Abstract: A pressurized-water coolant reactor installation includes a reactor pressure vessel, a steam generator containing a heat exchanger through which coolant from the vessel is circulated, and a single pipe interconnecting the vessel and generator and internally divided into two conduits for conducting the coolant to the steam generator and from the latter back to the vessel. This single pipe is divided into these two conduits by a partition wall extending longitudinally for the length of the pipe inside of the pipe. This horizontal partition is formed by at least two sections extending radially from the inside of the coolant pipe towards each other and having inner edges spaced from each other and interconnected by an expansion joint.