The safe thermal conditions of spent nuclear fuel storage are the important component of complex safety of the dry spent nuclear fuel storage facility. The multistage approach for numerical definition of thermal fields in storage containers with spent fuel assemblies is proposed. The approach is based on solving of the series of the conjugate heat transfer problems with different geometrical detailing. The developed approach is used for estimation of thermal state of ventilated containers with spent nuclear fuel of WWER-1000 reactors of Zaporizhska nuclear power plant. The results of the thermal calculations for single-placed container on open-site storage platform were presented. The safety of containers usage in normal and extreme ambient temperatures was proven. Copyright (c) 2015 John Wiley & Sons, Ltd.