CANDU Nuclear Reactor Is Moderated with Heavy Water

How Canada's CANDU Heavy Water Nuclear Reactors Work

CANDU nuclear reactor diagram
Diagram of the basic CANDU nuclear reactor design. Image (c) Canadian Nuclear Association

The CANDU nuclear reactor got its name because this heavy water reactor design was developed in Canada -- it stands for CANada Deuterium Uranium. Deuterium is the primary element in heavy water, and uranium is the fuel used in this reactor class.

CANDU Heavy Water Nuclear Reactors Worldwide

All of Canada's 20 nuclear reactors are of the CANDU design. Other nations with CANDU reactors include Argentina, China, India, South Korea, Pakistan, and Romania.

India also has 16 “CANDU derivatives,” or generic heavy water moderated reactors. The 48 CANDU reactors and CANDU derivatives comprise nearly 11 percent of reactors worldwide.

It is estimated that power plants using the CANDU design generate more than 23,000 megawatts, about 21 percent of the electricity produced by nuclear energy. A megawatt is generally enough to power 750 average-sized homes.

How CANDU Reactors Differ from Light Water Reactors

Heavy water nuclear reactors and light water nuclear reactors differ in how they create and manage the complex physics of nuclear fission or atom-splitting which produces the energy and heat to create steam to drive generators. The nuclear reactors in use in the U.S. are all light water designs. Several major differences distinguish between light water reactors and the CANDU heavy water moderated design, which include the following design features.


Core: The core of a CANDU reactor is kept in a horizontal, cylindrical tank called a calandria. Fuel channels run from one end of the calandria to the other. Each channel within the calandria has two concentric tubes. The outer tube is the calandria tube and the inner one is the pressure tube. The inner tube holds the fuel and pressurized heavy water coolant.

This design allows refueling during operation.

By contrast, the core of a light water reactor is vertical and contains vertical fuel assemblies which are bundles of metal tubes filled with fuel pellets. The reactor core is kept in a containment vessel.

Fuel: Unlike other nuclear reactors designed to use enriched ​uranium fuel and light water as a moderator, CANDU heavy water reactors use non-enriched, natural uranium oxide as fuel and heavy water as a moderator.

Moderator: The moderator is the material in the reactor core that slows down the neutrons released from fission so they cause more fission and sustain the chain reaction. The moderator in light water reactors is ordinary water, but the CANDU heavy water reactor uses heavy water or deuterium oxide, which has a chemical formula of D20.

Unlike ordinary water with its familiar chemical composition of H20, heavy water includes two atoms of deuterium. Unlike ordinary hydrogen which has no neutron and a proton in its most common form, deuterium has a neutron at its center. 

Coolant: Coolant circulates through a nuclear reactor core to transfer the heat away from it and prevent a meltdown that would halt energy production. The water moderator also functions as primary coolant in light water reactors.

The CANDU reactor uses either light or heavy water coolant.

How a CANDU Reactor Works to Make Electricity

The heavy water coolant is pumped through the reactor core’s tubes in a closed loop. The tubes contain fuel bundles to pick up heat generated from the nuclear fission taking place in the core. The heavy water coolant loop passes through steam generators where the heat from the heavy water boils ordinary water into high-pressure steam. The heavy water, now cooler, is circulated back to the reactor as the closed loop cooling cycle continues.

The high-pressure steam from the steam generator is piped outside the reactor containment building to power conventional turbines.

These turbines drive generators to produce electricity that is then distributed to the grid. The nuclear reactor is separate from the equipment used to produce electricity. The steam coming out of the turbine is condensed back into water and is pumped back into the steam generator.