Various core performances, such as power and void distributions in a core, reactivity change, and shifts of control rods, are predicted for the JPDR-1 in a three-dimensional framework by means of the one-energy-group coarse-mesh approximation of the boiling water reactor (BWR) core. The predictions are checked in detail with experimental data that were accumulated by “the core-performance assessment experiments” done throughout the life of the JPDR-1 core. The data include y-probing data of the core at the exposure of each 1000 MWd/ton (approximately) core outlet void fractions measured directly by voidmeters, logbook records of the control rod patterns, heat-balance data for the precise core outputs, and others. In conclusion, the one-energy-group coarse-mesh approximation of the BWR is proved to be satisfactory for describing the global core performances of the JPDR-1 for burnup cases. The global power distributions can be calculated, e.g., with 4% standard deviation in the channel power sharing, and this is accurate enough to predict the core reactivity within 0.3% Δk/k error at 6000-MWd/ton exposure. The observed discrepancy, 0.9% Δk/k in the core reactivity at 6000-MWd/ton exposure (i.e., 10 to 15% error of the burnup change of reactivity), is discussed, with the final suggestion that the local power and exposure distribution in a core should be studied first, among others, for better achievement of the global core description.