Question

heavy water in nuclear reactor

Answer 1

Heavy water (D2O) is used to regulate the temperature of the reactor and to condense the gaseous sodium. 

Answer 2

heavy water is used in certain types of nuclear reactors, where it acts as a neutron moderator to slow down neutrons so they are more likely to react with the fissile uranium - 235 than with uranium-238, which captures neutrons without fissile . the CANDU reactor uses this design.