Underground nuclear weapons (not for testing; as offensive weapons)

There were some post-WWII plans to bury atomic weapons in Germany, to be detonated by timer or remote control in the event of a Soviet invasion. This would mess up the Soviet plans by contaminating large areas and generally being a bummer (project Blue Peacock/Blue Bunny/Brown Bunny). There were also some schemes to use underground atomic bombs to widen the Panama Canal, or cause fractures to encourage natural gas release, or create artificial harbors. And of course there were quite a few underground atomic tests so as to minimize fallout (with varying degrees of success).

I’m curious as to whether there was ever any consideration to using fission or fusion weapons, not as airburst or ground-blast weapons, but as deep, offensive mines so as to minimize fallout. Never you mind the delivery system - maybe a deep penetrating bunker-buster type thing, or saboteur teams or something. Call the sappers and start them digging from a couple of miles outside of town. We’re brainstorming here. IF you could get a bomb into place under Moscow or Washington DC, would it make an effective weapon IF you wanted to keep fallout from escaping? Or does making sure the blast stays contained keep it from being a big enough boom at the surface? In my imagination, the rationale behind this is getting the biggest possible explosion with the minimal radioactive fallout, and maybe for the psychological effect.

You could potentially drop a lot of buildings directly over the blast, and wreck the city’s sewer and water and subway systems, and probably power distribution (do you get an EMP from underground?). But you wouldn’t put a lot of fallout into the air to blow places you don’t want irradiated. OTOH, maybe you’d just wreck a single city block on the one hand (too deep) or lose containment and catch all the international flack on the other hand (too shallow), with no margin for error.

Brings to mind the Battle of Messines - (Mines in the Battle of Messines (1917) - Wikipedia)

Loved the British officer’s quote:

“Gentlemen, I don’t know whether we are going to make history tomorrow, but at any rate we shall change geography”

This goes back to the sappers who undermined castle walls in medieval days. Problem is getting that close to enemy lines - and underground mikes listening for the incredibly slow process of tunneling is pretty standard wherever that’s a risk.

Details on Project: Blue Peacock/Blue Bunny/Brown Bunny

This has definitely been studied. It’s late so I’m not going to look for a report, but there’s decent unclassified stuff on nuclear earth penetrators.

Judging from historical records of underground nuclear tests, if you detonate a nuclear device deep enough to avoid ejecting fallout to the atmosphere, the damage to the surface doesn’t appear to be very widespread.

Pictures of most underground tests show a subsidence crater at the surface that’s maybe 100 meters in diameter. Even the Sedan test, which was intended to excavate a crater, only produced a crater about a 1/4-mile wide, but that doesn’t fit your scenario as it released quite a bit of fallout.

So a weapon buried deeply enough to prevent atmospheric fallout would be enough to destroy the US Capitol building if detonated directly beneath it, but I think the surrounding area would be subjected to merely tolerable earthquakes, i.e. some cracks and broken windows, but not utterly shredded infrastructure on a city-wide scale.

Put a subsidence crater under a large building and the building is (non-dramatically) destroyed. Especially the rigid and heavy edifices typically used for government and military purposes.

But you still get a lot more bang for your nuclear warhead buck by detonating at altitude (for normal buildings) or at ground level (for hardened surface buildings and shallow underground works). Normally, fallout on hostile territory is a non-issue: your troops button up during the early phases (while the fallout is still fresh and hot), and after a few weeks it’s just a background nuisance that you don’t concern yourself with (since it’s just your cannon fodder and the enemy exposed to it).

There are three mechanisms by which nuclear weapons are destructive. Radiation, flash/heat, and bladt/overpressure.

Subterranean bombs effectively negate the first two, and severely curtail the third which is why that was the best way to test them.

Generally speaking, the cratering or retarcing* is likely to be the most destructive thing a buried nuke would cause. But they’re not huge - you could seriously mess up a city block or facility, but that’s about it.

(a retarc is when the nuke detonation generates rock rubble with more volume than it originally had, forming a bulge instead of a crater- hence retarc, crater spelled backward)

See: Effects of Nuclear Earth-Penetrator and Other Weapons |The National Academies Press

Fallout is the most valuable part of a “strategic” (= counter enemy homeland) weapon. Why would any sane adversary want to limit that?

If you were trying to annex the enemy territory and were planning on occupying it yourself?

I don’t believe this is true. Airburst is designed to maximize overpressure and minimize fallout. You might use groundblasts to maximize groundshock in order to destroy silos, but you’re not doing it to maximize fallout.

You’ll be perfectly safe occupying fallout-contaminated land in weeks. Especially if you’re occupying with expendible troops.

Fallout loses about half its energy within the first hour.

https://remm.hhs.gov/nuclearfallout.htm#fig2

(Trying to keep this hypothetical firmly hypothetical)

But if a belligerent nuclear Lithuania wants to annex the Kaliningrad Oblast for some lebensraum, and have its own people live there, it would seem that the best way to do that would be with zero fallout. So as not to enrage the Poles, Finns, Estonians, Belorussians, Latvians, and Swedes, or get fallout on Vilnius, if nothing else. Or maybe the Lithuanians don’t want to wreck the world amber market for (four half-lives of something) years?

I would not have been keen on being the first to buy new postwar housing stock in Nagasaki.

Radiation levels in Hiroshima and Nagasaki dropped pretty quickly after the bombings.

Fallout also gets everywhere. Maybe you don’t care if the enemy country gets contaminated, but you don’t want yours contaminated, too.

There were also nuclear weapons that were designed to penetrate the ground before exploding in order send shock waves into hardened bunkers.

Exactly. Not the same thing, but the first clue about Chernobyl in the west was when workers leaving a Swedish nuclear plant set off the radiation detectors. Turns out the detectors were picking up the outside air.

One problem that limited the use of gas in WWI was the unpredictability of the wind - and that was for just a temporary local ground-hugging situation. Radioactivity going tens of thousands of feet in the air, thanks to a large thermal updraft, is rarely a good idea.

Maybe it is possible to make defensive buried nuclear defense. If you can calculate the variables, you might be able to create a barrier against heavy vehicles. Recently disturbed ground, such as being up heaved from below can be very soft. Explode it up and when it comes back down it is no longer compacted. So heavy vehicles will bog down. Just a wild guess.

Might work, but a conventional anti-armor minefield is cheaper and has fewer lasting effects.*

Plus also potentially destroying enemy vehicles instead of just bogging them down.

*if deployed by a responsible combatant who will record placement and then de-mine after hostilities end.