Modern life is becoming increasingly dependent on electric energy, and the need for energy production is constantly increasing. At the same time, it is becoming obvious that the use of fossil fuels is causing irreversible damage to the environment, while renewable sources still fail to offer a constant and powerful supply of energy. Nuclear energy is an almost unlimited source of power which produces no carbon dioxide, but extreme situations in nuclear power plants need to be evaluated. With the aging of nuclear reactors and resulting embrittlement of the RPV steel, the possible damage caused by pressurized thermal shock (PTS) resulting from an emergency procedure is becoming increasingly important. Being able to predict the consequences of such events is of key importance for the nuclear industry, and CFD may be an option for such assessments. To become accepted, CFD codes need validation against experimental data. This thesis describes the process of simulating the experiments of a density driven situation of emergency core cooling (ECC) in the ROCOM facility by using ANSYS Fluent 16.2. ROCOM is an experimental facility modeling a pressurized water reactor and capable of reproducing ECC situations by injecting a higher density tracer fluid which acts as colder water, making it ideal for benchmarking CFD codes. During simulations, special consideration was taken in analyzing the influence of boundary and initial conditions, as well as time step and turbulent model. Apart from work on this case, the fundamental mathematical principles of simulating fluid flow are described in this thesis. The results acquired in the final simulation are a good match to the experimental measurements, even though slight differences are evidenced. The displayed data shows that CFD has a potential to become a tool for the nuclear industry.