Investigations on thickness and velocity of water films at the fuel rods in the two-phase system air-water

The design of reactor components or systems with water cooling under high thermal loads should avoid the development of critical heat flux (CHF) situations. Since complete analytical forecasting of CHF is not possible for complicated geometrical situations there is a need for experimental investigations. AREVA is working on simplified experimental methods using the BESON (boiling and evaporation for nuclear research and conventional needs) facility to study CHF events in the two-phase system air-water at a pressure of 1 bar. The behavior of water films on fuel rods is of importance for the operation of BWR fuel elements. Concerning the transferability of water-air experiments on the real SWR situation it has to be taken into account that the evaporation of the water film and thus the decrease of film thickness cannot be simulated. The attachment or detachment of water droplets at the water film can be simulated using spacers or turbulent flow occurrence in water-air experiments. The demonstrated measuring techniques allow relative statements on the mode of action of different spacers.