[en] Experimental work has been carried out with air and water in a 0.114 times scale model of the proposed Sizewell 'B' PWR hot leg. The model was accurate from the entry from the reactor vessel to the tube plate in the steam generator. Visual observations are summarized in a flow map in the coordinates of an air Froude number versus a water Froude number. The map shows a predominantly stratified system with spray occurring at higher air flowrates and surges appearing at higher water flowrates. Also, at very low air flowrates, long bubbles of air passed along the top of the hot leg through water filling the pipe. A previous theory predicts the water level in the reactor vessel in terms of the air and water flowrates. This theory also agrees with data from other sources on the level in the hot leg in terms of the discharge of two phases through a horizontal hot leg break. The data includes results for steam and water at pressures up to 62 bar. Correlations for the interface level in the hot leg are developed. Data for lower air flowrates agrees substantially with a theory for the onset of slugging. (author)