Stability of natural circulation with gravity-induced flashing

The recent development of low-power, boiling water district-heating reactors has drawn attention to some two-phase flow phenomena related to the low-pressure and low-quality operation characterizing this type of reactor. Several proposed designs include a long adiabatic riser above the core to enhance natural circulation. Under such conditions, an important part of the steam generation takes place as flashing driven by the reduction of the hydrostatic pressure in the riser. Investigation of the transient behavior and of the so-called gravity-domain density-wave instabilities of such natural-circulation loops is an important part of their analysis. There is a large body of theoretical and experimental research and state-of-the-art computer codes for stability analysis of conventional and nuclear power systems, but there is no known analysis including the effects of time and space-dependent flashing that characterizes the systems considered here. A linearized density-wave stability analysis model fully taking into account time and space-dependent vaporization is presented here. It is based on the one-dimensional, thermal-equilibrium mixture conservation equations obtained using the widely used drift-flux model