Introduction - Chapter 1

The composed word thermal-hydraulics, here preferred to thermo-hydraulics, thermal-hydraulics, and thermalhydraulics, derives from two Greek terms also used by Romans, which indicate heat and water, respectively. Thus, thermal-hydraulics is the discipline which studies the transient evolution of engineering systems where (hot-heated) water is the coolant fluid and, typically, heating and cooling or boiling and condensing processes occur. Nuclear thermal-hydraulics addresses the performance of water-cooled reactors, typically water-moderated, i.e., the majority of research reactors (RR) and power plants. Selected features for the discipline can be listed as follows: • design and safety of nuclear reactors highly depend upon thermal-hydraulics: this has been established since the beginning of the nuclear technology era; • looking at the numerical simulation of phenomena, it shares with fluid-dynamics and gas-dynamics the difficulty associated with turbulence: in thermal-hydraulics the problem is even more complex because of the presence of two-phases; time and space averaging is unavoidable: discontinuities including the interface between the phases are solved by continuum equations; • approximation of results from numerical models and the need to bound the errors of calculations, i.e., the uncertainty, are a consequence of what written above; • experiments are necessary and highly dependent upon geometric configurations: they are not conclusive for a suitable knowledge because performed at reduced scale in the case when power reactors are concerned; • controversy in judging the quality, the applicability and the acceptability of experimental and numerical data to situations of technological interest is not surprising.