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High-resolution gamma-spectrometry (HRGS) has been used for monitoring burn-up of VVER-440 spent fuel assemblies. The objective is to support burn-up calculations by an independent experimental method. Measurements were carried out using a collimator tube built into the concrete wall of the service pit in the reactor block at the Paks Nuclear Power Plant. Fuel assemblies were sunk down into the service pit, positioned in front of the collimator, and moved down and up under water by the refueling machine. The measured Cs-134/Cs-137 activity ratio is in good correlation with the calculated burn-up of the assemblies. Experimental results were simulated by model calculations and excellent agreement was found between calculation and measurement.