Emergency water-cooling system for a steam generator for a pressurized-water coolant nuclear reactor

A steam generator for a pressurized-water coolant nuclear reactor has a housing with its lower portion closed by a horizontal tube sheet with an inverted U-shaped tube bundle extending upwardly within the housing and having inlet and outlet legs with ends mounted in the tube sheet, the latter having inlet and outlet manifolds below the tube sheet for the inlet and outlet legs of the tube bundle. The housing has a steam output outlet and a feed-water inlet to which feed-water is constantly supplied under pump pressure, and the manifolds are connected to a pressurized-water coolant nuclear reactor via a pipe line, a pump effecting circulation of the coolant with the latter flowing through the tube bundle. If the steam generator's feed-water supply fails for any reason, emergency measures must be taken quickly and this is done by a system for introducing cold water into the steam generator's housing to keep the tube bundle and tube sheet from dangerously overheating. To avoid severe thermal shock, particularly to the tube sheet and the adjacent portions of the tube legs mounted therein, this system introduces the cold water initially to parts within the steam generator's housing which are not so highly thermally stressed but which are hot and preheat the introduced emergency cold water to a temperature high enough so that when this water reaches the tube sheet and the portions of the tube bundle legs adjacent thereto, the emergency water has a high enough temperature to avoid causing dangerously high thermal shock on the part of the tube sheet and the adjacent tube bundle leg portions