Pressurized heavy-water reactor

Some machines make power.

These machines use special water. This water is called heavy water. It is different from the water we drink.

Inside the machine, tiny bits move fast. The heavy water slows them down. This helps the fuel work better.

Because of this, the machine can use natural fuel. This fuel is easy to find in the ground. It does not need to be changed much.

Heavy water costs a lot of money. But the fuel is much cheaper to use. It is a smart way to make energy.

Some machines make power using a special way. We call this a pressurized heavy-water reactor.

These reactors use heavy water to help them work. Heavy water is different from the water we drink. It is used as a coolant to keep things from boiling. It is also used as a moderator. A moderator is a material that slows down tiny particles called neutrons.

In a reactor, neutrons must move at just the right speed. They need to hit the fuel to keep a chain reaction going. Most fuel comes from natural uranium. This fuel has a lot of 238U. That part of the fuel does not help much. It needs a moderator to slow down neutrons so they can hit the 235U. This is the part of the fuel that makes power.

Heavy water is great because it does not soak up too many neutrons. This lets the reactor use natural uranium. This means we do not need expensive enrichment plants. Enrichment is a way to make fuel stronger. Heavy water is very expensive to make. However, using natural uranium saves money.

Some reactors, like the CANDU type, are very common.

A pressurized heavy-water reactor, or PHWR, is a special kind of machine used to make power.

These reactors are important because they use a unique type of liquid called heavy water. This liquid acts as both a coolant and a moderator. A moderator is a substance that helps slow down tiny particles called neutrons. In a nuclear reactor, these neutrons are needed to keep a chain reaction moving. By using heavy water, these reactors can work in a very efficient way. This helps provide electricity to many people around the world.

To make power, the reactor needs a steady chain reaction. This happens when neutrons from one atom hit another atom to release more neutrons. Natural uranium is often used as the fuel for this process. Natural uranium is mostly made of a type called 238U, which does not help much with the reaction. It contains a tiny amount of 235U, which is the part that can support a chain reaction. The heavy water slows down the fast neutrons so they can hit the 235U atoms.

The reactor keeps the heavy water under high pressure so it does not boil into steam.

Scientists have studied how to use different materials to control these neutrons for a long time. In 1937, Hans von Halban and Otto Frisch discovered that heavy water does not absorb many neutrons. This was a very important finding for building reactors. During the Manhattan Project, people used graphite to slow down neutrons instead. However, the German wartime nuclear project tried to use heavy water because they knew it worked well. These early discoveries helped engineers design the PHWRs we see today. They learned how to balance the fuel and the moderator to keep the reaction stable.

There are many different types of these reactors in use today. The most common ones are called CANDU and IPHWR reactors. As of 2025, there are 43 of these reactors working around the world. They have a total capacity of 23.430 GW(e) of electricity. This means they provide about 6.5% of all the power from operating reactors globally. Some other designs exist, like the PHWR KWU design used in Argentina. Each design has its own way of managing the fuel and the heavy water.

Using a PHWR is a bit like a trade-off in a game. Heavy water is very expensive to make, sometimes costing hundreds of dollars per kilogram. However, using it allows the reactor to use natural uranium, which is much cheaper than enriched uranium. Enriched uranium is fuel that has been specially processed to be stronger. Because natural uranium has less energy, the reactor must replace its fuel more often. This is done using a special system that replaces fuel while the reactor is still running. This clever way of working helps make the whole system more useful.

A pressurized heavy-water reactor, known as a PHWR, is a type of nuclear reactor used to generate electricity.

These reactors are unique because they use deuterium oxide, commonly called heavy water, as both a coolant and a neutron moderator. A moderator is a substance used to slow down neutrons to the correct speed for a reaction. In a PHWR, the heavy water is kept under high pressure to prevent it from boiling. This allows the liquid to reach high temperatures without turning into steam bubbles. This design is similar to a pressurized water reactor, but the choice of liquid changes how the fuel works.

To understand how a PHWR works, one must understand the nuclear chain reaction. This reaction requires a steady supply of neutrons to stimulate the fission of atoms. Natural uranium is a common fuel for these reactors, but it is difficult to use. Natural uranium consists mostly of the isotope 238U, which makes up the bulk of the material. It contains only about 0.72% of the isotope 235U by weight. While 238U can be fissioned by very energetic neutrons, it tends to absorb more neutrons than it releases. This makes it impossible to achieve a self-sustaining reaction, or criticality, with 238U alone. The 235U isotope can support a chain reaction, but it is too rare in natural uranium to work without help.

The heavy water solves this problem by acting as a moderator. When neutrons are released during fission, they move very quickly. To cause more fission in the small amount of 235U available, these neutrons must be slowed down. The moderator absorbs the kinetic energy of the neutrons without absorbing the neutrons themselves. This process brings the neutrons to thermal equilibrium with the surrounding material. In a PHWR, the fuel is often arranged in separate solid segments surrounded by the moderator. This prevents the 238U from absorbing neutrons through a process called resonance absorption.

By keeping the moderator at a lower temperature, the reactor increases the likelihood of fission.

There are several distinct types of PHWR designs used around the world. The most common families are the CANDU reactors and the IPHWR reactors. Other specific designs include the German PHWR KWU, which is used at the Atucha Nuclear Power Plant in Argentina. These different models use various methods to manage fuel and moderation. For example, some CANDU reactors can use specialized fuels like reprocessed uranium or spent fuel from light water reactors. There is even ongoing research into using mixed oxide fuel, or MOX fuel, in these systems. This flexibility makes the PHWR a versatile tool for different energy needs.

The history of these reactors is tied to early discoveries in nuclear physics. In 1937, Hans von Halban and Otto Frisch discovered that heavy water has a low absorption of neutrons. This discovery was vital for creating efficient reactors. During the Manhattan Project, scientists used graphite as a moderator instead. However, the German wartime nuclear project correctly identified heavy water as an excellent moderator. They had difficulty because they dismissed graphite due to impurities. Later, the Soviet program developed the RBMK reactor, which used graphite to produce both electricity and plutonium. These historical developments shaped how modern PHWR technology was eventually perfected.

Using a PHWR involves a complex economic trade-off. Heavy water is very expensive to isolate from ordinary water, often costing hundreds of dollars per kilogram. However, this high cost is offset by the ability to use natural uranium. Natural uranium does not require expensive enrichment facilities to increase its 235U concentration. In contrast, light water reactors must use enriched uranium, which is typically between 3% and 5% 235U. As of 2025, there are 43 PHWRs in operation globally. They have a total capacity of 23.430 GW(e). This represents about 11% of all operating reactors by number and 6.5% by generating capacity.

Despite their benefits, PHWRs present certain challenges and risks. Because natural uranium has less energy than enriched uranium, the fuel must be replaced more frequently. This is managed through an on-power refueling system. This frequent movement results in higher volumes of spent fuel, though it generates less heat per bundle. Another concern is the production of tritium, a radioactive isotope of hydrogen. Tritium is created when deuterium nuclei in the heavy water absorb neutrons. There is also a significant nuclear proliferation risk associated with these reactors. If the fuel is changed frequently, significant amounts of plutonium-239 can be extracted through reprocessing. This plutonium is a fissile material that can be used in nuclear weapons.