uranium-35 need a moderator to maintain a chain reaction and why the removal of a moderator can shut down a reactor.
The probability of new fission events is determined by the cross section of the fission, which depends on the speed (energy) of the incident neutrons. For thermal reactors, high-energy neutrons in the MeV range are much less likely to cause more fission. Newly released fast neutrons, which move at approximately 10% of the speed of light, should be reduced or "moderated", usually at speeds of a few kilometers per second, if they are likely to cause more fission in the nuclei of the neighboring atoms of uranium-235 and, therefore, continue the chain reaction. This speed becomes equivalent to temperatures in the few hundred degrees Celsius.
In all moderate reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more thermalised than others; for example, in a CANDU reactor almost all fission reactions are produced by thermal neutrons, whereas in a pressurized water reactor (PWR), a considerable part of the fissions are produced by higher-energy neutrons. In the proposed water-cooled supercritical water reactor (SCWR), the proportion of fast fissions can exceed 50%, which technically is a fast neutron reactor.
A fast reactor does not use any moderator, but is based on fission produced by fast, unmoderated neutrons to sustain the chain reaction. In some designs of fast reactors, up to 20% of the fissions can come from direct fission with fast neutrons of uranium-238, an isotope that is not fissionable with thermal neutrons.
Moderators are also used in neutron sources that are not reactors, such as plutonium beryllium and spallation sources.