The purpose of this work was to simulate the conditions of direct steam generation in a vertical tube receiving concentrated solar radiation. The modified RPI model was used for the conditions of critical heat flux coupled to a Eulerian two fluid model. The mathematical model was solved with CFD software ANSYS FLUENT v15. The results were validated with experimental data reported in the literature and a parametric study was carried out to determinate the effect of the mass flux, the heat flux incident on the wall and the heat transfer area on: the steam quality, the volumetric fraction and the temperatures of liquid and steam. From the results obtained, it was determined the required conditions to obtain a steam quality of 100%.