In order to ensure that data from vessel model flow tests are reliable, it is necessary to simulate hydrodynamically the various reactor components within the vessel, including the core. Modeling of the core is complex because, unlike other internals, a three-dimensional flow distribution must be taken into account. Core simulation criteria have been derived from the basic conservation laws governing fluid motion within the rod bundles that constitute the core. Using the criteria derived, along with any constraints imposed by the test loop, methodology for the design of a model core which hydrodynamically simulates the prototype core to a good degree of accuracy has been developed. Economic and fabrication considerations are taken into account in selecting the final design.