81 gigaton bomb?! Holy cow! It sounds like a planet buster! It seems to be the energy equivalent somewhere around an 8.5 magnitude or higher earthquake if the internet is to be believed?
No, the fireball size is proportional to the cube root of energy released, which is why giant nukes are generally wasteful compared to many smaller ones.
But the goal isn't to make a giant fireball, the goal is to bury it 7 kilometers under the seafloor bed at detonate it, pulverizing a massive volume of rock. Nearly all of the energy is being captured by rock and water, not being blasted out into space. Im not sure why you started talking about military nukes when half the point of this thing is how completely infeasible it is for military usage.
This borehole would effectively be a mineshaft. High yield nuclear weapons achieve around 5 Mt/ton, so a 81 Gt device would have a mass of 16,200 tons, about 1/3rd the displacement of a WW2 Iowa class battleship.
The deepest mine (a gold mine in South Africa) reaches a depth of 4 kilometers.
