In the framework of feasibility studies of future pressurized water reactors, a thermal-hydraulic program has been established in order to qualify computer codes in three fields: critical heat flux and reflooding heat transfer in triangular rod bundle arrays with tight lattices and the coolability of movable fertile rods in their guide tubes. Tests carried out at the Commissariat à I’Energie Atomique in cooperation with Electricité de France and Framatome are now in progress, and preliminary results are shown for these three studies.