Nuclear reactor and corresponding maintenance method

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;
• and
• drive 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 sleeve 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.

A method for maintaining a nuclear reactor, the 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, each guide tube comprising a tubular guide structure;
• 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;
• drive 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 sleeve end resting on an upper step arranged on the corresponding tubular adapter;
• a lower axial sleeve end of the tubular sleeve projecting axially into the vessel beyond the tubular adapter and being located above the guide tube;
• the method comprising:
• fastening an end part to the guide structure of at least one guide tube, the end part defining an upper axial guide tube end, the lower axial sleeve end being separated from the corresponding upper axial guide tube end by an axial gap with an axial height smaller than 50 millimeters.