The OECD/NRC boiling water reactor (BWR) turbine trip benchmark has been calculated by the coupled thermal-hydraulic neutronics system code ATHLET-QUABOX/CUBBOX developed by Gesellschaft für Anlagen- und Reaktorsicherheit. The results obtained for all three exercises and for the additional four hypothetical cases are presented. The physical phenomena determining the BWR pressure transient are studied and explained. The sensitivity of results to variations of the initial steady-state conditions and of parameters of the two-phase flow model is discussed. A comparison is also performed for exercise 2 between the reactor core model with 33 thermal-hydraulic channels (THCs) as specified and a reactor core model with 764 THCs using a 1:1 mapping scheme.