Light-water reactor

A light-water reactor (LWR) is a common type of nuclear reactor that uses ordinary water in two important ways. First, it uses water to keep the reactor cool, which is called the coolant. Second, it uses water to help control the speed of the neutron reactions inside the reactor, which is called the neutron moderator. This is different from some other reactors that use a special kind of water called heavy water.

Light-water reactors are the most common kind of thermal-neutron reactor, which means they are reactors that use slow-moving neutrons to produce energy. There are three main types of light-water reactors. The most common are pressurized water reactors and boiling water reactors. There is also a newer type called the supercritical water reactor, though it is still mostly in the design stage.

These reactors are important because they provide a steady source of energy for many countries around the world. They help create electricity without producing air pollution during operation. Understanding how they work is key to learning about safe and efficient energy production.

History

Early concepts and experiments

After scientists discovered how atoms can split apart and how reactions can continue on their own, they tested if regular uranium could keep reacting using materials like graphite or heavy water. Early reactors, such as CP-1 and X10, succeeded, but to build an atomic bomb for the Manhattan Project, scientists needed to enrich uranium. In May 1944, a small amount of this enriched uranium was used in a special reactor at Los Alamos to help measure how much was needed for a bomb. This reactor used uranium dissolved in water, not solid uranium, so it is not considered a light-water reactor, but it was an early step.

After World War II, scientists began testing if regular water could be used in reactors. They placed natural uranium in water inside the X10 reactor and found that with a little more enriched uranium, they could keep the reaction going. This was the first real move toward light-water reactors. With more enriched uranium available, new ideas for reactors appeared. In 1946, scientists built a test reactor called the Material Testing Reactor (MTR) in Idaho, which used enriched uranium and light water, reaching a successful reaction in 1952. Before building it, they tested a smaller version called the Low Intensity Test Reactor (LITR) in 1950, which was the world’s first light-water reactor.

Pressurized water reactors

Right after World War II, the United States Navy began working on using nuclear power for ships. Led by Captain (later Admiral) Hyman Rickover, they created the first pressurized water reactors in the early 1950s. This led to the launch of the first nuclear submarine, the USS Nautilus (SSN-571).

The Soviet Union also made its own version of these reactors in the late 1950s, called VVER. While similar to American designs, it had some unique features.

Boiling water reactor

A researcher named Samuel Untermyer II led efforts to develop another type of reactor called the boiling water reactor at the US National Reactor Testing Station, now known as the Idaho National Laboratory, through a series of tests named the BORAX experiments.

PIUS reactor

PIUS, short for Process Inherent Ultimate Safety, was a Swedish idea for a light-water reactor created by ASEA-ATOM. It aimed to use natural safety features that did not need someone to operate it or extra power to keep it safe. However, no PIUS reactors were ever built.

OPEN100

In 2020, the Energy Impact Center shared an open design for a pressurized water reactor that could produce 300 MWth/100 MWe of energy, called OPEN100.

Overview

Light-water reactors (LWR) are a common type of nuclear reactor that uses regular water to keep the reactor cool and to help control the reaction. Because they are simpler and cheaper to build, they are used in most nuclear power plants and on nuclear-powered ships around the world as of 2009.

There are three main types of light-water reactors: pressurized water reactors (PWRs), boiling water reactors (BWRs), and supercritical water reactors (SCWRs). The SCWR is still just an idea as of 2009; it is a new design that would have some features of a faster type of reactor.

Many countries lead in building these reactors. The United States, Russian Federation, Republic of France, and Japan are major builders of PWRs. For BWRs, the United States and Japan are leaders, working together to build advanced models. Light-water reactors are favored for new nuclear power plants because they are reliable and safe. They are also used in most naval ships, providing a safety feature that helps stop the reaction if the reactor is damaged.

As of 2022, most of the world's power reactors—379 out of 441—were light-water reactors. Of these, 306 were PWRs, 61 were BWRs, and 12 were another type. Together, they could produce about 361,888 megawatts of electricity.

Reactor design

The light-water reactor creates heat through a process called nuclear fission. The core of the reactor, where the reactions happen, contains nuclear fuel and control elements. The fuel rods, about 12 feet long, are filled with uranium oxide pellets. Control rods, made from materials like hafnium or cadmium, can absorb neutrons. When these rods are lowered, they slow down the reaction; when lifted, the reaction speeds up. All of this is inside a strong steel container filled with water.

In a boiling water reactor, the heat turns water into steam that powers turbines right away. In a pressurized water reactor, the heat is moved to another loop of water to make steam, which then powers the turbines. After passing through the turbines, the steam cools back into water to start the process again.

Control

Main article: Control rod

Control rods are grouped together and placed into the reactor core to manage how many neutrons cause uranium atoms to split. This controls the amount of heat and electricity made. In some designs, special chemicals in the water also help control the reaction.

Coolant

Main article: Nuclear reactor coolant

Light-water reactors use ordinary water to keep cool. The water takes away heat from the reactor and helps make steam. In some reactors, the same water makes steam for the turbines; in others, a separate water loop is used.

Fuel

Main article: Nuclear fuel

Light-water reactors need uranium fuel that is enriched to about 3 percent uranium-235. The fuel is made into small ceramic pellets, stacked into metal tubes called fuel rods. These rods are grouped into bundles to form the reactor core. The tubes are usually made of a special zirconium alloy.

Moderator

Main article: Neutron moderator

Light-water reactors use ordinary water to slow down neutrons, which helps sustain the reaction. Because water absorbs some neutrons, the uranium must be enriched. This is different from reactors that use heavy water. The use of water as a moderator helps keep the reactor safe; if temperatures rise, the water expands and slows the reaction down. If cooling is lost, the reaction stops, though some heat remains for a few years.