Abstract This paper presents the thermal-hydraulic investigation of spent fuel behavior during its transferring from reactor vessel through refueling cavity (RC) to spent fuel pool (SFP) in case of dry out for VVER440/V230 units 3 and 4 at Kozloduy NPP. The fuel transfer canal connects the refueling cavity and spent fuel pool and this way set up a command pool. The presented analysis has been performed up to the moment of fuel heat up in case of spent fuel pool (SFP) dry out during the first stage of refueling activities. The main feature during this stage is: the maximum decay power of “new” spent fuel; the “new” spent fuel is still in the reactor vessel; the fuel transfer canal connects the refueling cavity and spent fuel pool. In this way the coolant have maximum volume and the spent fuel with maximum decay power is still in the reactor vessel. The “old” spent fuel in SFP has significantly low decay power. The main purpose of this analysis is to estimate the time for dry out of SFP, heat up of spent fuel and time for recovery actions from the operators. In the performed analysis are defined the following stages during the accident. ∘ Termination of natural circulation after decreasing of water level in reactor vessel below the hot nozzles. ∘ Beginning of coolant heat up in the reactor core. ∘ Reaching the temperature of saturation at the outlet of the assembly. ∘ Startup of the reactor core uncover. ∘ Loss of critical safety functions. The analysis has been performed with the thermal-hydraulic computer code RELAP5/MOD3.2. The RELAP5/MOD3.2 model for Kozloduy NPP VVER-440 have been developed and validated at the Institute for Nuclear Research and Nuclear Energy – Bulgarian Academy of Sciences (INRNE-BAS) Sofia.