What is the longest time an unexploded atomic bomb could survive underground?

Question

In the story I'm building, a 3rd atomic bomb (same spec as Fat Man) was created for use on Japan - and dropped.

However, for whatever reason, the 3rd bomb never detonated - impacted the ground and became burried in the ground (as many unexploded munitions from WW2 have been).

In the story, the bomb is to unexpectedly detonate years later - causing political turmoil and accusations.

What is the longest this atomic bomb could be left underground, and still have the potential to detonate?

To extend the timeframe, it is perfectly acceptable to use extremely unlikely but technically possible circumstances that explain away blockers things like "atomic weapons had a 3-day activity due to needing batteries replaced".

It is also acceptable for the bomb to have detonated due to a natural event, or regular construction event (or similar), so long as nobody knows about the bomb untill it detonates.

Answer 1

You can always get a fizzle, and that is bad enough

The Fat Man design depended on an initiator. The initiator sat in the middle of the plutonium core, and when the plutonium was imploded, this crushed the initiator, that then gave an initial "spark" of neutrons. This spark kick-started the chain reaction and gave it a sort of initial boost that was essential to get the full effect of the weapon.

The "urchin" — as it was called — was based on polonium. That polonium isotope has a very short half-life of only ~138 days (this is necessary to get the neutrons needed, something that does not decay quickly does not give off neutrons!). This means that its efficiency drops by a factor of 10 about every 450 days. So in only 4 years, it would only produce 1/1000 of the number of neutrons it did when it was dropped. So you will not get a full blast out of this weapon, probably not even after one year.

However...

There are two factors that will still make this thing a very hot potato...

1. It can still go "prompt critical" if it is initiated. And even if the result is only a fizzle — as a failed bomb is called — the chain reaction will still cause "dirty" fallout.

   And even if you do not get any chain reaction, by having only part of the conventional explosives go off, or going off with poor efficiency after that long time, you will still make powder out of the plutonium core. Although plutonium is not much more toxic than lead _(*), the psychological effect of that will be gigantic and have people fleeing the area as if Death itself was standing there.

2. People are terrified of nuclear bombs and radiation — especially so in Japan — and the scars of Hiroshima and Nagasaki run very deep in the Japanese nation. The "soft" fallout of this will be enormous, even without the bomb itself making another mushroom cloud.

   And in any case: no-one will know for sure that it will not blow up. Do you(!) want to be the explosive ordnance disposal person that wanders up to this thing and tries to find out if it will blow up or not? The conventional explosives in this thing are enough to make you into fine red dust. And the usual method of disposing of old explosives — which is to vapourise them with other explosives — is completely out of the question.

So to summarise: you cannot get a full yield of the same kind that destroyed Hiroshima and Nagasaki, and that state will happen probably within a couple of days when batteries and capacitors run out, for sure within a year as the initiator decays. But(!) you will have a very effective plot hook in that this this thing is still political dynamite in the kiloton range, and everyone will be going around it on tippy-toes both figuratively and literally because the materials still left in it are in an unknown state and are still enough to kill a few people and cause a mass panic if it does blow up.

_{(*) The myth about plutonium being the most toxic substance on Earth is completely bogus. The botulinum toxin for instance is at least 1000 times more potent per unit of mass... and people inject that stuff into their faces to smooth out wrinkles.}

Answer 2

This is unlikely enough that if you want to go down this path, you should just handwave it and forget about it.

The first thing to note is that if you dropped a bomb like that, it won't just bury itself. It will decelerate rapidly, breaking things. You will not have a functioning bomb after it hits the ground -- you will have a pile of radioactive junk.

But let's say the bomb wasn't dropped. Let's say we hiked it in on a humvee, and burried it by hand.

Now you have to deal with the issue that there is an enormous amount of interest from nuclear powers to not have their bombs go off if they fall into enemy hands. This means they are unlikely to go off without an intelligent adversary actively trying to make it happen.

Setting off a nuclear bomb is actually not easy. To do this, we invented the "exploding bridgewire" initiator, capable of detonating several charges all over the bomb with a tolerance of about 100ns. These are not easy to set off intentionally. Having them go off by accident is highly unlikely.

So basically, we're going to have to assume the following scenario: The US military hands all of our nuclear weapon secrets to an unstable mad scientist, who intentionally creates a bomb designed to sew confusion. He designs a bomb with a timer that is designed to go off as late as possible.

Then he buries it.

Now the limit is going to be the shelf life of the material. The nuclear fuel isn't going anywhere. U-235 has a half life of 70 million years. Plutonium's half life is lower, ranging down to as low as 14 years if you use P-241, so there's no reason he'd choose this.

Atomic bombs are dependent on conventional explosives to set them off. If you use RDX, it's shelf life is about 5 years. That would be a major limiting factor. Fat boy used RDX and a few other carefully balanced explosives to achieve its goals. As they decay, this balance would be thrown out of whack.

To simplify things, you could design your bomb after Little Boy, rather than Fat Man. Little Boy's design was much simpler. Little boy only needed a "gun" to fire a ring of material down the body of the weapon. This gun was powered by cordite, and we've got documentation suggesting cordite can stay "good" for 100 years or longer. Also, because it's not a complicated timing-critical process, you'll have more slop room for it to continue functioning as the cordite decays.

Beyond this, your main limit will be the lifespan of the battery. If you use a thermal battery, you may be able to get this into the decades.

Answer 3

So let's say our hypothetical fat man used a wind safety, but instead of driving the detonator into the charge, for some reason, this one-off was designed so that the spinning propeller charges the detonator. This leaves the bomb failure plausible, and the detonator could be in any state depending on static charge buildup.

The explosive (RDX) is stable at room temperature. It is biodegradable, but that would depend on that case being compromised.

A charge failed and a "soft" landing through tree canopy, roof, hay, and/or mud makes this plausible.

The uranium in nuclear weapons is generally enriched to 90% U-235, but only 20% is required for an explosive reaction. I don't know how well fat boy was enriched. The half life of uranium is 4.5 billion years, so about 9 billion years for the fuel to decay due to half life.

The electronics and the detonators themselves are likely the weakest links. Depending on insulation and the presence of oxidizers it's extremely variable. In ideal conditions (such as the Edison bulbs) one hundred years is amazing, but not without precedent.