[en] A theoretical and experimental assessment is described of the value of gamma-scanning measurements of the ¹³⁴Cs/¹³⁷Cs activity ratio for the non-destructive determination of burnup in irradiated CANDU fuels. The modes of formation of the cesium isotopes during irradiation are discussed, and the computer codes LATREP and ISOGEN are used to calculate the effects of neutron flux level, neutron spectrum and irradiation time on the ¹³⁴Cs/¹³⁷Cs activity ratios. The method of calculation of burnup from gamma spectral measurements is described, and the factors on which an accurate burnup determination depends are discussed. These include the neutron absorption cross section of ¹³³Cs, the decay of ¹³⁴Cs during interruptions in the irradiation and correction of the experimentally-measured cesium activity ratio for gamma attenuation in the fuel matrix. Twenty fuel elements from the Pickering Generating Station have been analyzed by gamma-scanning, and by mass spectrometry. The results of the two methods agree within approximately 2.2%. It is concluded that the γ-scanning method can be valuable for laboratory studies where non-destructive burnup analyses are required but, for Safeguards measurements on whole fuel assemblies, the procedure is limited by the need for a detailed knowledge of the reactor operating conditions. (author)