Tritium is also produced in heavy water-moderated reactors whenever a deuterium nucleus captures a neutron. This reaction has a quite small absorption cross section, making heavy water a good neutron moderator, and relatively little tritium is produced. Even so, cleaning tritium from the moderator may be desirable after several years to reduce the risk of its escaping to the environment. Ontario Power Generation's "Tritium Removal Facility" processes up to  of heavy water a year, and it separates out about  of tritium, making it available for other uses.
CANDU reactors typically produce  of tritium per year, which is recovered at the Darlington Tritium Recovery Facility (DTRF) attached to the 3,512 MW Darlington Nuclear Generating Station in Ontario. The total production at DTRF between 1989 and 2011 was  – with an activity of  – which averages to about  per year.
Deuterium's absorption cross section for thermal neutrons is about 0.52 millibarns, whereas that of oxygen-16 () is about 0.19 millibarns and that of oxygen-17 () is about 240 millibarns. While  is by far the most common isotope of oxygen in both natural oxygen and heavy water, depending on the method used for isotope separation, heavy water may be slightly to notably higher in  and  content. Due to both neutron capture and (n,α) reactions (the latter of which produce , an undesirable long-lived beta emitter, from ) they are net "neutron consumers" and are thus undesirable in a moderator of a natural uranium reactor which needs to keep neutron absorption outside the fuel as low as feasible. Some facilities that remove tritium also remove (or at least reduce the content of)  and , which can – at least in principle – be put to use for isotope labeling.
India, which also has a large fleet of pressurized heavy water reactors (initially CANDU technology but since indigenized and further developed IPHWR technology), also removes at least some of the tritium produced in the moderator/coolant of its reactors but due to the dual use nature of tritium and the Indian nuclear bomb program, less information about this is publicly available than for Canada.