Abstract: In a nuclear reactor having the fuel disposed within a large number of pressure tubes fed in parallel with liquid coolant which is partly vaporized while flowing through the pressure tubes, the invention provides a coolant-supply header assembly comprising parallel-extending sub-header pipes which at their ends are connected by end pipes and which have water outlets (for connection to the individual pressure tubes) distributed along their lengths, and header pipes each extending transversely across the sub-header pipes at spaced intervals therealong and having a water inlet and a plurality of branch pipes each of which connects to a respective one of the sub-header pipes.