CANDU Reactors

The CANDU (CANada Deuterium Uranium) reactor is a Pressurized Heavy Water Reactor (PHWR) designed and built by Canadian Nuclear Laboratories since the 1950s. All nuclear power plants in Canada are powered by CANDU reactors.

Heavy Water as a Moderator

The CANDU reactor uses heavy water as a moderator.  Heavy water has a heavier isotope of hydrogen, t1, or deuterium, instead of regular hydrogen, t2. The difference is that deuterium has a neutron and a proton in its nucleus, whereas hydrogen only has a proton.  Many of the physical properties of heavy water are somewhat different than those of light water, but the most important difference is that heavy water does not readily absorb neutrons.  This makes heavy water one of the most effective neutron moderators available.

Properties of Heavy Water and Light Water
Heavy Water (D₂O)
Light Water (H₂O)
Freezing point (⁰C)
Boiling point (⁰C)
Density (at 20⁰C, g/ml)
Temp. of max. density (⁰C)
Viscosity (at 20⁰C, g/ml)
Surface tension (at 25⁰C, mPas)
Heat of fusion (cal/mol)
Heat of vaporization (cal/mol)
pH (at 25⁰C)
7.41 (sometimes “pD”)

Only about 0.015% of all naturally-occurring hydrogen is in the form of deuterium. Only about one water molecule in 3,200 is semi-heavy water (DHO). Heavy water (D2O) needs to be extracted from regular water for use in CANDU plants.

As a PHWR, the CANDU reactor uses heavy water as the moderator. The advantage of using heavy water as a moderator is that it absorbs fewer neutrons than light water. This allows the use of natural uranium (0.711% 235U) instead of enriched uranium (2–5% 235U) for fuel. Natural uranium fuel is less expensive than enriched fuel, although somewhat more fuel must be fed through the reactor to produce the same amount of energy.

CANDUs make more efficient use of mined uranium than enriched fuel reactors. The disadvantages are that heavy water is expensive to make, representing about 20% of the capital cost of each reactor, and the reactor core size is larger.

How the CANDU Reactor Functions

reactor assembly
CANDU reactor assembly.

The CANDU reactor functions in a manner similar to a pressurized water reactor (PWR). Pressurized coolant is passed through the fuel bundles to cool them. This hot, pressurized cooling water is carried to a steam generator where the heat energy is transferred to light water and converts it into steam. This steam is then used to turn the steam turbines which turn the generator, creating electricity.

One of the unique features of a CANDU reactor is that it allows on-line fuelling. The fuel bundles are placed in horizontal tubes (called pressure tubes). These tubes can be loaded remotely from either end while the reactor is running (on-line). This avoids scheduled shutdowns to replace the fuel. The CANDU design requires significantly more “plumbing” than a PWR reactor, as each pressure tube has high pressure heavy water passing through it.


The typical lifespan of a fuel bundle in the reactor is one to two years. As a fuel bundle is loaded in one end of the pressure tube, a spent fuel bundle is pushed out of the other end. The picture to the right taken during a shutdown shows the machine that loads the fuel bundles.

Next Generation CANDU Designs

Canada’s leadership in peaceful nuclear research and development is contributing to the design of two innovative next-generation nuclear reactors.

Enhanced CANDU 6

The Enhanced CANDU 6 (EC6) is a 700-megawatt (MWe) Generation III reactor based on the highly successful CANDU 6 reactor, with a number of safety enhancements included to meet the latest Canadian and international standards.

The EC6 uses natural uranium as a fuel, a heavy-water moderator and heavy-water coolant in a pressure-tube design. It can be refueled on power and has one of the highest lifetime capacity factors among the world’s reactors.

These features make the EC6 ideal for small and medium electric grids.

Advanced Fuel CANDU Reactor

The Advanced Fuel CANDU reactor (AFCR) is an evolutionary 700 MWe Generation III reactor based on the highly successful CANDU 6 and Enhanced CANDU 6 (EC6) reactors with a number of adaptations to meet the latest Canadian and international standards.

The AFCR’s fuel flexibility allows it to use recycled uranium or thorium as fuel. It has a heavy-water moderator and, a heavy-water coolant, and a pressure tube design.

Like all CANDU reactors, it can be refuelled on-power and has one of the highest lifetime capacity factors among the world’s reactors.


Chris Waltham, An Early History of Heavy Water, Department of Physics and Astronomy, University of British Columbia, June 2002.
Hans Tammemagi and David Jackson, Unlocking the Atom: The Canadian Book on Nuclear Technology, McMaster University Press, 2002, p. 73-88.