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.