NUFREQ-NP: a digital computer code for the linear stability analysis of boiling water nuclear reactors

The phenomena of nuclear-coupled density-wave oscillations are of considerable importance in boiling water nuclear reactor (BWR) stability analysis. A state-of-the-art linear frequency domain digital computer code, NUFREQ-NP, has been developed for either forced or natural circulation BWR stability analysis. The NUFREQ-NP code can be excited by many external perturbations, including system pressure perturbation. It is based on one dimensional drift-flux thermal hydraulics, and allows for subcooled boiling, arbitrary nonuniform axial and radial power shapes, distributed local losses (e.g., spacers), point or multi-dimensional neutron kinetics, and detailed fuel element dynamics. It has been compared with both out-of-core and in-core data, and good agreement has been found.