In heavy water reactors, tritium is formed by the (n,γ) reaction on deuterium and by the (n,p) reaction on the tritium decay product 3He. The relative contribution of the latter reaction depends on the retention time of 3He in the heavy water system. If the retention is at least 10 days, then, with an effective neutron flux of 1014 cm−2·s−1, the activity of tritium produced by the 3He reaction is at least 4% of that produced by the deuterium reaction after operation for 5 yr and is at least 22% after 30 yr. Complete retention of the 3He would result in similar contributions from both reactions to the concentration of tritium in a heavy water system after 30 yr.