The present study focuses on a thermal model describing a rotary kiln reactor. Several applications can be foreseen for this reactor, for example high temperature heat storage for thermal solar power plants. The energy is provided by concentrated solar radiation that heats up the cavity walls. A thermal model, describing the reactor behavior, is developed and validated. Particular attention is given to the radiation model, which constitutes the most important heat transfer. An innovative way of modeling the reactor aperture through a fictive surface at an imposed equivalent temperature leads to a significant decrease of the simulation time, without decreasing the precision of the solution. The model is validated by comparison first with other models, which make different assumptions and second with experimental results. After the validation, the model can be used for simulating the behavior under different operating condition or to define the possible improvements by a change of the reactor geometry such as the insulation's thermal conductivity or thickness. (C) 2013 Elsevier Ltd. All rights reserved.