AbstractA coarse mesh nodal coupling method, a well-known technique often used in steady-state neutronics analysis of light water reactors, is extended to a problem of transient phenomena of boiling water reactors (BWRs). Spatial collapse is attempted to develop a multiregion neutronics model and the associated axially one-dimensional and one-point models.These models are numerically solved through the use of two approximations, quasi-static and prompt jump. The results as applied to a reference BWR core for transient analyses, initiated by artificial thermal-hydraulic disturbances, are presented to show the practicality of the approach.The nature of the optimal weighting function necessary for the spatial collapse and for the quasi-static approximation is also discussed.