Theoretical Variations in the Concentration of Tritium in the Heavy Water in CANDU Reactors Caused by the ³He(n,p)³H Reaction

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.