Pressurized water reactor

A pressurized water reactor (PWR) is a common type of light-water nuclear reactor used to generate electricity. It is the most popular kind of nuclear power reactor in the world, making up almost 70% of the world's commercial reactor fleet. Many countries, such as the United States, France, and China, use PWRs to produce power.

In a PWR, water plays two important roles: it helps control the reaction inside the reactor and carries away heat. The water in the reactor gets very hot because of the energy released when tiny parts of atoms called fission happen. Special high pressure, about 155 bar, keeps the water as a liquid. This hot water then moves to a place called a steam generator, where it gives its heat to another set of water. That second set of water turns to steam and spins steam turbines connected to an electric generator to make electricity.

Unlike another type called a boiling water reactor, a PWR keeps the water under very high pressure so it does not turn to steam until it leaves the main reactor area. Most PWRs have been built to work in many countries around the world. Newer designs, known as Generation III and III+ reactors, such as the AP1000 and EPR, are being used in places like China since 2018. PWRs were first created to power nuclear submarines and later became the basis for many power plants, like the Shippingport Atomic Power Station.

History

Development of pressurized water reactors started in 1946 with the U.S. Naval Nuclear Propulsion Program at Oak Ridge National Laboratory in Tennessee. This program aimed to create nuclear power for submarines. The first submarine power plant was at the Idaho National Laboratory, and further work was done by Westinghouse at the Bettis Atomic Power Laboratory.

Rancho Seco PWR reactor hall and cooling tower (being decommissioned, 2004)

The first commercial nuclear power plant, at Shippingport Atomic Power Station, was designed as a pressurized water reactor. In the United States, the Army also used these reactors from 1954 to 1974. A serious issue happened at Three Mile Island Nuclear Generating Station in 1979, which slowed down new nuclear plant construction in the U.S. for many years. In 2016, a new reactor named Watts Bar unit 2 began operating, the first new U.S. reactor since 1996. In 2020, NuScale Power received approval for a smaller reactor design, and the Energy Impact Center shared open-source plans for building a 100 MW_electric nuclear power plant.

Design

Nuclear fuel in the reactor pressure vessel starts a controlled fission chain reaction, which creates heat. This heat warms the water in the primary coolant loop. The hot water moves to a heat exchanger called the steam generator, where it passes through many small tubes. Here, the heat moves to another loop of water on the other side of the tubes, turning it into pressurized steam without the two liquids mixing. This keeps the secondary coolant from becoming radioactive.

The pressurized steam powers a turbine connected to an electrical generator, which makes electricity. After the turbine, the steam is cooled and turned back into water in a condenser. This water is then pumped back into the steam generator. Sometimes, it is warmed up first to prevent sudden temperature changes.

The steam can also be used for other things, like powering ships and submarines, operating aircraft catapults, or heating buildings and facilities.

Two key features of pressurized water reactors are: they have separate coolant loops for the water that goes through the reactor and the water that makes steam, and the pressure in the primary coolant loop is very high, usually around 15–16 megapascals, which is much higher than in other types of nuclear reactors. This high pressure means only small areas boil, and the steam quickly turns back into water. In contrast, boiling water reactors are designed to let the coolant boil.

Pictorial explanation of power transfer in a pressurized water reactor. Primary coolant is in orange and the secondary coolant (steam and later feedwater) is in blue.

Nuclear fuel

reactor pressure vessel

fission chain reaction

Primary coolant system showing reactor pressure vessel (red), steam generators (purple), Pressurizer (blue), and pumps (green) in the three coolant loop Hualong One design

Reactor

Coolant

Water is used to keep things cool in a pressurized water reactor. It goes into the reactor at a very hot temperature but stays liquid because it is under a lot of pressure. This pressure helps keep the water from turning into steam until it reaches a certain temperature.

PWR reactor pressure vessel

Pressurizer

Main article: Pressurizer

A special container called a pressurizer helps keep the pressure just right in the reactor. It is filled with water and heated to stay close to the point where it would turn into steam. This way, the pressure in the reactor stays steady even when things change.

Pumps

Strong pumps move the cooling water around the reactor. The water picks up heat from the reactor and then gives off that heat to make steam, which can be used to make electricity. After giving off the heat, the water goes back to the reactor to be heated again.

Moderator

Main article: Passive nuclear safety

Pressurized water reactors need to slow down fast neutrons so they can interact with the fuel and keep the reaction going. In these reactors, water is used to slow the neutrons down. When neutrons bump into the hydrogen atoms in the water, they lose speed. This slowing down happens more when the water is denser.

One safety feature of these reactors is that if the temperature gets too high, the water expands. This creates more space between water molecules, which means fewer chances for neutrons to slow down. As a result, the reaction slows down and produces less heat. This helps the reactor stay stable. The hotter the water gets, the less active the reactor becomes, which helps it control its temperature naturally.

Fuel

PWR fuel bundle This fuel bundle is from a pressurized water reactor of the nuclear passenger and cargo ship NS Savannah. Designed and built by Babcock & Wilcox.

Main article: Nuclear fuel

After processing, uranium is made into small ceramic pellets and then placed inside metal tubes. These tubes are grouped together to form fuel bundles that go into the reactor. A big reactor can have many of these bundles, holding a lot of uranium. These reactors can produce a lot of power, from about 900 to 1,600 megawatts. The fuel needs to be replaced every 18 to 24 months, with about a third of it changed each time.

Control

In pressurized water reactors, the power can change based on how much steam is needed. When there is less demand for steam, the reactor slows down naturally. Special materials like boron and cadmium are used to help control the temperature. Operators can also adjust the flow of water and the chemicals in it to keep the reactor running smoothly. Control rods can be moved to start or stop the reaction and to handle changes in power quickly.

Advantages

Pressurized water reactors (PWRs) are very stable because they produce less power when temperatures rise. This makes them easier to operate. The water used to create steam in a PWR is kept separate from the water that carries the nuclear reaction, so it does not become radioactive.

PWRs are also used in nuclear ships and submarines because their design is compact and fits well in these vehicles. They are the most common type of nuclear reactor around the world, which means many companies can supply new plants and parts for existing ones. Because we have used PWRs for a long time, we know a lot about how to operate them safely.

Water is used as a coolant in PWRs because it is safe, clear, and easy to find. It is also easier to check and maintain than other materials used in reactors. PWRs can use special types of fuel, which helps manage the materials used in the nuclear process.

Disadvantages

The water used to keep a pressurized water reactor cool must be kept under very high pressure so it stays liquid at high temperatures. This means the reactor needs strong pipes and a heavy container, which makes it more expensive to build. The high pressure can also make accidents more serious if the cooling water leaks out.

The reactor needs extra parts like pumps and a special container to keep the water pressurized, which adds to the cost and makes the reactor more complicated. The water, which has a special chemical added to it, can wear down some of the metal parts of the reactor over time. This can lead to problems and costs for fixing or replacing these parts.

Pressurized water reactors also need special treated fuel, which costs more to make. Because of how these reactors work, they cannot be built to make more fuel than they use. They are not as efficient as some other types of reactors and cannot provide heat hot enough for many industrial uses. In rare cases, certain accidents could cause explosions from hydrogen gas, although this is not common.