[en] The cooling time for exposed nuclear fuel in a light-water-moderated research reactor of 10% enriched fuel was studied for 30 EK-10-type fuel assemblies. For this purpose, the scanning of the fuel assemblies was done with a mobile mechanical device and a Ge(Li) detector coupled to a multichannel analyser. The fuel assemblies were immersed in the cooling poind under a 3-m-thick layer of water, and the measurements were performed at various angles. The systematic experiments indicated that the immersion angle makes an important contribution to the quality of the results. The optimum value of this angle was also established. The experimental results show that many fission-product isotopes can be used for the cooling-time determination; in this report the optimum choice of these isotopes for this type of reactor is given. It was also found that the total time interval can be divided into a few subgroups, each of which has an equation of a specific analytical form. The use of the combined experimental results and of theoretical calculation allows cooling times, which agree well with the real values, to be predicted. (author)