[en] Highlights: • A Fuel Burnup Measurement System was designed and developed in TRR using a non-distractive gamma-ray spectroscopy method. • The irradiated fuel burnup was measured in TRR through absolute activity determination of ¹³⁷Cs indicator. • The operating histories of the irradiated fuel assemblies of TRR were obtained carefully. • Results from reactor physics calculations were verified using non-distractive gamma-ray spectroscopy. In this work, a nondestructive gamma scanning technique has been applied to determine the irradiated fuel burnup of Tehran Research Reactor (TRR). Thereby, a system was designed and installed at the rim of the TRR pool, which includes a High-Purity Germanium (HPGe) detector and associated electronics-devices, a proper shield, a collimator, and an elevator to radioactive fuel handling for any longitudinal and transverse fuel movement. In the system, it also was possible to measure fuel burnup for fuels with short cooling times at the minimum distance between the fuel and the detector. Five Standard Fuel Elements (SFEs) have been studied with U3O8Al fuel in burnup range of 14%–60% FIMA and cooling time range of 60 days–550 days. Then, by analyzing the gamma-ray emitted from 137Cs isotope as a fuel burnup indicator, an axial profile of fuel burnup was measured in the active fuel length of 61.5 cm and an active width of 6 cm. Finally, verification of results from reactor physics calculations was conducted based on fuel burnup measurements using a nondestructive gamma scanning technique that represents a well enough agreement between calculations and measurements.