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Old 2020-08-10, 03:02   #221
Dr Sardonicus
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Feb 2017

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Originally Posted by ewmayer View Post
I hate such later hagiography where the role of the actual other 'lesser' scientists who made the discoveries get written out in favor of the Big Names. In this case, Joliot-Curie, Enrico Fermi and Leo Szilard, who approached the by-then-elder-statesman Einstein because they figured someone of his stature would be more likely to catch the president's ear.
Yes, this is the story I learned. WRT discovering the potential of uranium, the name Lise Meitner also comes to mind.

You might be amused at a misdiagnosis by Dr. Einstein in this regard, mentioned, e.g. here:
In 1932, the great physicist claimed that "there is not the slightest indication that nuclear energy will ever be obtainable. That would mean that the atom would have to be shattered at will."
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Old 2020-08-13, 23:35   #222
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Sep 2002
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Re. the Shinkolobwe mine, my memory got mixed up re. the natural fission reactor - it's actually at Oklo, Gabon, a couple W African statelets NW of the Shinkolobwe mine:
The natural nuclear reactor formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a nuclear chain reaction took place. The heat generated from the nuclear fission caused the groundwater to boil away, which slowed or stopped the reaction. After cooling of the mineral deposit, the water returned, and the reaction restarted, completing a full cycle every 3 hours. The fission reaction cycles continued for hundreds of thousands of years and ended when the ever-decreasing fissile materials no longer could sustain a chain reaction.
A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope U235 made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 96.9% was non-fissile U238.) Because U235 has a shorter half-life than U238, and thus decays more rapidly, the current abundance of U235 in natural uranium is about 0.70–0.72%. A natural nuclear reactor is therefore no longer possible on Earth without heavy water or graphite.

The Oklo uranium ore deposits are the only known sites in which natural nuclear reactors existed. Other rich uranium ore bodies would also have had sufficient uranium to support nuclear reactions at that time, but the combination of uranium, water and physical conditions needed to support the chain reaction was unique, as far as is currently known, to the Oklo ore bodies.
Note that the U235/U238 isotopic ratio at the time of Earth's formation was a global constant, due to whatever primordial supernova(e) which created the heavy elements incorporated into the Earth having mixed said elements more or less uniformly throughout the presolar nebula.

Getting back to the question of the % of U and Pu atoms which fissioned in the Hiroshima and Nagasaki A-bombs - this would fit better in the Science subforum, but by way of continuity:

Hiroshima: Wikipedia says: "[Little Boy] contained 64 kg (141 lb) of enriched uranium, although less than a kilogram underwent nuclear fission."

For the Nagasaki A-bomb, Wikipedia says "6.2 kg of Plutonium-239, [of which]about 1 kg fissioned", thus more than 10x more efficient than Little Boy.

Modern fission weapons (most often used as the initial small 'trigger' stage of a fusion bomb) achieve significantly higher efficiencies even than Little Boy via use of fusion boosting, so we could conceivably be talking of on the order of half the atoms of fission core undergoing fission.

Last fiddled with by ewmayer on 2020-08-13 at 23:36
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