Explanatory notes
… Part (I) of Explanatory Note to heading 28.44. This heading covers stable isotopes, that is, non‑radioactive isotopes … . This is separated from normal hydrogen in which it occurs at a level of 1 part in 6,500. (2) Heavy water , … of water. Used as a source of deuterium, and in nuclear reactors for slowing down the neutrons which split the …