Heavy water of an atomic reactor is :

Heavy water, chemically known as deuterium oxide (D2O), is a form of water where the standard hydrogen atoms (protium) are replaced by deuterium, which is a heavier isotope of hydrogen. While normal water (H2O) consists of two protium atoms and one oxygen atom, heavy water contains two deuterium atoms, each having a neutron in addition to the proton found in protium [1]. This isotopic substitution increases the molecular mass of the water. In atomic reactors, heavy water is primarily used as a neutron moderator and coolant because its nuclear properties allow it to slow down neutrons effectively without absorbing them excessively, facilitating a sustained nuclear chain reaction. It is distinct from deionised water, which is simply water stripped of mineral ions, and is not a mixture of ice or an oxide of a heavier oxygen isotope.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > 8.3.2 Compounds > p. 124