Experimental investigation of natural convection of a two-component gas in a simplified PWR model primary coolant loop

Two-component gas natural convection flow (air mixed with helium) under ambient pressure is investigated in a semicylindrical test vessel (1.08 m diameter x 1.7 m height), which is representative for the core of a pressurized water reactor. The vessel contains a bundle of 544 rods, on which a radial temperature difference is imposed, and it is provided with three simulated steam generators. Thereby the effect of a hot leg can be studied. The experiments are used for the validation of the 2D-thermohydraulics code FRECON, which is employed in the analysis of severe reactor accidents. Temperature and flow patterns in the test vessel as well as in one hot leg pipe are measured by thermocouples and Laser-Doppler-Velocimetry, respectively. Samples of the gas mixture are withdrawn from the test vessel and one hot leg pipe, respectively, and fed to a gas analyzer. Compared to a blocked hot leg, experiments with active hot leg show an overall reduction of the temperature level and an increase of the in-vessel flow rate. Both, hot leg and blocked hot leg conditions result in a homogeneous helium distribution all over the test vessel and in the hot leg, respectively. (author)