How does a nuclear reactor work

A nuclear reactor is defined as an installation capable of initiating, maintaining and controlling the fission chain reactions that take place in the reactor core, composed of the fuel, the coolant, the control elements, the structural materials and the moderator in the case of thermal nuclear reactors.

There are two ways to design a nuclear reactor: thermal or fast.

Difference between a thermal and fast reactor
As we have said, there are two ways of designing a nuclear reactor, either by slowing down (moderating) the fast neutrons or by increasing the proportion of fissile atoms. For the task of retarding the neutrons, a moderator (light water, heavy water, graphite) is used and the resulting slow neutrons are called thermal, so that the reactors based on this technique are known as Thermal Reactors, unlike those that they use fast (fast) neutrons, called Fast Reactors.

What does Critical Mass mean?
When building a reactor, it is necessary to have a critical mass of fuel, that is, enough fissile material, in an optimal arrangement of the fuel and the rest of the core materials, to maintain the chain reaction. The arrangement of the neutron absorbers and the control rods makes it possible to maintain criticality in operation and subcriticality in stop-and-go.

Components of the reactor core
The core is the part of the reactor where the nuclear chain reaction occurs and is maintained. Its objective is to heat the water in the primary circuit. It is designed to operate in a safe and controlled manner, so that the amount of energy extracted from the fuel is maximized.

Each component of the reactor core plays an important role in generating heat:


The fuel of a nuclear reactor is fisional material in quantities such that the critical mass is reached, and placed in such a way that it is possible to quickly extract the heat that is produced inside it due to the nuclear chain reaction.

The fuels used in nuclear power plants are in solid form, although they vary from slightly enriched ceramic uranium dioxide, uranium in magnesium alloy tubes to enriched or natural uranium dioxide in zirconium alloy tubes, it all depends on the type of reactor. .

Control rods

The control rod bundles provide a rapid means of controlling the nuclear reaction, allowing rapid power changes of the reactor and its eventual shutdown in case of emergency. They are made of neutron absorbing materials (boron carbide or silver, indium and cadmium alloys, among others) and usually have the same dimensions as fuel elements. The reactivity of the core increases or decreases by raising or lowering the control rods, that is, by modifying the presence of neutron absorbing material contained in them in the core.

For a reactor to work for a period of time, it must have an excess of reactivity, which is maximum with fresh fuel and decreases with its life until it is canceled, at which time the fuel is recharged.

In normal operation, a nuclear reactor has the control rods totally or partially extracted from the core, but the design of nuclear power plants is such that in the event of a failure in its safety or reactor control system, it always acts in the sense of safety. of reactor, fully introducing all the control rods in the core and bringing the reactor to a safe shutdown in a few seconds.

In general, a fuel element is constituted by a quadrangular arrangement of the fuel rods, although the hexagonal arrangement of the Russian pressurized water reactor should be mentioned.

Attaching the guide tubes to the fuel support grids ensures that the centers of the fuel rods and the guide tubes are the same distance. All fuel elements have the same mechanical design. Some contain bundles of control rods and others contain consumable poisons or neutron sources.

To ensure the quality of the fuel elements, numerous inspections and tests are carried out on both raw materials and the final product.


The neutrons produced in fission have high energy in the form of velocity. It is convenient to slow down their speed so that the probability of other atoms fission increases and the chain reaction is not stopped. This is achieved by elastic collisions of the neutrons with the nuclei of the moderator.

Among the most used moderators are light water, heavy water and graphite.