[en] BEAGL-01 is a computer program for calculating the conditions in a light water reactor core at steady state and while undergoing a transient. It couples a neutron kinetics model in R,Z geometry with a two-phase thermal-hydraulics model for multiple parallel channels. Appropriate applications include reactivity insertion accidents in both pressurized and boiling water reactors and transients in boiling water reactors initiated by perturbations in flow rate, inlet temperature, or pressure. In this volume the modeling of the neutron kinetics, coolant thermal-hydraulics and fuel rod heat conduction are discussed. This includes the basic equations and the numerical algorithms used to solve these equations