A high-temperature solar chemical reactor for the processing of solids is scaled up from a laboratory scale (5 kW concentrated solar power input) to a pilot scale (200 kW). The chosen design features two cavities in series: An upper cavity has a small aperture to let in concentrated solar power coming from the top. It serves as the solar receiver radiant absorber, and radiant emitter to a lower cavity. The lower cavity is a well-insulated enclosure. It is subjected to thermal radiation from the upper cavity and serves in our application as the reaction chamber for a mixture of ZnO and carbon. Important insight for the definition of the geometrical parameters of the pilot reactor has been generated by a radiation heat transfer analysis based on the radiosity enclosure theory. The steady-state model accounts for radiation heat transfer within the solar reactor including reradiation losses through the reactor aperture, wall losses due to thermal conduction and heat consumption by the endothermic chemical reaction. Key results include temperatures of the different reactor walls and the thermal efficiency of the reactor as a function of the major geometrical and physical parameters. The model, hence, allows for a fast estimate of the influence of these parameters on the reactor performance.