Heavy water-moderated and cooled reactors have been extensively developed in Canada, and form the basis of the nuclear power programme in that country. They are called CANDU-PHW (Canadian Deuterium Uranium Pressurized Heavy Water). The CANDU reactors have several features that distinguish them from other types. The moderator is contained in a cylindrical steel vessel, called the calandria, with a large number of zircaloy tubes through it parallel to its axis, which is horizontal. The active core region is approximately 6 m high with a diameter of 7 to 8m. The D_2 O coolant enters the regular array of pressure tubes at 2600C and 110 bar, flows through the fuel elements, and leaves the pressure tubes at 3200C, and the net efficeiency is about 29%. Like PWR, there is no bulk boiling of coolant.
The heavy water coolant pressure in the reactor is 88.3 bar, and the inlet and outlet temperatures are 2500 C and 2900 C, respectively. In heat exchangers, steam is generated at 41 bar pressure, 2510C.
The thermal power of each reactor (there are 8 at Pickering at Canada) is 1744 MW, and the net electrical output is 515 MW, giving a thermal efficiency of 29.5%
The calandria contains up to 380 horizontal pressure tubes, called calandria tubes, which are welded to the tube sheets each end of the vessel. The moderator temperature is maintained at about 700 C and low pressure to reduce heavy water losses. The fuel assembly contains 37 fuel rods. As shown in figure. Each rods contains natural uranium dioxide (UO2) fuel pellets with 0.38 ziracloy cladding. Each rod bundle is about 0.1 m in a diameter and 0.5 m long.
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