[en] About 95% of the ¹⁴C generated in CANDU reactors is produced in the heavy water moderator by neutron activation of the ¹⁷O of the heavy water molecules. Most of this radioisotope, present as ¹⁴C bicarbonate, is removed by ion exchange. If the ion exchange column is 100% efficient in the removal of ¹⁴C, then a small predictable concentration of ¹⁴C is expected in the moderator water and the helium-filled cover gas. The cover gas is in contact with the moderator water, where ¹⁴C gaseous exchange occurs. Airborne release of ¹⁴C takes place through venting and leakage of the cover gas. Airborne releases of ¹⁴C from stations are very low, much below the emission limits, but releases higher than expected have been observed from several CANDU reactors. Based on laboratory simulations, station data and other reactor information, the cause of this was related to the following sequence: deuterium peroxide, produced in-core, oxidizes and depolymerizes the resins; resin fragments released from the column are converted to inactive bicarbonate in-core; this additional bicarbonate saturates the column resulting in premature ¹⁴C accumulation in-core. Changes in awareness of the problem, resin management and other reactor operation, particularly related to the moderator, resulted in a reduction in ¹⁴C emissions by a factor of approximately 6 from one CANDU reactor. (author)