The development of predictive capabilities for the phenomena governing fluid flow and heat transfer in nuclear reactors encompasses a wide spectrum of methods and levels of detail, from using simple heuristic formulae or correlations to small-scale extremely complex computational models and algorithms. The objective of this paper is to demonstrate that it is not uncommon that specific existing modeling methods and assumptions which are taken for granted are not necessarily always correct, and their applicability to particular problems should be carefully examined and assessed. Also, selected issues are discussed concerning the importance of identifying appropriate spatial and temporal scales for a meaningful and consistent formulation of mechanistic models. Examples are shown illustrating the assessment and robustness of selected modeling assumptions using both experimental data of reference and parametric studies. These examples are based on the research performed by the author and his collaborators. Additional information about each example can be found in the references to the original publications on the corresponding issues.
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