A solar reactor consisting of a cavity-receiver containing a reticulated porous ceramic (RPC) foam made of CeO2 is considered for effecting the splitting of H2O and CO2 via a thermochemical redox cycle. A transient 3D heat and mass transfer model of the reduction step is formulated and solved using Monte-Carlo ray-tracing coupled to computational fluid dynamics. Experimental validation is accomplished in terms of measured temperatures and O-2 evolution rates obtained with a solar reactor prototype tested under high-flux radiative power inputs in the range 2.8-3.8 kW and mean solar concentration ratios up to 3024 suns. Critical temperatures of up to 2250 K induced CeO2 sublimation, which in turn affected detrimentally the solar reactor performance. The model is applied to analyze an improved geometrical design with alternative flow configuration, enabling more uniform radiative absorption and temperature distributions, and resulting in a higher solar-to-fuel energy conversion efficiency. (C) 2015 Elsevier Ltd. All rights reserved.