Nuclear reactor head, nuclear reactor comprising such a nuclear reactor head and method of maintaining a nuclear reactor

A nuclear reactor head comprising:

• a vessel top head;
• a penetration extending through the vessel top head along a penetration axis for allowing passage of a control shaft of a control rod drive mechanism through the vessel top head and to a corresponding control rod guide tube of the nuclear reactor, the penetration comprising a penetration tube extending through the vessel top head and a thermal sleeve extending inside the penetration tube and coaxially with the penetration tube with an axial play between the thermal sleeve and the penetration tube;
• and
• a spacer attached to a lower end portion of the thermal sleeve for maintaining axial spacing with an upper end of the corresponding control rod guide tube, the spacer comprising a seating ring attached to the lower end portion of the thermal sleeve via a plurality of legs of the seating ring, flow openings each being circumferentially delimited by a corresponding two of the plurality of legs, the flow openings each being delimited axially by the lower end portion of the thermal sleeve and the seating ring.