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Situation:

  • there is an are of likely attack, something in line of Fulda Gap which looks indefensible in case of serious war, so classical approach would be just a barrage of tactical nukes
  • the value of this territory is actually low and there are no political objections to openly declaring scorched earth policy
  • the technology level is modern, however its a back up to plan that is supposed to work even in case being confronted with one or two generation more modern equipment (like for example point blank laser defence against tactical missiles or some really good bunker busting bombs)

Weapon designed:

  • U235 (cheaper than plutonium), not boosted (no maintenance), implosion
  • usual lithium deuteride for second stage
  • XXL U238 casing
    (yes, so classical Teller-Ulam design, but with uranium as weight is not longer limit and casing can be made thicker, possible to add another stage if necessary)

The idea is to build big inexpensive simple bomb and bury it deep enough so it could not be easily neutralised by any attacking force. Possible to set it in case of emergency to explode after loosing contact with HQ

No, not tiny Atomic demolition munition

OK, so the question what's a reasonable depth to avoid demining efforts even if enemy had good intelligence (with bunker busting bombs) and what's the biggest realistic yield to avoid wasting the fissile material in pointless overkill? (after adjusting for the fact that it would be an underground explosion)

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    $\begingroup$ @b.Lorenz the question implies that the enemy has "good intelligence" so saturation of the whole area is not required. Using much smaller warheads than 1MT would be practical, so the overall destruction to the terrain would be less. Cold-war era battle tanks are quite resistant to fallout, even fresh fallout, and are good in broken terrain, so the principle job of this landmine would be to destroy a tank formation in the initial blast. $\endgroup$
    – Starfish Prime
    Jun 9, 2019 at 13:36
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    $\begingroup$ One megaton? No offense, but are you kidding? The bomb the U.S. dropped on Nagasaki was estimated to be about 18 kilotons. It destroyed a city. Did you perhaps mean "one ton?" Because I frankly can't imagine any reason for such a large device being designed for a foot soldier to step on. A one-megaton (or kiloton) land mine is unbelievable by definition. Even my suggestion of a one-ton nuclear land mine might be unreasonably large. $\endgroup$
    – JBH
    Jun 9, 2019 at 18:39
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    $\begingroup$ A megaton land mine is a salted bomb: en.wikipedia.org/wiki/Salted_bomb Atomic demolition munition were deliberately tiny to provide only local effects. The fallout of a megaton bomb would reach the stratosphere, and from it, almost anywhere in the world. Since it would take weeks to months to all of it to come down, there's no way to predict where the hell the fallout is going to concentrate the most. That's playing russian roulette with radiation. $\endgroup$
    – Rekesoft
    Jun 10, 2019 at 11:12
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    $\begingroup$ don't forget to ensure your chickens have enough food and water to survive en.wikipedia.org/wiki/Blue_Peacock $\endgroup$
    – mgh42
    Jun 11, 2019 at 4:28
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    $\begingroup$ @mgh42 Consensus is to bury it much deeper, thus for heating it up bats would be more suitable ;) $\endgroup$
    – Shadow1024
    Jun 11, 2019 at 4:30

2 Answers 2

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Davy Crockett!

Atomic demolition munitions even though ruled out by the OP would be fine for this use although they are apparently less fusion more fission because the radiation is an undesirable output. Not sure why these were ruled out. Want big boom?

Clicking on the link to Fulda Gap brought me to one of my favorites, the Davy Crockett.

Both recoilless guns proved to have poor accuracy in testing, so the shell's greatest effect would have been its extreme radiation hazard. The M-388 would produce an almost instantly lethal radiation dosage (in excess of 10,000 rem, 100 Sv) within 500 feet (150 m), and a probably fatal dose (around 600 rem, 6 Sv) within a quarter mile (400 m).

Perfect for this application. Each one is about 50 lbs. Field tested and tech works. It is not the fault of the nuke that they don't fly straight. The Fulda Gap is not amenable to clearing with bunker busting bombs: bunkers are small and you know where they are; the Fulda Gap is many square miles. Many small mines offers redundancy - if you big one is taken out you are all done, but even if half the little ones get taken out you still have a serious deterrent. Even if you got your big one to work you would need more than one.

You could rig these with electromagnetic sensors to detonate when something big and metal went over them. Better would be to detonate manually by satellite or drone but if you had those you would probably not be doing a 1960s tank attack.

If you do not want to worry about clearing mines later and you have a deadman switch allowing these mines to detonate "if they lose contact with base" then you have the ability for an observer to detonate them manually one by one as with an IED. I am wearing my gold-lined shorts for that job but I have been looking for an excuse.

Really the Fulda Gap is not easily blocked by mines of any sort, as far as I can tell. Fulda to Kassel is 100 km; the width of the gap. That is a boatload of mines, big or small.

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  • $\begingroup$ Yeah, I was thinking that short-range ballistic missiles would be the way to go, and lo and behold the Fulda Gap planners thought pretty much the same thing. Big nuclear landmines seem a little bit silly! $\endgroup$
    – Starfish Prime
    Jun 9, 2019 at 13:37
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    $\begingroup$ @Starfish Prime: I could imagine them being laid as a desperation move, and then posing a problem for the inheritors of the land. Or a solution for some ragtag band of misfits willing to use a decades old finicky big nuke they exhume. $\endgroup$
    – Willk
    Jun 9, 2019 at 13:44
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I estimate from this and from this description stating that the greatest US conventional bunker buster has penetration of 61 m (presumably in earth), and that quite large (340 kt, so its explosion at a shallow penetration would be little better than the mine itself) nuclear bunker busters can destroy hardened facilities (reinforced concrete) at 70 m depth, that a mine being buried 200 m deep, using modern hardening (shock absorbing cabin, etc) would be quite safe from near term demining efforts.

Being buried so deep would of course reduce the effectivenes of the mine once activated. You can not count on prompt radiation (heat, UV, gamma, neutron or otherwise) and primary blast damage, but the above sources indicate that a 1 MT bomb would be more than enough to "unbury" itself, (300 m of dry hard rock being specified to fully contain a 100 kt explosion), and thus shower radioactive gravel on a large area. To increase effects, the bunker in which the warhead is hidden could incorporate high-stability conventional explosives, designed to crack it open seconds before the 1 MT mine is activated

As for wastefully high yield, I do think that mine yields up to Castle Bravo (15 MT) would not be wasteful. Very large airburst thermonuclear weapons "waste" by radiating out their energy into space, but this is virtually prevented in the underground detonation case. So a bigger explosion would just throw more debris and farther, and produce more fallout, which is desirable if it is to block a large zone even if it is triggered before the main force of the enemy comes

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  • $\begingroup$ Thanks! May you link me source for this 300m of rock to contain 100 kt? BTW - in this weapon size gamma and neutron radiation would not matter much anyway, as their effective range grows slower than blast radius. $\endgroup$
    – Shadow1024
    Jun 9, 2019 at 19:29
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    $\begingroup$ Soory, it is the first link, which itself references this: Glasstone and Dolan, The Effects of Nuclear Weapons, 3rd Edition (Washington, DC: U.S. Government Printing Office, 1977), p. 255, Figure 6.72a. (of course I did not look up that) $\endgroup$
    – b.Lorenz
    Jun 10, 2019 at 8:12

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