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Heavy water - what is it and how it is obtained from nature

Heavy water is used in atomic reactors to cushion emission of radioactive neutrons. By preventing neutrons from escaping, heavy water allows fission reaction to retain enough neutrons to continue in sustained mode. If a critical mass of uranium is used, the heavy water would not be needed, but then again, the reaction would get out of hands and we'd see an explosion form almost instantly.

So what is this heavy water? From chemistry we know that hydrogen (H) has one proton and no neutrons. There are other isotopes of hydrogen, namely deuterium (often marked as D or H2,1) and tritium (T or H3,1). These two isotopes contain one proton and one neutron (in case of deuterium) or two (in tritium). Tritium is highly radioactive and it is estimated that at any given moment there is not more than about 3 grams of it high in our atmosphere where it forms while being bombarded by gamma rays. Deuterium on the other hand is not radioactive and for all intents and purposes it is nontoxic (read more for caveats).

Scroll down for additional info labeled How do we obtain heavy water and deuterium in particular

More info: How do we obtain heavy water and deuterium in particular

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Deuterium, when replacing hydrogen in water, forms D2O. This is called heavy water. Heavy water is nontoxic as long as at least 90% of body weight consists of H2O. Once the 10% threshold is exceeded, chemical reactions are slowed to the point that body can no longer function. Deuterium is naturally occuring with about 1 in 5000 chance as compared to hydrogen. This means that in ordinary water, the ratio between H20 and HDO (one hydrogen and one deuterium) is about 1:4500. Ratio between H2O and D2O is about 1:20,000,000. This heavy water can be extracted through electrolysis (different weight contributes to different speed of movement) but that is highly inefficient.

Atomic power plant requires large amounts of heavy water, so in order to obtain it, oridinary water is first split into hydrogen and oxigen. The hydrogen gas contains deuterium and is cooled to liquid point. Then using distilation process, the gas is slowly warmed. Ordinary hydrogen boils at -252.8 celsius whereas deuterium boils at -249.6, which is 2.8 degrees higher. When mixture is warmed to -250 degrees, only deuterium is left. This gas is then burned with oxigen to form D2O.

What about oxigen? Well, the oxygen is not important for the properties needed in reactor, so natural mixture of common O16,8 and rare O17,8 and O18,8 is used as extracted from original water.