A mathematical treatment has been developed to describe the activity levels of 129 I as a function of time in the primary heat transport system during constant power operation and for a reactor shutdown situation. The model accounts for a release of fission-product iodine from defective fuel rods and tramp uranium contamination on in-core surfaces. The physical transport constants of the model are derived from a coolant activity analysis of the short-lived radioiodine species. An estimate of 3×10 −9 has been determined for the coolant activity ratio of 129 I/ 131 I in a CANDU Nuclear Generating Station (NGS), which is in reasonable agreement with that observed in the primary coolant and for plant test resin columns from pressurized and boiling water reactor plants. The model has been further applied to a CANDU NGS, by fitting it to the observed short-lived iodine and long-lived cesium data, to yield a coolant activity ratio of ∼2×10 −8 for 129 I/ 137 Cs. This ratio can be used to estimate the levels of 129 I in reactor waste based on a measurement of the activity of 137 Cs.