See explanation below.
Work Step by Step
The water slows down the neutrons and results in a fission reaction. Heavy water is a better moderator as it does not absorb neutrons as much as the light water. Hence more neutrons are available to initiate the fission process. But D2O does not occur naturally in high amounts, so production of D2O adds to the cost of a heavy water reactor. If heavy water absorbs a neutron, it becomes the isotope tritium, which is radioactive in nature.