Patent Number: 
Section: claims

1. A nuclear reactor comprising:a vessel having a vertical central axis, the vessel including a vessel head having a plurality of openings;a core positioned inside the vessel;guide tubes positioned inside the vessel;a plurality of control members for controlling the reactivity of the core, axially movable inside the guide tubes;vessel head penetrations each comprising a tubular adapter fixed in one of the openings and defining an inner passage, each vessel head penetration further comprising a tubular sleeve engaged in the inner passage and axially extending in line with a corresponding one of the guide tubes;control rods secured to the control members and each extending through one of the tubular sleeves; anddrive mechanisms of the control members, located outside the vessel, each drive mechanism being configured to drive one of the control rods axially,each tubular sleeve being suspended by an upper axial end resting on an upper step arranged on the corresponding tubular adapter,a lower axial end of the tubular sleeve projecting axially into the vessel beyond the tubular adapter and being separated from an upper axial end of the corresponding guide tube by an axial gap having an axial height of less than 50 millimeters,wherein the upper step is outside of the vessel. 2. The nuclear reactor according to claim 1, wherein the lower axial end of the tubular sleeve is separated from the upper axial end of the corresponding guide tube by a radial gap with a radial width smaller than 20 millimeters. 3. The nuclear reactor according to claim 1, wherein the lower axial end of the tubular sleeve has a shape flared toward the corresponding guide tube, the upper axial end of each of the guide tubes comprising an axially protruding part engaged in the corresponding lower axial end of the tubular sleeve, the axial gap being defined between an inner surface of the lower axial end of the tubular sleeve and the axially protruding part of the corresponding guide tube. 4. The nuclear reactor according to claim 3, wherein the lower axial end of the tubular sleeve is separated from the corresponding upper axial end of the guide tube by a radial gap with a radial width smaller than 20 millimeters, the radial gap being defined between the inner surface of the lower end of the tubular sleeve and the axially protruding part. 5. The nuclear reactor according to claim 3, wherein the lower axial end of the tubular sleeve has a frustoconical shape, the axially protruding part having a frustoconical shape conjugated with that of the lower axial end of the tubular sleeve. 6. The nuclear reactor according to claim 1, wherein the upper axial end of the guide tube has a lower step, a free edge of the lower axial end of the tubular sleeve being located axially above the lower step, the axial gap being defined between the free edge and the lower step. 7. The nuclear reactor according to claim 6, wherein the lower step is defined by a plurality of closed-contour surfaces, separated from one another by passages, and distributed around the control rod. 8. The nuclear reactor according to claim 6, wherein the lower axial end of the tubular sleeve is separated from the upper axial end of the corresponding guide tube by a radial gap with a radial width smaller than 20 millimeters, the upper axial end of the guide tube including at least one radial stop located radially toward an inside and/or toward an outside of the lower step, the radial gap being defined between the free edge and the at least one radial stop. 9. The nuclear reactor according to claim 1, wherein each guide tube comprises a tubular guide structure and an end part fastened to the guide structure and defining the upper axial end of the guide tube. 10. The nuclear reactor according to claim 9, wherein the end part has a central axial orifice passed through by the control rod, and fluid circulation ducts emerging in the central axial orifice. 11. The nuclear reactor according to claim 10, wherein the tubular guide structure comprises an upper plate having a passage orifice for the control rod placed to coincide with the central axial orifice, the end part being fastened to the upper plate, the upper plate and the end orifice having, perpendicular to the vertical central axis, inner and/or outer sections that are identical. 12. A method for maintaining the nuclear reactor as recited in claim 1,each guide tube comprising a tubular guide structure;the method comprising:fastening an end part to the guide structure of at least one guide tube, the end part defining the upper axial end of the guide tube, the lower axial end of the tubular sleeve being separated from the upper axial end of the corresponding guide tube by the axial gap with the axial height smaller than 50 millimeters. 13. The method according to claim 12, wherein the lower axial end of the tubular sleeve is separated from the upper axial end of the corresponding guide tube by a radial gap with a radial width smaller than 20 millimeters.