What is Nuclear Reactor and How Does it Work?

What is Nuclear Reactor?

A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion.

The heat from nuclear fission is passed to a working fluid (water or gas), which in turn runs through steam turbines. These either drive a ship’s propellers or turn electrical generators’ shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating.

Some reactors are used to produce isotopes for medical and industrial use, or for the production of weapons-grade plutonium.

How does a nuclear reactor work?

Nuclear reactors are, fundamentally, large kettles, which are used to heat water to produce enormous amounts of low-carbon electricity. They come in different sizes and shapes and can be powered by a variety of different fuels.

A nuclear reactor is driven by the splitting of atoms, a process called fission, where a particle (a ‘neutron’) is fired at an atom, which then fissions into two smaller atoms and some additional neutrons. Some of the neutrons that are released then hit other atoms, causing them to fission too and release more neutrons. This is called a chain reaction.

Engineering Choice | The biggest engineering Blogs

The fissioning of atoms in the chain reaction also releases a large amount of energy as heat. The generated heat is removed from the reactor by a circulating fluid, typically water. This heat can then be used to generate steam, which drives turbines for electricity production.

In order to ensure the nuclear reaction takes place at the right speed, reactors have systems that accelerate, slow or shut down the nuclear reaction, and the heat it produces. This is normally done with control rods, which typically are made out of neutron-absorbing materials such as silver and boron.

Nuclear reactors come in many different shapes and sizes – some use water to cool their cores, whilst others use gas or liquid metal. The most common power reactor types use water, with more than 90% of the world’s reactors being water-based.

Further information on the many different types of reactors around the world can be found in the Nuclear Power Reactors section of the Information Library. Nuclear reactors are very reliable at generating electricity, capable of running for 24 hours a day for many months, if not years, without interruption, whatever the weather or season.

Additionally, most nuclear reactors can operate for very long periods of time – over 60 years in many cases. In 2019, units 3&4 at the Turkey Point plant in Florida were the first reactor in the world to be licensed for 80 years of operation.

nuclear reactor

What fuels a reactor?

A number of different materials can be used to fuel a reactor, but most commonly uranium is used. Uranium is abundant and can be found in many places around the world, including in the oceans. Other fuels, such as plutonium and thorium, can also be used.

Most of today’s reactors contain several hundred fuel assemblies, each having thousands of small pellets of uranium fuel. A single pellet contains as much energy as there is in one ton of coal. A typical reactor requires about 27 tons of fresh fuel each year. In contrast, a coal power station of a similar size would require more than two-and-a-half million tons of coal to produce as much electricity.

How about the waste?

Like any industry, the nuclear industry generates waste. However, unlike many industries, nuclear power generates very little of it and fully contains and manages what it does produce. The vast majority of the waste from nuclear power plants is not very radioactive and for many decades has been responsibly managed and disposed of.

If nuclear power was used to supply a person’s electricity needs for an entire year, only about 5 grams of highly radioactive waste would be produced, which is the same weight as a sheet of paper. The used fuel which comes out of the reactor can be managed in different ways, including recycling for energy production or direct disposal.

As a matter of fact, many countries have been using recycled fuel for decades to partially fuel their reactors. 

Types of Light-water Reactors in the United States      

All commercial nuclear reactors in the United States are light-water reactors. This means they use normal water as both a coolant and neutron moderator.

There are two types of light-water reactors operating in America.

1. Pressurized Water Reactors

More than 65% of the commercial reactors in the United States are pressurized-water reactors or PWRs. These reactors pump water into the reactor core under high pressure to prevent the water from boiling.

The water in the core is heated by nuclear fission and then pumped into tubes inside a heat exchanger. Those tubes heat a separate water source to create steam. The steam then turns an electric generator to produce electricity.

Nuclear Reactor Diagram

The core water cycles back to the reactor to be reheated and the process is repeated.

2. Boiling Water Reactors

Roughly a third of the reactors operating in the United States are boiling water reactors (BWRs).

BWRs heat water and produce steam directly inside the reactor vessel. Water is pumped up through the reactor core and heated by fission. Pipes then feed the steam directly to a turbine to produce electricity.

Nuclear Reactor Diagram

The unused steam is then condensed back to water and reused in the heating process.

New Nuclear Reactor Technology

Innovative entrepreneurs and startups are developing new types of reactors to be more efficient and flexible in operations, reach remote and developing areas, and even turn seawater into drinking water.

  • Advanced reactors include many types of reactors, including small modular reactors (SMRs), now in development. Several of these new designs do not use water for cooling; instead, they use other materials like liquid metal, molten salt or helium to transfer heat to a separate supply of water and make steam.
  • SMRs are advanced reactors that produce 300 megawatts or less of electricity. They will be less costly to construct and can be built in factories and shipped to where they are needed, so they can help power remote areas or developing nations with carbon-free energy. SMRs also can scale in power output to meet electricity demand, making them ideal partners to support intermittent renewable energy sources.
  • Some advanced reactors will operate at higher temperatures or lower pressures than traditional nuclear reactors. They also will offer other applications like water desalination and hydrogen production. Other reactors will be very fuel efficient by producing less waste or by having extended fuel cycles and not having to stop and refuel for a decade or more.