How nuclear reactors work

A nuclear reactor is where nuclear fission is used to produce energy. There are many variants: most immerse enriched uranium rods in water and use graphite to modulate the reaction, others use natural uranium as fuel and heavy water as coolant (= to transport heat) … This is the most important part of a nuclear power plant.

This is the heart of nuclear power plants, where nuclear fission takes place: the nuclear reactor. Here, fissionable material will be agitated and kept under control to produce heat. Let’s take a look:

  1. How a nuclear reactor works in general
  2. The different generations of nuclear reactors
  3. A recent innovation: SMRs (Small and Medium Reactors)

How a nuclear reactor works in general

To “launch”, fissile fuel must be struck by electrons. A radioactive material such as californium is used. The reaction is then controlled by “control rods” made of electron-retaining material. In the event of an incident, all the control rods are lowered, instantly stopping the chain reaction. The chain reaction produces a great deal of heat, so a liquid coolant is needed to capture and transport it until it is used by the turbine. In France, this is water. We call this a pressurized water reactor (PWR). The fuel must be regularly renewed. On the one hand, it can run out after a long period: 3 to 5 years in a PWR. Secondly, corrosion and radiation can damage the fuel sheath.

This triptych – fuel, moderator and coolant – defines the essence of the technology used: the reactor technology.

Nuclear reactor technologies

In principle, a nuclear reactor chain includes the entire production cycle (extraction, transformation, utilization, processing), but for simplicity’s sake, we use this term to designate the main types of nuclear reactor, based on three elements:

  • Fuel. This may be natural uranium, which was common in the early days of civil nuclear power, but more often it will beenriched uranium. It can also be a reprocessed mixture known as MOX. New reactor technologies will use other fuels, such as HALEU, TRISO, thorium and others.
  • The moderator, in “slow” neutron reactors: neutrons are slowed down. In general, this is the role of water, but graphite (notably in the Chernobyl power plant) has also been used for this purpose (and is used, in water reactors, as control rods).
  • Heat transfer fluid. This can be not only water, but also liquid metal or gas (e.g. carbon dioxide, helium).

The main reactor types in operation today are:

  • PWR and BWR, pressurized water or boiling water reactors. These are the main technologies, accounting for over 85% of installed capacity worldwide (67.4% and 22.5% respectively). Pressurized water in a closed circuit, the primary circuit, serves as both moderator and coolant. The fuel is enriched uranium.
  • RMBK, “Reactor Bolchoie Molchnastie Kipiachié”. Represents 3.4% of installed capacity and is still in use in Russia, where it was the technology that led to the Chernobyl accident in 1986.
  • Candu, “CANada Deuterium Uranium”. Developed by Canada, this technology uses natural uranium and heavy water as a moderator. It accounts for 6.1% of installed capacity.
  • Gas-cooled reactors (GCR), of which only the AGR (Advanced gas-cooled reactor) remains, British gas-cooled reactors inspired by the Magnox. The first French and British reactors were of this type, with the UNGG (“Uranium naturel graphite gaz”) and BritishMagnox models respectively. The latter two used graphite as moderator, CO2 as coolant and unenriched uranium metal as fuel. They are now obsolete. Only 8 British AGRs (6 were closed in 2021 and 2022), representing a power output of 4.9GW, remain.

Finally, there are emerging technologies:

  • Fast Neutron Reactors (FNR) began to be studied in the 1950s, with the first prototype in the USA in 1951 (the EBR-1). The technology began to develop in the 1980s, but was halted by the discovery of new uranium deposits and the general slowdown in the industry. Reactors of this type are in operation in Russia and Japan. They are unique in that they do not use moderators, enabling them to use a wide range of fuel types. There are many different types of RNR.
  • High-temperature reactors (HTRs), using enriched uranium as fuel, helium as coolant and graphite as moderator.Pebble BedReactors (PBR) are a type of High Temperature Reactor (HTR) that use pebbles containing TRISO nuclear fuel to generate power.

The different generations of nuclear reactors

There are 4 generations of reactors.

First-generation reactors

First-generation reactors include those commissioned from the 1950s onwards: UNGG, Magnox and the first American PWRs. In France, Chooz A was the first to use this technology, coming on line in 1967 as a test for the second generation launched by the Messmer plan.

Current 2nd-generation reactors

Most of today’s reactors, designed after 1970, are so-called second-generation reactors. This is the case, for example, of the pressurized water reactors launched under the Messmer plan in France.

3rd generation reactors (EPR …)

Designed in the wake of the Fukushima accident, the third generation of reactors puts the emphasis on safety, even taking into account risks such as hijacking. There are currently three models:

  • A French one, Areva’s EPR
  • an American-Japanese one, the AP1000
  • the Russian VVER-TOI

More and more of these reactors are currently under construction.

4th generation reactors: infinite fuel

The 4th generation of nuclear reactors promises to be more durable and efficient than previous generations, with radically different systems. For example, 3 of the 6 reactors selected as 4th generation reactors by the Generation IV Forum are “fast neutron”. They use no moderator. This type of reactor can fission a wide range of fuels: all types of plutonium, all types of uranium and even transmute certain minor actinides, which are among the main contributors to the radioactivity of nuclear waste. The result would be a virtually infinite fuel supply, and nuclear waste would be even easier to manage than it is today.

This was the technology behind the now-defunct French Superphénix project.

Small Modular Reactors (SMRs)

Numerous projects are currently underway to develop small reactors that can be mass-produced in factories: small modular reactors (SMRs). This would greatly facilitate access to nuclear energy: reactors would be cheaper, better and quicker to build, less labor-intensive to install, and it would be possible to decentralize electricity production even further, favoring cogeneration. A rather fantastic horizon that I detail in the article on Small Modular Reactors.

To find out more:

  • CEALuclear reactor systems
  • Tristan Kamin, “Nuclear reactor technologies”,