The function of the moderator in a nuclear reactor is to slow down the neutrons in atomic fission reactions through elastic collisions between its nuclei and fission neutrons.
During nuclear fission reactions, neutrons collide with fissile uranium atoms, causing new reactions. With each fission reaction, one or two neutrons are released at high speed, collide with other atoms, and generate a nuclear chain reaction.
To take advantage of the nuclear energy of the atom, the objective of the nuclear power plant is to maintain a nuclear chain reaction within the nuclear reactor. Moving the neutrons too fast in the reactor core reduces the chance of colliding with another atom.
Fast neutrons released after a fission reaction moves at approximately 10% of the speed of light. Therefore, for the proper functioning of the reactor, its speed must be reduced to a few kilometers per second. This speed becomes equivalent to temperatures in the few hundred degrees Celsius.
What Is the Neutron Moderator Doing Inside the Reactor?
Neutrons, due to their speed, have significant kinetic energy. Therefore, if a high neutron collides with an atom of the material used as a moderator, part of its kinetic energy is transferred to the atom of the moderator, losing energy and momentum.
The chemical elements of the periodic table suitable to perform the function of the moderator are those that have a low atomic mass to maximize the energy transferred in each collision.
The moderator mustn't absorb the neutrons as we only want to slow down. Thus, the moderating materials must have a low cross-section of capture.
How Is a Fission Reaction Generated?
In a thermal neutron reactor, the atomic nucleus of a heavy fuel element such as uranium 235 (U-235) absorbs a slow-moving free neutron. With the new neutron, the atom becomes unstable and breaks down.
What Is the Result of a Nuclear Fission Reaction?
In the fission process, the nuclear reaction produces:
Two fission products
Two or three fast-moving free neutrons
An amount of energy that manifests itself primarily in the kinetic energy of fission products.
Why Do Nuclear Reactors Need a Neutron Moderator?
Thermal nuclear reactors need a moderator to reduce the speed of the neutrons because the probability that a neutron will hit a uranium-235 atom decreases with increasing neutron speed.
It explains why most uranium-35 fueled atomic reactors need a moderator to sustain a chain reaction and why removing a moderator can shut down a reactor.
Differences Between Different Types of Nuclear Reactors
In all moderate reactors, some neutrons of all energy levels will produce fission, including fast neutrons.
Some reactors are more thermal than others. For example, in a CANDU reactor, almost all fission reactions are produced by thermal neutrons. On the other hand, in a pressurized water reactor (PWR), a considerable part of the fissions are produced by higher-energy neutrons.
Gas-cooled reactors (GCR) and advanced gas-cooled reactors (AGR) use graphite as the moderator and carbon dioxide as the coolant.
Light water reactors (LWR), which englobe boiling water reactors (BWR) and pressurized water reactors (PWR), use the light water as the moderator in the nuclear reactor.
Pressurized heavy water reactors (PHWR) use heavy water as the moderator. Unlike
The light water graphite reactor (LWGR) uses graphite as a moderator of neutrons which is a good moderator.
A fast reactor does not use any moderators but relies on fission produced by unmoderated fast neutrons.