The origin of this image is unknown, could be anything. But note the control rods penetrating the vessel from below… a design flaw noted for years because the seals could leak. The control rods are pushed up, between the fuel rods, absorb neutrons, and stop the chain reaction between neighbouring rods. The RPV lives high in the containment vessel
Cutaway of reactor pressure vessel (RPV) - origin unknown
Fukushima 