Wouldn't the "surrounding materials" we're worried about here be the rock itself? The Tsar Bomba was proportionately the cleanest nuke ever, but I thought that also relied on it being an air burst. The Sedan test (creating the Sedan crater using a 70%+ clean fusion warhead) released more fallout than any other US test & I thought that was because it was done in the ground at a depth where reaction products could escape. But you're saying with a clean design & boron based neutron absorbing buffer layer, we could limit neutron activation of the surrounding rock? That does sound plausible, although I don't know what it would actually take & I'm doubtful their estimated $10B budget would hold.
> But you're saying with a clean design & boron based neutron absorbing buffer layer, we could limit neutron activation of the surrounding rock?
Yes, but if the bomb is surrounded by a blanket of boron, it will absorb most of the fusion neutrons. Not _all_, but a significant part of them.
The budget is actually not unreasonable. Fusion warhead scaling is easy, especially if you are not worried about practicalities for combat applications. You just keep on adding bigger stages.
And a boron blanket is literally a boron blanket. You can't do any real engineering for it, it's going to become a highly ionized plasma within the first moments of the explosion, held together only by inertia.
