• Distortions
• Fractional Rate of Change
• FSA

The plan of this thesis is as follows. After a brief introduction about nuclear energy future perspective and Generation IV, IRIS is described stressing its characteristics which make it a Generation 3+ nuclear system (chapter 2). Then, the SPES3 facility is described with particular attention to the design criteria important for the scaling issue (chapter 2). In chapter 3, a review on the Hierarchical, Two-Tiered Scaling Analysis is presented and then attention is focused on Fractional Scaling Analysis. Chapters 4 and 5 present the application of FSA to Reactor Vessel and Containment Vessel pressure response after pressure equalization; in particular IRIS and SPES3 time and space decompositions are described, IRIS and SPES3 fractional rates of change (see chapter 3 and 5) are calculated, distortions are evaluated and actions to reduce or eliminate the distortions are proposed. Conclusions and future perspectives of the work are finally proposed.