The present work is concerned with the stability investigation of open natural circulation systems as a basis for use as a passive heat removal system in the containment of a boiling water reactor. The operation of such a system with natural circulation is based solely on the formation of a density gradient which, as a result of free convection, leads to a flow within this system. This density gradient in the working fluid is caused by the supply and removal of heat. When the saturation temperature is reached, the developing and continuously increasing mass flow changes into mass flow oscillations, the so-called
two-phase flow instabilities. As the temperature of the working fluid increases, the mass flow returns to a stable flow and continuous heat removal, but as two-phase flow.
A test facility was erected at the Technische Universit¨at Dresden, which simulates the containment cooling condenser of the KERENATM (formerly BWR-1000) reactor concept, in order to evaluate the system and operating characteristics with regard to geometric influences. With the help of high-resolution temperature and void fraction measurement, it was found that, with risers arranged in parallel, the pressure surges caused by condensation
induced water hammers could be greatly reduced or even prevented. One of the riser thus functions as a buffer for subcooled fluid flowing back from the heat sink in the other. The operating characteristics were also summarized in a stability map, which clearly differentiates between the stable single-phase flow, the unstable two-phase flow and the stable two-phase flow. The prediction of the stability boundary between unstable and stable two-phase flow using an analytical approach has been successful. The underlying model for such an open natural circulation system was reduced from a model solved by the weighted residual method and the finite volume method to a low-order model (ROM) with
the help of a proper orthogonal decomposition. Comparative calculations with a developed model of the GENEVA test facility using the already validated system code ATHLET from the GRS (Gesellschaft f¨ur Anlagen- und Reaktorsicherheit (GRS) gGmbH) confirmed the calculated operating conditions and ultimately the stability boundary detected by the linear stability analysis. This ROM maps the two-phase flow by means of the Drift-flux mixture model, which takes into account the relative velocities of each phase. The nonlinear stability study of this ROM resulted in supercritical Hopf bifurcations at selected reference operating points, which could only be demonstrated by the detection of occurring stable
limit cycles during the numerical integration. With the help of this ROM, parameter studies for stability analysis can be carried out with a considerable reduction in computational effort.