Heavy Water (D₂O) or deuterium oxide is made up of two atoms of deuterium and one atom of oxygen. Deuterium is a stable isotope of hydrogen with double the mass of hydrogen due to presence of an extra neutron in its nucleus. Deuterium is present in hydrogen and hydrogen bearing compounds like water, hydrocarbons, etc. and has a small natural occurrence (D/D+H) of about 140 to 160 ppm. The same is extracted and enriched to > 99.85 mass % IP used in Pressurized Heavy Water Reactor (PHWRs).

Heavy Water displays similar physical and chemical properties but differs in nuclear properties when compared to ordinary water which makes it an extremely efficient material for use as moderator and coolant in Pressurized Heavy Water Reactor (PHWRs).

Moderator is required in a Nuclear reactor to slow down the neutrons produced during the fission reaction so that the chain reaction can be sustained. Heavy Water is an excellent moderator due to its high moderating ratio and low absorption cross section for neutrons.

Heat energy produced in the core of reactor due to fission reaction is required to be carried away by a coolant. Heavy Water is used as a primary coolant to transport heat generated by the fission reaction to secondary coolant, light water. The light water takes the heat energy from Heavy Water and generates steam at an appropriate pressure for running steam turbines. Steam turbines drive generators to generate Electricity.

No, Heavy Water is not radioactive. It is oxide of deuterium (D₂O) which is stable isotope of hydrogen.

Few Deuterium in Heavy Water gets converted to Tritium by absorbing a neutron in nuclear reactor. Tritium is a radioactive isotope of hydrogen having two neutrons in the nucleus. Further, some of the ionic impurities that are carried by heavy water as moderator or coolant also get activated in course of irradiation. Hence the radio activity is observed in Heavy Water used in Nuclear reactors.

A small quantity of heavy water is not harmful to human health. In fact, human body contains few grams of Heavy Water as natural abundance. Small quantities of Heavy Water have been used in clinical studies and treatment in infants, lactating women and adults without any reported adverse effect. Toxic effects of Heavy Water begin to appear only when deuterium content of body fluids exceeds 20%, which is the most unlikely event.

Water containing Deuterium less than its natural abundance ( <125 ppm) is called Deuterium Depleted Water. It is getting popularized as adjuvant therapy for cancer.

Heavy Water has greater affinity for Light Water and can get degraded in its IP if it comes in contact with Light Water in any form. It can readily absorb light water present in air as moisture.

There is no fixed life for Heavy Water.