Deuterium (or heavy water) is a non-radioactive isotope of hydrogen and a key component of CANDU reactors. The heavy water "moderates" or slows down neutrons to allow fission to occur. All water contains deuterium in low concentrations. Hydrogen also has a heavier and mildly radioactive isotope, tritium, which is produced in very small quantities in the heavy water used in CANDU reactors.

AECL is widely recognized around the world for its expertise in separating hydrogen isotopes. In particular, the company has developed several new processes based on water-hydrogen exchange, using a special "wetproofed" catalyst. To produce heavy water at large-scale hydrogen production plants, an industrial prototype plant has been built and is currently operating in the Province of Ontario. This technology has also been successfully demonstrated for use in maintaining the isotopic purity of heavy water and for removing tritium from heavy water. Enhancement of the wetproofed catalyst is an ongoing development activity at AECL.