Various NC systems are relevant within the nuclear technology. As already mentioned in the present report, various computational tools have been applied to the prediction of NC features of those systems. The complexity level of the concerned tools largely varies depending upon the simulated system and the objectives of the simulation. Simple analytical models solving approximate equations resembling the fundamental principles of the physics making reference to systems assumed in steady state conditions up to sophisticate codes including numerical solutions of partial differential equations in the three-dimensional space and in transient conditions can be distinguished.

In the following discussion, attention is focused on the application of thermal-hydraulic system codes. Under this category codes like APROS, ATHLET, CATHARE, RELAP5 and TRAC are included, all based upon the solution of a main system of six partial differential equations. Two main fields, one per each of the two phases liquid and steam are considered and coupling is available with the solution of the conduction heat transfer equations within solids interfaced with the fluid phases. A one-dimensional solution for the characteristics of the fluid is achieved in the direction of the fluid motion in time dependent conditions. It should be emphasized that more sophisticate models are also available including three­dimensional solutions and multi-field approaches in two and multiphase fluids. However the present qualification level of those sophisticated computational tools is questionable as well as their actual need in the design or in the safety applications.

The mentioned codes have been applied to the simulation of data measured in experimental facilities of different dimensions and complexity as well to the prediction of the NC performance of existing nuclear power plants and of advanced reactor concepts. The discussion is limited hereafter to the main findings from the applications of the system codes to the simulation of NC in Integral Test Facilities (ITF) that are simulators of PWR, BWR and WWER-440. Mention is made of the scaling problem and of the uncertainty expected in the predictions of NC in NPP.