In a steam generator equipping a nuclear power station and having inverted U-Lubes connected to a tube plate or sheet (12) and surrounded by an inner envelope (26) having a lower flow distribution plate (41) in its bottom part, the space (46) known as a waterway between the tube plate (12), the flow distribution plate (41) and the hot and cold branches of the tubes contains a deflection and purging box (44). The latter extends over the entire length of the waterway and in height up to the upper edge of the lower envelope (26). It is provided on its lower face with purging orifices (50) and fulfils both a deflection function and a secondary fluid drainage function in the vicinity of the tube plate (12).

The flow distribution baffle (14) is placed inside the bundle wrapper (7) of the steam generator, at its lower part,-near the steam generator tube plate. Passing through the flow distribution baffle (14) are openings (17, 18) for the passage of tubes of the steam generator bundle and feedwater. The flow distribution baffle (14) comprises a central zone (15) of more or less circular shape with, as its axis, the axis of the steam generator and a radius (R.sub.1) smaller than 40% of the internal radius (R) of the bundle wrapper (7), and a more or less annular peripheral zone between the central zone and the outer edge of the flow distribution baffle (14). In the central zone (15) the openings (17) for the passage of tubes and feedwater in the flow distribution baffle have a cross section (S.sub.1), the cross-sectional area of which is greater than the cross-sectional area of the cross section (S.sub.2) of the openings (8) for the passage of tubes and feedwater in the peripheral zone. The total surface area for the passage of feedwater in the peripheral zone (16) is greater than the total surface area for the passage of water in the central zone (15).