Space-time neutronic behavior of CANDU reactors is of importance in the analysis and design of reactor safety systems. A methodology has been developed for simulating CANDU space-time neu-tronics with application to the analysis of postulated loss-of-coolant accidents. The approach involves the efficient use of a set of computer codes that provides a capability to perform simulations ranging from detailed, accurate three-dimensional space-time to low-cost survey calculations using point kinetics with some “effective” spatial content. A new space-time kinetics code based on a modal expansion approach is described. This code provides an inexpensive and relatively accurate scoping tool for detailed three-dimensional space-time simulations.