Abstract: A nuclear reactor with a reactor core having a plurality of individual core elements is provided with a clamping device for at least some of the core elements, which clamping device includes a plurality of pawls pivotally mounted to the interior of each wall of the hexagonal outer wrapping tube for movement between a position within the wrapper tube and a position extending through an adjacent hole in the side wall of the wrapper tube, and an operating tube axially slidably mounted within the wrapper tube for moving the pawls between their two positions. When the pawls are extended by the operating tube, they will move outwardly a predetermined distance transversely of the core element to cooperatively engage other core elements and clamp the assembly. A lock mechanism is provided to lock the operating tube and wrapper tube in one position of the pawls, preferably the withdrawn position, to assist in inserting and removing the core element.