Enclosed System
This drawing postulates the possible dimensions of a system that would convert 100% of the blast and heat energy into weight displacement. Like described in the fusion to electricity drawing the bubble of energy created by rapid vaporization of water would be flattened by weight energy until all the heat and blast energy is used to push the weight energy up out of the cavity. In this example all the water weight energy is pushed out over a wall and into an area between two walls sufficient enough to hold all the water. In the drawing the dimensions are one mile wide by two miles deep into the bedrock. The dimensions of such a cavity could be blasted by fusion or pre-drilled then blasted and or dug out gradually. The depth is just an estimate the only information I found in Wikipedia concerning this matter is that at a depth of two hundred feet a 37 kiloton explosive as it expands will collapse in on itself from water pressure then billow out even faster. It leaves the water as sort of mist then.