Pressurized water reactor

A pressurized water reactor (PWR) is a common type of light-water nuclear reactor that makes electricity. It is the most popular kind of nuclear power reactor in the world.

In a PWR, water has two jobs: it helps control what happens inside the reactor and takes away heat. The water gets very hot because of tiny parts of atoms called fission. Special high pressure, about 155 bar, keeps the water as a liquid. This hot water then moves to a place called a steam generator. There, it gives its heat to another set of water. That 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 high pressure so it does not turn to steam until it leaves the main reactor area. Many countries use PWRs to make power. Newer designs, called Generation III and III+ reactors, such as the AP1000 and EPR, are used in places like China. PWRs were first made 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 began 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 more 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 a pressurized water reactor. In the United States, the Army also used these reactors from 1954 to 1974. A problem happened at Three Mile Island Nuclear Generating Station in 1979, which slowed down new nuclear plant building in the U.S. for many years. In 2016, a new reactor named Watts Bar unit 2 started working, the first new U.S. reactor since 1996. In 2020, NuScale Power got 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. Here, the heat moves to another loop of water, turning it into pressurized steam. 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.

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. This high pressure means only small areas boil, and the steam quickly turns back into water.

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 very hot but stays liquid because it is under a lot of pressure. This pressure stops 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 keeps the pressure 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. The steam 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 use water to slow down fast neutrons. This helps the neutrons interact with the fuel and keep the reaction going. When neutrons bump into hydrogen atoms in the water, they lose speed.

One safety feature is that if the temperature gets too high, the water expands. This creates more space between water molecules. With more space, there are 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

Uranium is turned into small ceramic pieces called pellets. These pellets are put inside metal tubes. The tubes are grouped to make fuel bundles for the reactor. Large reactors have many of these bundles, holding lots of uranium. These reactors can make a lot of power, from about 900 to 1,600 megawatts. The fuel is replaced every 18 to 24 months, with about a third changed each time.

Control

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

Advantages

Pressurized water reactors (PWRs) are stable because they make less power when it gets too hot. This makes them easier to run. In a PWR, the water that makes steam is kept apart from the water used in the nuclear reaction, so it does not become radioactive.

PWRs are also used in nuclear ships and submarines because their design is small and fits well. They are the most common type of nuclear reactor in the world, so many companies can supply new plants and parts. Because we have used PWRs for a long time, we know how to operate them safely.

Water is used to keep things cool in PWRs because it is safe, clear, and easy to find. It is also easier to check and care for than other materials. PWRs can use special fuel, which helps manage the materials 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.