The CANDU (CANada Deuterium Uranium) nuclear reactor got its name because this heavy water reactor design was developed in Canada. Deuterium is the primary element in heavy water and uranium is the fuel used in this reactor class.
All the 22 reactors in Canada, some of which are offline for refurbishment, are of the CANDU design. Other nations with CANDU and “CANDU derivatives,” or generic heavy water moderated reactors, include: India, South Korea, Romania, Pakistan, Argentina, and China. The 47 CANDU and derivatives worldwide comprise 11 percent of the 423 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.
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 make steam to drive generators. Nuclear reactors in use in the U.S. are all light water designs.
The major differences between light-water reactors and CANDU/heavy water moderated design are:
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 one is called the calandria tube and the inner one is called 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 that are designed to use enriched uranium fuel and light water (H20) as a moderator, CANDU reactors use non-enriched (natural) uranium oxide as fuel and heavy water as a moderator.
Moderator – 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 in the CANDU reactor, the moderator is heavy water or deuterium oxide, which has a chemical formula of D2O.
Unlike ordinary water, with its familiar chemical composition of H20, heavy water includes two atoms of deuterium. Deuterium (D20) has a neutron at its center, unlike ordinary hydrogen, which in its most common form has no neutron and a proton.
Coolant – Coolant circulates through a nuclear reactor core to transfer the heat from it and prevent a melt-down that would halt energy production. In light-water reactors the water moderator functions also as primary coolant. The CANDU reactor uses either light or heavy water coolant.
How a CANDU Reactor Works to Make Electricity
A CANDU reactor core is cooled by heavy water or deuterium oxide, D2O, which also serves as the moderator.
In a closed loop, the heavy water coolant is pumped through the reactor core’s tubes containing the fuel bundles, picking 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, which drive generators to produce electricity which is distributed to the grid. In this way, 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.