Abstract: Nuclear boiler comprising a vessel sealed in its upper part by a removable cover, a basket-shaped member carrying the reactor core and separating the vessel into a hot inner area and a cold outer area, at least one steam generator positioned within the vessel and comprising a tightly sealed enclosure and a tube plate separating the enclosure into a primary part or water box and a secondary part, the tube plate carrying U-tubes, whose ends issue into a cold area and a hot area of the water box, defined by an inner structure, two linear, coaxial tubes defining an inner pipe and an annular outer pipe respectively connecting the hot areas and cold areas of the vessel and the water box, wherein a removable safety sleeve, whose wall is designed to resist the pressure in the primary circuit, is arranged coaxially within the outer annular pipe and defines with the latter a calibrated clearance making it possible to limit to a given value the leakage flow if the outer tube fractures.