Space- and time-dependent phenomena, mostly related to neutron flux oscillations, have been observed in several boiling water reactor plants, A time-dependent three-dimensional transient analysis code is indispensable for simulating such phenomena. In a joint effort between the General Electric Company and the Toshiba Corporation, a three-dimensional neutron kinetics model has been implemented into the best-estimate thermal-hydraulics code, TRACG. A neutronics model implementation and the applicability of the modified TRACG code for analyzing space-dependent phenomena are discussed. To verify the code, startup tests with selected rod insertions, where control rods are locally inserted, are simulated. Both corewide, spatially in-phase neutron flux oscillations and regional, spatially out-of-phase oscillations are modeled. The results show that the modified TRACG code has sufficient capability to simulate space-dependent transients and is also a useful tool for investigating the fundamental mechanisms behind such transients.