The advanced boiling water reactor (ABWR) has ten reactor-internal pumps peripherally mounted on the bottom of a reactor vessel. Analytical simulation of reactor-internal pumps unique to the ABWR requires new modeling because of the difference in core flow characteristics between the reactor-internal pumps and the two external-recirculation pumps of the primary outer loops with the jet pumps in a current boiling water reactor. Efforts in this work focused on modeling and simulation of reactor-internal pumps and core flow of the ABWR, using the RETRAN-3D code, the computer program for transient thermal-hydraulic analysis of a complex fluid flow system, without multidimensional kinetics. Included are modeling of the core and reactor pressure vessel with ten reactor-internal pumps, and simulation of the events of reactor-internal-pumps trip during the startup-phase tests, which are unable to be done in the simulation of a current BWR. Sensitivity analyses on the recirculation flow control and the slip model were also performed. The predictions by the RETRAN-3D code successfully tracked the measured data of reactor-internal-pump trip during the startup-phase test. The present analytical simulations could demonstrate the validation of the RETRAN-3D code applicable to the ABWR with the pump model of reactor-internal pumps in the program.