They also wanted to keep it from the Germans. Before war, much of the foundations of atomic physics were developed by people in the Axis countries. It was plausible that Germany had a nuclear program as big as the American one. Denying them fuel was important.

See the sabotage of Norwegian heavy water:

https://en.wikipedia.org/wiki/Norwegian_heavy_water_sabotage

>> It was plausible that Germany had a nuclear program as big as the American one.

Plausible only for those without knowledge of the extent of the US program. Even among those involved, few had perspective on exactly how much of the US economy was being funnelled into the nuclear program. The Manhattan Project famously at some points used roughly 10% of the electricity available in the united states, a country with an infrastructure relatively untouched by war. There is absolutely no way that WWII Germany could have sustained anything close to a similar effort. They had to brains to understand the physics, but not the raw industrial power necessary to turn that into a practical weapon.

The Manhattan project studied four different Uranium enrichment solutions (none of being centrifuges'; ultimately settled on diffusion) and pursued two completely different bomb designs. The uranium bomb required a high degree of enrichment, while the plutonium bomb required a reactor. I don't have the figures, but I imagine a great deal of the electric power consumed by the Manhattan project was for making heavy water and enriching uranium.

The Japanese had two different atomic bomb projects, as usual, one for the navy and one for the army. As I understand it, one of the (the navy's) got pretty close. But details of the Japanese effort are hard to find.

A much more focused project that consumed a smaller portion of Europe's industrial capacity was somewhat plausible, at least up until the scuttling of Norway's heavy water. A serious German bomb effort could have done the minimum uranium enrichment (using centrifuges, perhaps) needed to breed plutonium in a reactor, and then yielded plutonium bombs, at much lower cost than the Manhattan project. Necessity being the mother of invention, and with all the energetic and industrial constraints they had it's not too farfetched to suppose they would have landed an idea like that. Still, once they lost access to Norway's heavy water, it would have cost them a great deal of electric power to make more, so there's that.

>> Even among those involved, few had perspective on exactly how much of the US economy was being funnelled into the nuclear program.

Which also serves to highlight the incredible cost of the B-29 project — $1.9 billion for the Manhattan project, $3 billion for the B-29.

If you haven't seen it already, the Farm Hall transcripts provide a fascinating insight into the failure of the Nazi nuclear effort: https://www.nybooks.com/articles/1992/08/13/the-farm-hall-tr...

"The Winter Fortress" is a fantastic book about the Norwegian sabotage of the heavy water plant... really highly recommended.

Natural uranium is 0.7% U-235, and enrichments of 80%+ are needed for U-based weapons (like Little Boy). The enrichment process used during the Manhattan project (electromagnetic and then gaseous) not perfectly efficient, and there are losses during conversion, but assuming 100% efficiency that means just Little Boy required 8 tons of raw U.

If you add up all the work going on, all the experiments, all the piles (reactors) off consuming uranium, this figure doesn’t seem too surprising to me.

From the article:

Mines in the US and Canada were considered a “good” prospect if they could yield ore with 0.03% uranium. At Shinkolobwe, ores typically yielded 65% uranium. The waste pile of rock deemed too poor quality to bother processing, known as tailings, contained 20% uranium.

If it were 65% uranium, the mine itself would be a massive nuclear reactor. (A situation that did happen elsewhere in prehistory.) It would heat up and into a red-hot cauldron. 65% is the richness of the ore not the percentage of uranium in the rock overall.

This is NOT the case, even if the mine was 100% Uranium this would not happen, at least not today. As someone else pointed out, the world did have a natural reactor, Oklo, Gabon 1.5 billion years ago.

When Uranium is refined, they are enriching the U235 vs U238. The ratio is too low today to sustain any natural reactors, but 1.5 billion years ago this wasnt the case.

Not the case on day-1, but an open pit of 65% uranium isn't a typical lab experiment. Add some rain, some evaporation, some oxygen, countless other micro effects, and after a while you will get localized concentrations of U235. Then things start to get interesting as the fertile U238 is changed into other things. I'm not saying that that actually happened, just that a hypothetical multi-kiloton pile of 65% natural uranium isn't a neutral pile of sand.

Evaporation, oxygen and any number of other effects won't change the ratio of U235 and U238. Those isotopes are chemically identical. The only way they are different is by their different atomic mass, i.e. their gravitational effects. And since the atomic mass is different by only about 1%, even the gravitational effects are almost identical. So U235 constitutes between 0.71% and 0.72% of the Uranium everywhere in the world.

Exactly as the other guy posted. U235 doesnt do anything "interesting" until you get it to around 3%.

Due to somewhat obvious reasons (not a lot of net-new uranium being formed, and the half life is fairly stable), the Uranium concentration is pretty uniform around the world and is something like: uranium-235 0.720% uranium-238 99.275% uranium-234 0.006%

Evaporation and other factors wont change this, it hasnt changed this ratio in the muti-billion years Uranium has been on earth. What does change this is the half life of U235. U235 wont naturally 'accumulate', it doesnt move around in rock to form higher concentrations, it is generally 0.720 everywhere.

The natural reactor ran becaues back then there was simply more U235 (half life 700 million years) vs the half life of U238 (4.5 billion years)

No Hypothetical examples needed. Open pit mines wont do anything 'interesting' unless you can either go back in time, or reverse the U235 Decay and then add water and you will probably get something like what took place at Gabon.

Quite interesting, do you have an article, or a paper about the prehistoric episode you mentioned? I’m genuinely curious.

The only naturally occurring nuclear fission reactor was found to be in Oklo, Gabon in Central Africa and was thought to be active about 1.5 billion years ago.

https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...

This provides much better coverage: https://blogs.scientificamerican.com/guest-blog/natures-nucl...

Those are the yields of uranium in the ore. The 80%+ needed is the quantity of the Uranium-235 isotope.

This was something i read recently in the history of Belgium.

Since the World War II, Belgian shared their Uranium supply with the US and the UK ( https://en.wikipedia.org/wiki/Belgian_Congo_in_World_War_II#... ), done as part of repayment for allied support in the war.

So this little country was actually the main / sole supplier through Congo for the atom bomb / nuclear energy in the US.

Congo gained independence in 1960.