Canada was the first country to develop commercial power reactors based on heavy — water coolant. The heavy water, i. e. water in which the two hydrogen atoms are replaced by deuterium atoms, is an attractive coolant and moderator because it has a much lower tendency to absorb neutrons. This allows the reactor to be fueled by natural uranium rather than requiring the complex and expensive process of enriching the uranium. Because of the excellent ‘neutron economy’ provided by the low neutron absorption of the heavy water, this coolant became a favorite option for several ‘production’ reactors that were built by the United States and the former Soviet Union for the purpose of producing weapon-usable fissile material.

India imported the Canadian ‘ CANDU’ technology and developed an indigenous design of a 220 MWe plant. Although India subsequently up-scaled their plant design to capacities of 540 and 700 MWe, they continue to build 220 MWe plants. Because of the limited uranium resources but abundant thorium resources in India, they have developed a three-stage strategy for achieving a self-sustaining power production capability. The first stage utilizes the pressurized heavy-water reactors (PHWRs) (220, 540 and 700 versions) to generate sufficient plutonium

to support a fleet of fast spectrum reactors, which constitutes the second stage. The sodium-cooled fast reactors will include blankets of fertile thorium to produce 233U, which provides the feedstock for the third stage: advanced heavy-water reactors AHWRs. At equilibrium, the AHWRs will be self-sustaining and produce sufficient 233U from the U-Th or Pu-Th mixed oxide fuel to support subsequent fuel loads.

Table 2.2 provides a summary of the two available or planned SMR designs based on heavy water technology.