Nuclear reactor

A nuclear reactor is a special device that controls a nuclear reaction. It works by splitting tiny parts of materials, like uranium and plutonium. This makes a lot of energy as heat. The heat can be used to make electricity or power machines.

Nuclear reactors are efficient. A small amount of fuel can make a lot of energy, more than coal. They are used all over the world for electricity and to power ships. In the past, there have been some problems, like accidents. Because of this, people are very careful about building and running them.

Today, hundreds of nuclear reactors work around the world. They provide about 9% of the world's electricity. Most use a design called pressurized water reactors. But there are many other types being made to be safer and better. These reactors help reduce pollution compared to burning fossil fuels. However, they also create waste that must be handled carefully.

Terminology

In the early 1940s, scientists called early nuclear devices "atomic piles." This included any setup with uranium that tried to increase neutrons. After Chicago Pile-1 showed a lasting chain reaction, the term "reactor" became common. Other names like "nuclear pile" and "atomic reactor" were also used.

Sometimes, simple tests to find the right amount of material are called research reactors, like the Godiva device. The term "nuclear reactor" mostly means a device that uses nuclear fission. It can also mean a nuclear fusion reactor, but so far, these only use more power than they make. Devices like radioisotope thermoelectric generators and radioisotope heater units get power from nuclear decay but are not called reactors because they don't start new reactions.

Operation

Just like regular power plants that burn fuels to make electricity, nuclear reactors make electricity by using heat. But instead of burning fuels, nuclear reactors use a process called nuclear fission to create that heat.

Fission

Main article: Nuclear fission

When a big atom, such as uranium-235, uranium-233, or plutonium-239, takes in a tiny particle called a neutron, it can split into smaller atoms. This splitting, called fission, releases a lot of energy and more particles called neutrons. Some of these neutrons can cause more atoms to split, creating a chain reaction.

Heat generation

The reactor makes heat in a few ways:

• When atoms split, they bump into nearby atoms and turn movement into heat.
• The reactor also takes in some of the radiation made during splitting and turns it into heat.
• Even after the reactor stops, the split atoms keep giving off heat as they change over time.

One kilogram of uranium-235 releases about three million times more energy than a kilogram of coal.

Cooling

A special liquid, often water, moves through the reactor to take away the heat. This heated liquid is used to make steam, which spins a turbine to create electricity. In some reactors, the same water that makes steam also moves through the reactor, while in others, it’s kept separate.

Reactivity control

The speed of splitting can be changed by controlling how many neutrons are around to cause more splitting. Special parts called control rods can be moved in and out of the reactor to absorb neutrons and either slow down or speed up the reaction.

Electrical power generation

The heat made by splitting atoms is used to boil water and create steam. The steam turns a turbine connected to a generator, which makes electricity.

Life-times

Modern nuclear power plants are usually built to last about 60 years. Older ones were meant to last 30–40 years, but many have been updated to last longer. Even with updates, a plant might close early if it’s too expensive to fix or if there are safety worries.

History

See also: Nuclear fission § History, History of nuclear power, and Timeline of nuclear power

In 1932, a scientist named James Chadwick discovered a tiny particle called the neutron. This helped scientists learn how some atoms could break apart and release energy. A few years later, in 1933, a scientist named Leó Szilárd thought of a way to make a machine using these breaking atoms.

In 1938, scientists found that when uranium atoms were hit with neutrons, they broke apart and released more neutrons. This showed that a continuing reaction was possible. In 1939, Albert Einstein wrote a letter to the president of the United States to warn that this discovery could be used to make very powerful bombs. This started more research into these reactions.

In 1942, a team led by Enrico Fermi built the first artificial nuclear reactor in Chicago. This reactor worked by carefully controlling the breaking apart of uranium atoms. After this, many more reactors were built for research and to help make energy.

Table by country

Reactor types

All commercial power reactors work on the same basic idea: nuclear fission. They usually use uranium, and sometimes plutonium, as fuel. There are two main types of fission reactors, depending on the energy of the neutrons they use:

• Thermal-neutron reactors use slowed-down neutrons to keep the fission going. Almost all reactors today are this type. They have materials that slow the neutrons down, making them more likely to cause fission in certain fuels like uranium-235. These reactors can use lower-enriched or natural uranium as fuel. The slowing material is often water, which also acts as a coolant.
• Fast-neutron reactors use fast neutrons to cause fission. They don’t slow the neutrons down and need more enriched fuel. These reactors can produce less long-lived waste but are harder and more expensive to build. They are less common than thermal reactors.

Fusion power, using the fusion of elements like hydrogen, is still mostly in the research stage, with no self-sustaining fusion reactor built yet.

By moderator material

Thermal reactors use different materials to slow down neutrons:

• Graphite-moderated reactors were used in early reactors like the Chicago pile.
• Water moderated reactors include:
  • Heavy-water reactors, used in several countries, which can use natural uranium.
  • Light-water-moderated reactors, the most common type, which use ordinary water and need enriched fuel.
• Light-element-moderated reactors include designs using lithium or beryllium.
• Organically moderated reactors use materials like biphenyl and terphenyl.

By coolant

Most reactors today are cooled by water, but other coolants are also used:

• Water-cooled reactors make up most of the world’s nuclear power. These include:
  • Pressurized-water reactors (PWRs), the most common type in Western countries.
  • Boiling-water reactors (BWRs), where the water boils inside the reactor to produce steam.
  • Other types like supercritical water reactors and reduced moderation water reactors.
• Liquid-metal-cooled reactors use metals like sodium or lead, especially in fast reactors.
• Gas-cooled reactors use gases like carbon dioxide or helium.
• Molten-salt reactors use molten salt as both coolant and fuel carrier.
• Organic nuclear reactors use organic fluids like biphenyl as coolant.

By generation

Reactors are also grouped by generation:

• Generation I were early prototypes and research reactors.
• Generation II are most current nuclear power plants, built between 1965 and 1996.
• Generation III are improvements on existing designs, built from 1996 to 2016.
• Generation III+ are further improvements focusing on safety, built from 2017 to 2021.
• Generation IV are advanced designs still under development.
• Generation V are theoretical designs not currently being researched.

By type of fuel

Reactors can use different fuels, including uranium, plutonium, mixed oxide (MOX) fuel, and thorium.

By phase of fuel

Fuels can be solid (like ceramic or metal), fluid (like molten salt), or gas.

By shape of the core

Reactor cores can have different shapes, such as cubical, cylindrical, octagonal, spherical, slab, or annular.

By use

Reactors are used for:

• Electricity production, including small modular reactors.
• Propulsion, such as in nuclear marine propulsion or rocket proposals.
• Other uses like desalination, heating, and hydrogen production.
• Research and producing radioactive isotopes for medicine and industry.

Current technologies

Some of the most common reactor types today include:

• Pressurized water reactors (PWRs), which use high-pressure water as both moderator and coolant.
• Boiling water reactors (BWRs), where the cooling water boils inside the reactor.
• Pressurized Heavy-Water Reactors (PHWRs), a Canadian design using heavy water.
• RBMK reactors, a Soviet design that can be refueled while operating but has safety concerns.
• Gas-cooled reactors, including advanced designs with higher efficiency.
• Liquid-metal fast-breeder reactors, which can produce more fuel than they consume.
• Pebble-bed reactors, using ceramic fuel balls cooled by helium.
• Molten-salt reactors, which dissolve fuel in salt and have inherent safety features.

Future and developing technologies

Many new reactor designs are being developed, including:

• Advanced reactors like the advanced boiling water reactor and passively safe designs.
• Integral fast reactors, which recycle waste.
• Small, sealed, transportable, autonomous reactors (SSTAR) for remote areas.
• Thorium-based reactors, which can use more abundant thorium as fuel.
• Generation IV reactors, theoretical designs aiming for better safety and efficiency, expected after 2040.
• Generation V+ reactors, highly theoretical designs like liquid-core or gas-core reactors.

Fusion power, using the fusion of light elements, is still in the research phase, with projects like ITER aiming to make it a reality.

Nuclear fuel cycle

Main article: Nuclear fuel cycle

Most nuclear reactors use a special kind of uranium called enriched uranium. This starts with mining uranium from the earth. The uranium is then processed to increase the amount of a type called U-235, which can split and release energy. This enriched uranium is made into small pellets and placed into tubes called fuel rods, which are used in nuclear reactors.

Some reactors can use a mix of uranium and another material called plutonium. Most common reactors need uranium enriched to about 4% U-235. Special reactors can use natural uranium without enrichment. The uranium used in reactors can produce energy for many years, but the fuel needs to be replaced eventually. Used fuel is stored in special pools of water and later moved to secure, shielded containers for safekeeping.

Nuclear safety

Main article: Nuclear safety

See also: Nuclear reactor safety system

Nuclear safety means taking steps to stop accidents and keep things safe. People who build and run nuclear reactors work hard to make sure they are safe. Sometimes, mistakes can still happen. For example, at a place called Fukushima in Japan, an earthquake caused a big wave that stopped the machines meant to keep the reactor safe. This showed that even careful planning can sometimes fail. There are also worries that bad people might try to cause trouble at nuclear places. Experts have looked at these risks and think that more accidents could happen in the future as more nuclear power is used.

Nuclear accidents

See also: Lists of nuclear disasters and radioactive incidents

Sometimes, rare accidents happen at places that use nuclear power. Some of these events include places like Windscale fire, SL-1, Three Mile Island accident, the Chernobyl disaster, and the Fukushima Daiichi nuclear disaster. There have also been problems with reactors on nuclear-powered submarines, like the K-19, the K-27, and the K-431.

Nuclear reactors have also been sent into space. One well-known event was when a Soviet satellite named Kosmos 954 fell back to Earth in January 1978. This satellite carried a nuclear reactor and dropped nuclear material over northern Canada.

Natural nuclear reactors

Main article: Natural nuclear fission reactor

A long time ago, about two billion years back, something special happened in a place called Oklo in Gabon, West Africa. There, nature made its own simple nuclear reactors. These were not built by people but happened when certain conditions were just right. Scientists found evidence of these ancient reactors in 1972. They learned that these natural reactors worked for many hundreds of thousands of years, making a steady amount of energy.

These natural reactors worked because water helped control the reaction. As the reaction grew stronger, the water would heat up and move away, which slowed the reaction back down. This kept everything balanced for a very long time. Scientists study these ancient reactors to better understand how materials move through the ground, which helps with planning for safe storage of radioactive materials today.

Emissions

Nuclear reactors make a substance called tritium during normal work. Tritium is a type of hydrogen that can mix with oxygen to make a molecule like water. It is colorless and has no smell. The amount of tritium released by nuclear power plants is very small and safe.

Very small amounts of another substance, strontium-90, are also released from nuclear power plants. The amount is so small that it is hard to notice above natural background radiation. Most strontium-90 in the environment today comes from past weapons testing.