A nuclear reactor is disclosed. The nuclear reactor comprises a vessel closed at the top by a roof and housing a core, comprising a bundle of fuel elements. The core being supported by a lower support supporting a lower portion of the core below an active zone of the fuel elements; and an upper support supporting an upper portion of the core above the active zone of the fuel elements; the upper support is joined to the roof by a support structure extending from the roof and has an end element centrally open and internally provided with a plurality of jaws for vertical support and elastic radial constraint of the fuel elements.