In order to develop approaches to generalization of the nonstationary heat exchange data and to obtain the approximate thermal-model-testing criteria the numerical investigations of the nonstationary temperature patterns in the central zones of the nuclear reactor fuel assemblies are performed. The fuel assemblies of fast and water-cooled reactors with the different fuel compositions are investigated. The temperature patterns are found for instantaneous change in energy release and for steady coolant flow. 4 refs.; 11 figs.; 1 tab.