Production of hydrogen using solar thermal energy has the potential to be a viable alternative to other hydrogen production methods, typically fossil-fuel driven processes. Thermochemical reactions for splitting water require high temperatures to operate effectively, for which solar is wellsuited. Numerical modeling to investigate the concept of a solar-driven reactor for splitting water is presented in detail in this paper for an innovative reactor, known as the "counter-rotating-ring receiver/reactor/recuperator" (CR5) solar thermochemical heat engine that is presently under development. In this paper, details of numerical simulations predicting the thermal/fluid behavior of the innovative solar-driven thermo-chemical reactor are described in detail. These scoping calculations have been used to provide insight into the thermal behavior of the counter-rotating reactor rings and to assess the degree of flow control required for the CR5 concept.