How effectively recover enriched uranium from nuclear impact site?

Question

In the world(not Earth, but eath-like), I am building there was a nuclear exchange recently. Recently, I mean 25(subject to change) years into the past. I am describing a small clump of cities near the coast, they were protected enough that they were unharmed by nuclear impacts.

After surviving initial difficulties they now need power. Among those cities, there is one big city, like New York or Shanghai and there are some nuclear power stations.

Nobody knows if any more parts of the world maintained the pre-war level of technology.

Speaking of the level of technology.

It is similar to Saraksh from Prisoners of Power of Noon Universe. There is nuclear power, but there are no personal computers and no internet, there is some advanced crop designing, but everything is controlled via buttons and sliders. Computers are big, slow, and expensive.

The question is if no uranium or thorium mines are present and provided that significantly large quantities of depleted uranium are stored. How feasible it is to recollect enriched uranium particles from nuclear explosion sites to power a nuclear reactor?

The type of nuclear reactor is subject to change, there may be even different reactors in different blocks for pre-war research reasons. The same applies to the design of nuclear warheads, what is inside them is a degree of freedom to play with.

I know it is probably insanely inefficient, but if no other options are present, how much can be recovered?

I would really appreciate some upper bounds relative to the nuclear explosion center. My google-fu is not strong enough.

My thoughts so far:

• One can find unexploded warheads. That's a win, but one needs to have a special reactor if the warhead has plutonium in it.
• Found uranium is already highly enriched, so one can use it almost immediately after decreasing enrichment to the desired level. So only chemical extraction is required, no need for power-hungry centrifuges.
• Maybe it is a good idea to use something like a lunar harvester concept for harvesting helium-3 or spice harvester from Dune. Several things are the same: the dust, the preciousness, and the need for a mobile factory for the initial processing.

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Edit1 with some numbers:

• according to this only 4% of uranium is used in nuclear fuel
• according to this(2.1.4.3) only 25% of uranium is used in a nuclear explosion

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Edit2:

This is not Earth, but an Earth-like planet with one big continent that was nuked to hell, and many islands that were not, but are controlled by a very closed society with Japan-like culture. They have big naval forces(post-WWI-like vessels with a lot of big cannons). Their tech level is lower than on the pre-war continent and they probably do not know how to make energy via fission.

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Edit3:

I am dropping the hard-science tag because the idea of spice or helium harvester seems appropriate as a solution, but not appropriate in a hard-science setting.

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Edit4:

As I understood from the answers below there are some problems with the idea of collecting uranium in desired way:

1. Airborn explosion lifts everything too high up. All unreacted fissile material will be effectively scattered uniformly around the globe. No gathering will be possible here.
2. Pure fissile bombs were produced only in a very short window of our history. And most of them were plutonium ones.
3. It is far more productive to go to other cities and collect natural uranium or unused fuel rods.

The third one is easy: I know it is far more feasible to do. I just think that it is might be interesting to explore another possibility.

The solution to the first problem is provided by Starfish Prime in his answer below. Bunker busters and nuclear mines explode on the surface and leave very radioactive areas. If they are made from uranium it is possible to recollect some from them. Not much is needed if it is highly enriched uranium.

The solution for the second problem is the most complex one. As only one bomb design is feasible for all this to work, I need to create an environment where only one design is possible. A good way to do it is when such design is stolen, given, or captured.

Initial idea is that somewhere among the islands there was a former empire. With technology ahead of everybody else for about 35-50 years. A good way to set up this is to make the continent into a prolonged everybody-for-themselves-styled war.

After the war on a continent is settled there is tension between the former empire and the new upper dog of the continent. After some time another war bursts out. Due to superior numbers, the continent's armies are able to win. Not without paying a heavy price. The first nuclear explosion in this world appears on the outskirts of the capital city. It does not save it from capture, though. The former empire fell, but the effort is made not to let new destructive technology to fell into the wrong hands. Much of it is destroyed.

The winners are trying to replicate the technology, with a 35-50 year technology gap it is possible to replicate the existing technology, but not to improve it.

So the first bomb was made from uranium, resembling Little Boy. The enrichment process was likely the same as for early nuclear devices. Thus, no centrifuges are involved.

After the end of the second war in a row and many dead from the use of a new weapon, civil unrest is splitting territories of the former upper dog of the continent while newly acquired technology is leaked.

Each independent state is trying to replicate new technology and many eventually succeed. As design can not be improved, only stockpiled in large quantities while borders are mined with nuclear devices.

After some local conflicts, it is recognized that nuclear weapons are not very usage-friendly and many major countries agree to ban the usage and production of such weapons, while research continues.

After some time the situation is as follows:

• Each independent country has a stockpile of old nuclear warheads made from the same initial design. With some low quantity of illegal new ones.
• Each of them implemented some kind of dead hand protocol.
• Research is approaching the post-WWII level. Civil reactors are built and computers are invented.

Then some event triggers dead hand protocols and nuclear exchange happens. Making the world in a state it was before this post was made.

P.S. I know there are a lot of details to figure out for this scenario to be realistic. I just tried to come up with an initial painting to satisfy all constraints. I would like to hear suggestions though.

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Edit5:

How hard it is to fuel a nuclear power station using fuel from unexploded Little-Boy-type bombs?

Assuming these facts hold:

1. Reactor is 1GW PWR
2. Reactor eats 27 tonnes of 1% enriched uranium per year
3. Little Boy contains 64 kg of 80% enriched uranium

Math:

Little boy Fissile uranium mass = 64 * 0.8 = 50 kg

Fissile uranium needed for PWR  = 27'000 * 0.01 = 270 kg per year per GW

Total GW-year per Little boy    = 50 / 270 = 0.18

To summarize: It is hard to power a full-scale power station with unexploded little-boy-styled bombs. It is not impossible though.

P.S. If one has heavy water figured out then it is possible just to use a CANDU reactor and never think about fuel enrichment ever again :)

Answer 1

TL;DR:

• hardly anyone used pure uranium devices in history, especially by the time civilian nuclear reactors existed
• airbursts leave you no recoverable uranium
• only very specific weapon designs are going to be of use (pure uranium ground penetrating ones)
• you'd be better off reprocessing nuclear waste than scavenging nuclear blasts, if all you want is uranium (but plutonium might be another matter).

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So, there's a certain amount of ambiguity and possible anachronicity in your setting, so lets start by listing some real world things:

• Highly enriched uranium weapons exist in a very brief technological window. The Manhattan project showed that if you can make a HEU device like Little Boy, then you aren't far off being able to make a working plutonium device like Fat Man. The US did make a number of Little Boy-type weapon assemblies, but apparently retired them all by the early 50s and went with the plutonium flavor only until they had working thermonuclear devices.
• The British (initially working with the US) and the Soviets (spying on the US) went directly to plutonium-fuelled warheads.
• Working thermonuclear weapons appeared pretty promptly. Castle Bravo was in '54, just 9 years after the bombing of Hiroshima.
• Pure uranium-based warheads seem to have been phased out by 1960 by the US and USSR.
• Nuclear reactors were used to make weapons-grade plutonium before they were used for power generation.
• The first civilian power generating nuclear reactor, at Obninsk, went online in '54, having started construction in '51.

So what does this all mean?

There's a very brief window in real world history when it would have been possible to dismantle an active-service nuclear weapon and use it to fuel a conventional power-generating reactor, and suitable warheads would likely have been outnumbered by plutonium and thermonuclear devices.

• only 4% of uranium is used in nuclear fuel

4% is used per cycle. Real world uranium is cheap, so we don't make much effort to reprocess "spent" fuel. This means that nuclear waste depositories are actually surprisingly reach sources of uranium. The problem is that they're often hot and toxic and radioactive, and so if other options were available you probably wouldn't want to "mine" them, but you might consider that reprocessing waste is a much better way to get reactor-grade fuel than scavenging a battlefield.

• only 25% of uranium is used in nuclear explosion

If you're making a 40s-era pure-fission uranium bomb, it might be even worse... wikipedia suggests that burnup might have been under 2%. That means over 60 kilos of highly enriched uranium remained. It would have been thoroughly vaporised in the explosion though, and as uranium is highly pyrophoric it would have reacted with the air and formed fine uranium dioxide dust which will readily disperse in the atmosphere and spread over a large area. Wikipedia implies that there was little fallout from the Little Boy explosion, as it was an airburst... the uranium dioxide would likely have been lofted into the stratosphere and spread around the world.

In order to keep a reasonable amount of the unfissioned uranium in a small enough area to make it worth trying to mine it, I suspect that you'd need an underground blast. Weapons intended for this purpose have been called Atomic Demolition Munitions. The US-built W54 warhead would have been used as part of such a weapon, but it was a plutonium-based device. No pure-uranium devices of this sort were designed to my knowledge, but you could handwave such a thing if it seemed useful.

Your best option might be a nuclear bunker buster, a bomb optimised for penetrating ground or construction over a target before detonating. The US built the Mark 8 nuclear bomb for this purpose... this was apparently a gun-time uranium device, though I haven't found a better source for that information. The Mk. 8 was in service from 1951 to 1957, and so definitely existed and was ready for use at the same time as the earliest civilian power plants.

The cryptically named Buster-Jangle Uncle test was intended to simulate the use of a gun-type ground-penetrating device. The blast did breach the surface, as you can see from photos of the test:

but much more of the fallout (including unburnt uranium) would have been deposited in the ground at the blast site and on the surface immediately around the blast area, something confirmed by the very high levels of radioactivity around the site after the blast. This seems to be the most plausible way to get macroscopic quantities of enriched uranium from a nuclear blast site. I'm not going to estimate concentration of fissiles, or whether it is sensible or economical to recover them, because it seems like if you have nuclear power plants then going after nuclear waste is far easier and will likely yield far more uranium in greater concentration.

Answer 2

The temperature of a nuclear explosion is high enough to vaporise the core and scatter the dust over a wide area. You would be better off by restarting old mines that were abandoned (or never opened) because the ore concentration fell below the economically feasible threshold, such concentration would still be higher than the concentration over the site of an explosion.

Keep in mind that high radiactivity does over the area does not mean high quantity of useful material because the most radioactive elements are fission products not fissile materials.

Otherwise, as suggested by this question, you could add to the story some fighting involving a lot of depleted uranium ammunitions, after the fighting subsided it could be easier to recover than low concentration ore, but you would need thousands of projectiles. An alternative could be finding some depots of unused anti-tank ammunitions. The trouble is that DU needs some fissile material to start a reaction, but it could be pre-processed with a neutron source, costly, but feasible.

One can find unexploded warheads

One single warhead is not much, a between 5 and 7 Kilograms of plutonium, you would need several, but they would be useful. Plutonium can also be burned in a conventional reactor, there are a lot of reactor currently using mox recovered from nuclear waste. To improve the reaction you can replace the light water with semi-heavy water which is easier to distil than heavy water. The classic PWR would still work and require lower quality fuel.

Answer 3

Go to other cities inoperable power plants.

Spent fuel rods and Waste materials stored in cooling ponds. Most nuclear power plants, if I recall correctly, store spent materials on site in cooling ponds. That would provide a source of material that could be refined and might already be packaged in ways it could be transported.

If the reactors have SCRAMed then depending on the the design fuel rods could be pulled from the core. Perhaps on site there are also unused fuel being stored in anticipation of a planned refueling.

There are large amounts of depleted uranium that DOE has a waste product and several years ago. They kept trying to find uses for it, and one suggestion was counterweights for fork lifts. Older 747s also used it as counterweights to help balance the tail sections of the airplane. Anyway, perhaps a road trip to Oak Ridge in Tennessee…

747s and some types of ammunition have switched from depleted uranium to tungsten.

In Washington State there are a bunch of reactor cores due to arms control agreements that are buried… a different kind of road trip to Hanford.

I don’t think gathering explosion materials would be that feasible unless you have something like a tiny robots that swarm to find and maybe gather the materials.