In this master thesis three dimensional model of Nuclear power plant Krsko IB rooms on the elevation of 100.3 meters which could be under influence of the break in the blowdown processing system was built. It is essential for the safety operation of the nuclear power plant to preserve very strictly defined environmental condition in rooms with DC batteries and their chargers. If the batteries and chargers are preserved in good operating condition we have the strongest guarantee for the safety of nuclear instrumentation power supply. For the defined mass flow of discharged fluid and for the different scenarios of break point isolation and communication with environment, via turbine building or via IB001 room, with the CFD code FLUENT three dimensional contours of temperature were calculated. Also based on the results of this analysis optimal locations for temperature sensors were defined. In the first phase of the master thesis work with the aim to get a better comparison with results obtained with GOTHIC code first calculations with FLUENT were made on a simple geometry models. Results and experience obtained on this simple geometry models were essential for the quality of the calculation in quite complicated model of IB rooms. In the first chapters of this paper basic overview of the past analysis (Grgić, 2001) were made. This past analysis (Grgic, 2001) was the base and input for the analysis that was made in this paper. Also in the first, so called introducing chapters key set of fluid dynamics equations and basic capabilities of the CFD code FLUENT were presented. Through the analysis of the results obtained on the model of IB room optimal location for the temperature sensors were defined. Based on all scenarios, wall between room IB010 and containment building and wall between IB010 and IB007 which are in the room IB010 were declared to be optimal location for the sensors. As global conclusion after these analysis we may say that even regarding all simplifications of the model that were necessary to make introduction of the CFD code FLUENT in this class of problem was very valuable. Results of analysis preformed with FLUENT clearly present local distribution of temperature and velocity vectors. Also unsteadiness and three dimensionality of the resolving problem was ideal place for the implementation of CFD code FLUENT. Results of the analysis confirmed for the first time that in the moment when communication via turbine building would be opened, on the door between rooms IB001 and IB010 would occur a reverse flow. Also we can not neglect the fact that modelling multi-phase phenomena with FLUENT is a very complex problem. Very good advantage of GOTHIC code is defined in this last sentence that describes disadvantage of FLUENT. GOTHIC code has very good capabilities for the multi-phase phenomena modelling but limited three dimensional capabilities. Combination of these codes provides the best results when applied to the analysis of this class of problems. Perspectives for the future work in this field are underlined with the combined usage of CFD codes and codes with distributed volumes for the validation of the results obtained with CFD codes.