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NUCLEAR-COUPLED THERMAL-HYDRAULIC STABILITY AND BIFURCATION ANALYSES OF FORCED- AND NATURAL- CIRCULATION BWRS

We report our recent efforts in modeling nuclear-coupled thermal hydraulic instabilities in forced and natural circulation boiling water reactors characterized by single and two-phase flows. While density-wave type oscillations are captured with thermal hydraulic models with constant properties, simulation of flashing requires the water saturation enthalpy to be treated as a function of local pressure. Hence, separate thermal hydraulic models are developed and used for the two different kinds of instabilities. Models also include fuel rod heat conduction and modal neutron kinetics. Results of steady-state, stability and bifurcation analyses, as well as numerical integration are reported. Preliminary results of numerical integration to simulate intermittent oscillations in natural circulation heated loops with two-phase flow are also reported. Results show that these models can capture in-phase and out-of-phase oscillations as well as show suband supercritical bifurcations as stability boundaries are crossed.