The present work focuses on the development of a numerical model which simulates the hydrodynamics of forced and natural circulation boiling water nuclear reactor loops and demonstrates its capability to analyse density wave instability of the reactor undergoing in-phase and out-of-phase modes of oscillations. The numerical algorithm integrates a compressible and distributed fluid flow model used for simulating the two-phase flow phenomenon in the channels of the reactor core with an incompressible lumped parameter model which accounts for sub-cooled water flow in various ex-core and recirculation components of the reactor loop. The algorithm used in the present hybrid model explicitly addresses the uncertainties related to boundary conditions existing in the literature to simulate parallel channel instability analysis for forced and natural circulation boiling reactor loops. Extensive numerical experimentations are performed to search the stability thresholds for both the forced and natural circulation reactor loops undergoing in-phase and out-ofphase instability modes.
Keywords: boiling water nuclear reactor, thermal-hydraulic model, density wave oscillation, parallel channel instability, forced and natural circulation loops, boundary conditions