Heavy water

Heavy water is water in which most or all hydrogen atoms are deuterium, the stable hydrogen isotope with one neutron. Its greater mass changes some physical and nuclear properties relative to ordinary water.

Deuterium: A stable hydrogen isotope whose nucleus contains one proton and one neutron. Heavy water contains deuterium in place of ordinary hydrogen.
Neutron moderation: The reduction of neutron energy through collisions with nuclei in a material. Heavy water slows reactor neutrons while absorbing relatively few of them.
Heavy water reactor: A nuclear reactor that uses heavy water as a neutron moderator, often also as coolant. This reactor type exploits heavy water’s ability to moderate neutrons with little absorption.
Light water: Ordinary water, H₂O, whose hydrogen atoms are predominantly protium. Its lighter hydrogen isotope gives it different neutron-absorption and physical properties.
Hydrogen isotope: A form of hydrogen distinguished by the number of neutrons in its nucleus. Heavy water is defined by substituting a particular hydrogen isotope.
Neutron capture: A nuclear reaction in which an atomic nucleus absorbs a neutron. Heavy water’s low neutron-capture rate helps preserve the neutron supply in reactors.
CANDU reactor: A Canadian pressurized heavy-water reactor design that can use natural uranium fuel. CANDU reactors are a prominent application of heavy-water moderation.
Tritiated water: Water containing tritium, the radioactive hydrogen isotope with two neutrons. It differs from heavy water because tritium is radioactive and heavier than deuterium.
Isotopic substitution: Replacing atoms in a substance with isotopes of the same element. Replacing hydrogen with deuterium produces heavy water and alters molecular properties.
Kinetic isotope effect: A change in reaction rate caused by replacing an atom with a heavier isotope. Deuterium’s extra mass makes reactions involving heavy water proceed differently.

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