Water flashes to steam in case of rupture of primary system. Condenser leak may cause serious trouble. Although the reactors mentioned above are classified generally by the type of coolants used, the title heavy water refers to the use of heavy water as a moderator. The reactors discussed here are cooled by heavy water. Any fuel including natural uranium can be used.
Heavy water is excellent moderator.
Heavy water has high heat capacity. Specific power high. Negative temperature coefficients. Fission product are contained, not circulated. Radioactivity of coolant is short lived if kept pure.