Abstract: A nuclear steam generator vessel having a stayed tube sheet with an untubed circular central region in the upper surface thereof, a baffle plate disposed a predetermined distance above the tube sheet parallel thereto and a plurality of tube lane blocks spaced-apart longitudinally of a tube lane extending diametrically across the tube sheet, incorporates a system for controlling the flow velocity of secondary fluid along the tube sheet to reduce deposition of sludge thereon. The system includes a cylindrical flow boosting ring disposed coaxially with the central region and supported beneath the baffle plate a predetermined distance above the tube sheet. The ring may be used alone or in combination with one or both of two perpendicular vertical plates which extend diametrically across the central region in contact with the tube sheet, one of these plates lying along the tube lane axis and spanning the innermost ones of the tube lane blocks. When the ring is used with the vertical plates it is supported thereby.