The ability to use natural uranium fuel was one of the principal requirements in the design of the CANDU reactor. This necessitates a design in which high neutron economy is of paramount importance, and is achieved by:

• minimizing parasitic absorption through design and through the use of low neutron absorbing structural materials in the fuel and core

• using heavy water (D2O) as both coolant and moderator, which avoids parasitic absorption in water and provides excellent neutron thermalization (e. g., slowing down of neutrons to energies where they are more likely to be absorbed by fissile isotopes)

• refuelling the reactor on power, which results in very low excess reactivity in the core, obviating the need for burnable poisons and minimizing neutron absorption in control materials. Low core excess reactivity also results in safety benefits

• employing a pressure tube design, which helps reduce resonance absorption in U-238 while achieving a very thermal neutron spectrum. (Grouping the fuel elements into a cluster (fuel bundle) surrounded by a large amount of heavy water moderator increases the probability that a high-energy neutron produced through fission will escape from the fuel into the moderator before being captured by the intermediate and high-energy neutron resonances in U-238.) A pressure tube reactor also avoids the difficulty of manufacturing a large pressure vessel

A schematic of a CANDU power plant is shown in Fig. 11.1. The reactor consists of a horizontal, stainless steel cylindrical tank called a calandria, which is filled with heavy water moderator. Inside the calandria are several hundred fuel channels running the length of the tank, arranged in a square lattice with a pitch of 28.6 cm. The fuel channels consist of a zirconium-alloy pressure tube inside a thinner zirconium-alloy calandria tube, separated by an insulating gas gap. Twelve fuel bundles, cooled by the D2O coolant at high temperature (around 300°C) and pressure (around 10 MPa), sit inside each pressure tube (see Fig. 11.2). The fuel channel separates, and insulates, the unpressurized, cool moderator on the outside of the channel from the pressurized, hot coolant within. The CANDU reactor design is modular, in that the number of fuel channel assemblies can be varied to give the desired power output. The CANDU 6 reactor has 380 fuel channels and, therefore, 4560 fuel bundles.

The coolant is pumped through a large inlet header into small inlet feeders, which are connected to one end of the fuel channels. The coolant removes fission heat from the fuel bundles as it moves through the channels, and flows through outlet feeders to a large outlet header, which is connected to a steam generator where the heat from the D2O coolant is transferred to ordinary water on the other side of the steam generator tubes, which turns to steam. The coolant is then pumped into an inlet header at the other end of the reactor. Figure 11.3 illustrates

Heavy water coolant (in heat transport system)

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Cooling water from lake/ocean/river

Fuel ‘Heavy water Fuelling (uranium) moderator machine (in calandria)

77.7 Schematic of CANDU power plant (figure is copyright Atomic Energy of Canada Limited and is used with permission).

11.2 CANDU reactor fuel channel arrangement (figure is copyright Atomic Energy of Canada Limited and is used with permission).

11.3 Face view of reactor showing feeders (figure is copyright Atomic Energy of Canada Limited and is used with permission).

the face view of the reactor, showing the feeders. In a CANDU 6 reactor, the coolant flows in two separate figure-of-eight loops, each serving half of the fuel channels, with two heat transport pumps, two inlet and outlet headers and two steam generators at each end of the core. Coolant flow is bi-directional, meaning that coolant flow in adjacent channels is in opposite directions. The reactor core, primary heat transport pumps, steam generators and associated equipment are located in a containment building. The balance-of-plant, or secondary side, is outside the containment building, with the principal components being the steam lines, steam turbines, electrical generator, condenser and feed water lines to the steam generators. A summary of the parameters of the principal types of heavy water reactor is given in the Appendix in IAEA (2002).