|Fig. 1: A CANDU reactor design - 1- Fuel Bundle, 2 Calandria, 3 Adjuster Rods, 4 Heavy Water pressure reservoir, 5 Steam generator, 6 Light Water Pump, 7 Heavy Water Pump, 8 Fueling Machines, 9 Heavy Water Moderator, 10 Pressure Tube, 11 Steam going to steam turbine, 12 Cold Water returning from turbine, 13 Containment Building Source: Wikimedia Commons|
CANDU stands for CANada Deuterium Uranium is a type of nuclear reactor with technology to use non enriched fuel developed by Canada. Currently, seven countries in this world have CANDU type reactors: Canada, India, China, Pakistan, South Korea, Argentina and Romania.
The neutron which is emitted from a nuclear fission reaction is an ~MeV neutron. However, we know that the absorption cross section for Uranium of a thermal(0.025eV) neutron is orders of magnitude higher than MeV neutrons. Hence, one needs to thermalize these neutrons. At the same time, we do not want the thermalizeing agent to absorb the neutrons. It is also desirable that a neutron loses a lot of energy in each collision. These factors, are accounted in the number called the Moderating Ratio which is proportional to scattering cross section for neutrons, energy lost in each collision, and inversly proportional to absorption cross section. This number works out to be 58 for light water and 21000 for heavy water. It is this large value of the moderating ratio that allows CANDU reactors to use Natural Uranium without enrighment. This also reduces the fuel cost. Also, it allows one to use other fuels such as Uranium-Plutonium mixed fuels. [1,2]
The coolent is flown in pressure tubes horizontally as it provides higher symmetry. However, there is a large mass of the pressure tube inside the reactor and it can potentially absorb too many neutrons to sabotage reactor criticality. The solution to this problem came via Zirconium which happens to have a very low neutron absorption cross section. It is worth mentioning that this result came as a product of materials research in Chalk River for the US nuclear program. [1,3]
Although, in principle, the coolent can be anything, all operating CANDU reactors have heavy water as coolent as they wanted to maximise neutron economy for neutrons. Gentially-1 and WR-1 reactors tried light water and organic coolents but they both failed due to different reasons.However, advanced CANDU reactors use pressurised light water as coolent. 
The CANDU design is illustrated in the figure. The core of the reactor consists of several fuel channels embedded in a large heavy water moderator tank. This setup is called a calandria. One fuel bundle lasts for a year or two. One of the advantages of CANDU reactors is that fuel can be continuously changed while the reactor is in operation. This is very good as one one can continuously get power and hence these reactors have very high capacity factors. 
To control the power and to keep the reactor critical, control rods are dynamically controlled. In CANSU reactors, there is a redundancy in the control rod system and they have two independent control mechanisms to shutdown the system. This further provides safety. 
Yet another advantage of CANDU-type reactor is the possibility of burning Thorium. It is well known that Thorium reserves outnumber Uranium reserves by over a factor of 3. CANDU reactor gives two options for burning Thorium. In the first option, only one fuel type is used in the reactor core which is a mixture of slightly enriched Uranium and natural thorium. The other design, there are 3 layers. The innermost layer has 6.0% wt. of gadolinium to shape the flux distribution. The middle layer has same fuel as the first option. The peripheral outermost layer is designed to contain thorium bundles to achieve high burn ups. 
© Pranjal Bordia. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 P. Rinard, "Neutron Interaction with Matter," in Passive Nondestructive Assay of Nuclear Materials, ed. by D. Reilly et al., U.S. Nuclear Regulatory Commission, NUREG/CR-5550, March 1991, p. 357.
 W. C. Patterson, Nuclear Power, 2nd Ed. (Penguin, 1983).
 J. A. L. Robertson, The Basics of Nuclear Design," Atomic Energy of Canada Limited, August 1992.
 "Thorium Fuel Utilization: Options and Trends," International Atomic Energy Agency, IAEA-TECDOC-1319, November 2002.