Stability analysis of density wave oscillations with application to BRW and SCWR nuclear reactors

In this work, the phenomenon of Density Wave Oscillations (DWOs) will be studied. DWOs are the most common instability mechanisms in two-phase flows. Therefore, the phenomenon needs to be understood, in particular in the nuclear field, where safety is of key importance. In Chapter 1, a brief introduction to the two-phase instability phenomena will be presented and density wave oscillations will be introduced qualitatively. Then, in Chapter 2, the physical mechanism underlying the phenomenon will be studied analytically and it will be proved the role of the pressure drop in the occurrence of density wave oscillations. In Chapter 3, a mono-dimensional analysis of a two-phase flow will be presented and the effects of the different operational parameters on the channel stability will be pointed out. Thereafter, in Chapter 4 supercritical water will be introduced, and the same analysis of Chapter 3 will be applied to a supercritical channel. In fact, even if phase transition doesn’t take place in supercritical water, all the two-phase instability phenomena can be observed in supercritical flows too, due to the strong density change near the pseudocritical point. The results of this analysis will be of key importance in the design of different two-phase and supercritical systems such as boilers and steam generators. In these systems density wave oscillations are not desired, since they may induce mechanical vibrations, lead to control problems and affect the heat transfer. For this reason, it is important for the designer to ensure the largest possible margins of stability. This problem is even more critical in the field of nuclear power plants, an engineering area in which safety is of primary interest. For this reason, in Chapters 5 and 6, the Boiling Water Reactor (BWR) and the Supercritical Water Reactor (SCWR) respectively will be studied, in order to evaluate the stability of the core power following a reactivity insertion by the operator. All the above analysis will be carried out by investigating the poles of the transfer functions of the various systems.