Light-water reactor

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

Light-water reactors are the most common kind of thermal-neutron reactor. These reactors use slow-moving neutrons to make 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, but it is still being designed.

These reactors are important because they give many countries a steady source of energy. They help make electricity without creating air pollution while they work. Learning about them is important for safe and efficient energy production.

History

Early concepts and experiments

After scientists learned that atoms can split apart, they tested if regular uranium could keep reacting using materials like heavy water. Early reactors, such as CP-1 and X10, worked, 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 help control the reaction. They are simpler and cheaper to build, so they are used in most nuclear power plants and on nuclear-powered ships around the world.

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; it is a new design that might have some features of a faster type of reactor.

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

As of 2022, most of the world's power reactors were light-water reactors. Together, they could produce a lot of electricity.

Reactor design

The light-water reactor makes heat through a process called nuclear fission. The reactor core, where the reactions happen, holds nuclear fuel and control parts. The fuel rods, about 12 feet long, are filled with uranium oxide pieces. Control rods, made from materials like hafnium or cadmium, can soak up neutrons. When these rods go down, they slow the reaction; when lifted, the reaction speeds up. All of this sits inside a tough steel container full of water.

In a boiling water reactor, the heat changes water into steam that turns turbines right away. In a pressurized water reactor, the heat moves to another loop of water to make steam, which then turns the turbines. After the turbines, the steam cools back to water to start again.

Control

Main article: Control rod

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

Coolant

Main article: Nuclear reactor coolant

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

Fuel

Main article: Nuclear fuel

Light-water reactors need uranium fuel enriched to about 3 percent uranium-235. The fuel is made into small ceramic pieces, 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 regular water to slow down neutrons, helping keep the reaction going. Because water soaks up some neutrons, the uranium must be enriched. This is different from reactors that use heavy water. Using water as a moderator helps keep the reactor safe; if temperatures go up, the water grows and slows the reaction. If cooling is lost, the reaction stops, though some heat stays for a few years.