HKNIC - Principles of Nuclear Power Reactor

Principles of Nuclear Power Reactor

In a nuclear power reactor, material that may undergo nuclear fission is used in a controlled manner to produce a steady supply of heat. Uranium containing a low enrichment of uranium 235 (U-235) – typically below 5% – is the most common material, although a much higher enrichment of U-235, or plutonium 239 can be used with slightly different principles.

Fission of U-235 produces two lighter elements and heat, together with either two or three neutrons containing high energy. The fission process involves splitting the nuclei of atoms using neutrons. A “controlled” chain reaction is said to occur when the number of neutrons used to bombard U-235 atoms remains constant over time.

Image Credit: DNMC


At a low level of U-235 enrichment, a high energy neutron has a low probability of splitting a U-235 atom. For the neutron to split the atom, it must be slowed down. This is done by placing the neutron in a medium, so that as the neutron hits the atoms in the medium it gives away its energy so as to come into "thermal equilibrium" with the medium. After losing much of its kinetic energy, the neutron then slows down to become a "thermal neutron", although it still travels at some 2.2 kilometres per second.

Thermal neutrons are used to maintain the nuclear chain reaction. Other neutrons are absorbed by the U-238 and its transmutation products, and a small amount will escape from the reactor core.

The medium to slow down the neutrons is called a "moderator" which should ideally slow down the neutrons without absorbing too many, otherwise there will not be enough neutrons to maintain the nuclear reaction. Several common materials can be used for the task. Graphite and ordinary water are good moderators and are readily available. Heavy water is less commonly found but it is also a good moderator. A heavy water molecule differs from a molecule of ordinary water, by replacing the hydrogen atoms in the water molecule with deuterium atoms, a nuclide of hydrogen that contains a proton and a neutron in its atomic nucleus instead of a single proton. As a molecule of ordinary water is physically lighter, ordinary water is also known in the nuclear industry as light water.

A nuclear power reactor that makes use of thermal neutrons to maintain the nuclear chain reaction is called a thermal-neutron reactor.


A cooling medium is necessary to take the heat out of the reactor core for use and to keep the material within the desired temperatures.

A suitable coolant has to be chemically stable and it should not interfere with or be affected by the nuclear chain reaction. In a thermal reactor, ordinary water, heavy water or carbon dioxide can be used as a coolant, depending on the reactor design.

As the highly enriched U-235 fuel in a fast-neutron reactor produces more heat per unit volume than in a thermal-neutron reactor, it is necessary to use a more effective cooling medium. Some liquid metals such as sodium, which have good conductivity and low melting point, are suitable choices.

The fission process of U-235 has to be regulated to ensure steady heat production. This is achieved by using a material to absorb any excess neutrons produced during the fission process of U-235. Material such as boron, cadmium and indium can be placed inside tubes, known as “control rods”, which can then be inserted into the reactor to reduce the reaction. In the case of a water-cooled reactor, a solution of boron can also be added to the reactor coolant to absorb the neutrons to achieve the same effect.

In a modern nuclear power reactor, the control rods can be quickly inserted into the reactor to stop the nuclear chain reaction and shut down the reactor completely. For a water-cooled reactor, a concentrated solution of boron can be added to the reactor coolant as an additional measure to secure the same result.


When the U-235 or Pu-239 has a high level of enrichment, the high energy neutrons emitted after nuclear fission have a high probability of splitting a fissile atom. These are called “fast” neutrons and in this case, no moderator is necessary to maintain the nuclear chain reaction.

A “blanket” of “fertile” material, such as U-238, can be placed around the reactor core to absorb the neutrons emitted from the core to become Pu-239. In this way, the reactor produces fissile material faster than it has consumed, so that the energy stored in U-238 can be accessed thus increasing the energy resource of uranium by a factor of about sixty. Such a reactor is called a fast breeder reactor. A reactor constructed without the blanket of fertile material – in which case it consumes more fissile material than it produces – is sometimes called a fast burner reactor.