Q. Heavy water is used as a coolant in nuclear reactors. Heavy water is:
Answer: Water containing heavy isotope of hydrogen
Notes: Heavy water, formally called deuterium oxide (D2O), is a form of water that contains a larger than normal amount of the hydrogen isotope deuterium, (also known as heavy hydrogen). In nuclear reactors, heavy water acts as a neutron moderator to slow down neutrons so that they are more likely to react with the fissile uranium-235 than with uranium-238.