A nuclear reactor head (12) comprising a vessel top head (14), a penetration (36) extending through the vessel top head (14) along a penetration axis (B) for allowing passage of a control shaft (34) of a control rod drive mechanism (32) through the vessel top head (14) and to a corresponding control guide tube (30) of the nuclear reactor (2), the penetration (36) comprising a penetration tube (38) extending through the vessel top head (14) and a thermal sleeve (40) extending inside the penetration tube (38) and coaxially with the penetration tube (38) with an axial play between the thermal sleeve (40) and the penetration tube (38), wherein the nuclear reactor head further comprises a spacer (60) attached to a lower end of the thermal sleeve (40) for maintaining minimal spacing with the upper end of the corresponding control rod guide tube (30).
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