There is a common understanding among thermal-hydraulic experts that the system analysis codes have currently reached an acceptable degree of maturity. Reliable application, however, is still limited to the validated domain. There is a growing need for qualified codes in assessing the safety of the existing reactors and for developing advanced reactor systems. Under conditions involving multi-phase flow simulations, the use of classical methods, mainly based upon the one dimensional approach, is not appropriate at all. The use of new computational models, such as the direct numerical simulation, large-eddy simulation or other advanced computational fluid dynamics methods, seems to be more suitable for more complex events. For this purpose, the European Commission financed NURESIM Integrated Project (as a part of the FP6 programme), was adopted to provide the initial step towards a Common European Standard Software Platform for Modeling, recording and recovering computer data for nuclear reactor simulations. Some of the studies carried out at the University of Pisa within the framework of the NURESIM project are presented in this paper. They mainly concern the investigation of two critical phenomena connected with jet instabilities and direct contact condensation that occur during emergency core cooling. Through these examples, the state-of-the-art and the need for model improvements and validation against new experimental data for the sake of getting a better understanding and more accurate predictions are discussed.
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