Abstract: Upper internals for a pressurized nuclear reactor include an assembly for collecting the coolant leaving the core and a separating device. The device has guides for the control clusters and their drive shafts, a lower plate formed with passages for the coolant leaving the core and an upper plate formed with holes for passage of the same coolant into the collection assembly or plenum. A peripheral shroud may connect the plates together. Each cluster guide of the flow separation device has an internal casing devoid of openings, having a practically coolant-tight connection with the plates, whereby the control clusters and their drive shafts are protected against the high speed coolant flow from the core to the outlet nozzle.