Abstract: An integral nuclear reactor having between the core and the reactor shell an annular passage in which is situated at least one bundle of tubes leading to and from an outlet and inlet header respectively to which they are butt welded. Heated fluid from the reactor core flows around the tubes causing a cross-flow heat exchange with the fluid in the tubes. Preferably some of the tubes in the upper region of the or each bundle near where they are welded to the headers are bent so that they lie in a higher plane than others in the bundle, thereby offering a greater welding area to the headers.