The invention relates to a novel architecture for a nuclear reactor of the integrated type.The invention comprises:realising the hot area and cold area separation device for the primary sodium flow in the form of two walls with cuts,providing two pumping groups hydraulically in series, one for the flow of sodium from the hot area to the cold area through the intermediate exchangers and the other in the cold area;providing outlet windows of the intermediate exchangers below the lower wall;providing outlet windows of the removal exchangers of the decay heat above the cold area, wherein all of clearances between the walls with cuts and the heat removal exchangers and the height between the two walls with cuts are previously determined so as to, during normal operation, take up differential movements between the walls, exchangers and vessel and to make it possible to establish during normal operation a thermal stratification of the primary sodium in the space defined between the horizontal portions of the two walls and so as to reduce, in case of an unexpected stop of a single pumping group, the mechanical stress applied to the walls and due to the portion of the primary sodium flow passing between said clearances.