Abstract: A closing device for removably securing and sealing engaging a closure around an opening in a transportation cask for radioactive materials is disclosed herein. The closing device comprises a plurality of shear key assemblies uniformly spaced around the outer portion of the closure, each of which includes a shear key having an elongated bolt portion movably mounted in the closure, as well as a latch portion connected to one end of the bolt portion that is insertable into and retractable out of a slot located in the edge of the cask that defines the cask opening. The latch portion is beveled so as to wedgingly engage the slot when inserted therein in order to apply a sealing pressure around the outer edge of the closure. The width of the latch portion of the key is substantially larger than the bolt portion, thereby giving the shear key a T-shaped profile. The broad width of the bolt portion allows the shear key to more uniformly distribute a sealing pressure around the closure.