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. Heavy water expensive. Special precautions must be taken to make primary loop leak proof and to prevent loss of heavy water during refueling operations. Primary loop must be highly pressurized to achieve even high temperatures without boiling.