Heavy Water

Google WWII, NAZI Germany, Hitler, the H bomb etc ...

Are we not men? We are D2O.

I know "heavy water" in relation to nuclear weapons. I believe it came into use around 1940, when the Germans had a heavy water plant in Scandinavia, Norway IIRC, and the allies, the English specifically I believe it was, successfully attacked it about 1944 preventing the Nazi Germans from developing the atom bomb.

Until your use, I do not think I've heard the term "light water".

"Light water" in the context of Nuclear power is ordinary water.

http://en.wikipedia.org/wiki/Light_water_reactor

Heavy water (D2O)

Water containing significantly more than the natural proportions (one in 6,500) of heavy hydrogen (deuterium, D) atoms to ordinary hydrogen atoms. Heavy water is used as a moderator in some reactors because it slows down neutrons effectively and also has a low probability of absorption of neutrons.

http://www.nrc.gov/reading-rm/basic-ref/glossary/heavy-water-d2o.html