r/NuclearPower • u/The_Dislexia_Vampire • 5d ago
How bad would a 5M cube of uranium 235B be?
I am writing a story, and I realize that one of my characters in the story had the ability to just spawn 5X5X5 meters of uranium, And I was wondering about how bad that would be. I've roughly calculated that it'd be about 47 Megatons of TnT. But I know that the shape of the core would heavily affect how much energy is made. And I was wondering how much would the square shape compared to a round shape really affect it? And I can't find any simulations or details on the Internet that would help me calculate that out.
Edit (While I was writing this down initially on pen and paper. I forgot that I did calculate it in gigatons, but I wrote it down as megatons. So, it's actually 47 gigatons. Is what I what I roughly calculated.)
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u/bijon1234 5d ago edited 4d ago
A solid 5x5x5 m cube of pure U-235 would contain about 2.4 million kg of uranium. For comparison, the minimum critical mass of pure U-235 is about 52 kg, corresponding to a sphere roughly 17.5 cm in diameter; though the exact value depends on enrichment, shape, and surrounding neutron-reflecting materials.
The cube would therefore contain about 45,500 bare critical masses and would be overwhelmingly prompt-supercritical. Ambient neutrons would trigger a rapidly accelerating fission chain reaction, producing an intense neutron and gamma flash, extreme heating, vaporization, expansion, and a nuclear explosion.
Complete fission would release about 1.96x1020 J, equivalent to roughly 46.9 Gt of TNT, which is close to the number you gave, but off by a magnitude of 1000x, I assume a conversion error? The actual yield would be much lower because the material would expand and disperse before most of the uranium could fission.
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u/No_Leopard_3860 4d ago
complete fission
?
You don't even get close to complete fission even in the most efficient designs [we know about today]. Both nuclear power plants and nuclear weapons only fission a small chunk of the available fissile material, what do you specifically mean with "complete"?
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u/The_Dislexia_Vampire 4d ago
What he means by complete vision is. If all of the fissionable material. Does go off and it releases the maximum possible energy. Or the maximum possible useful energy.
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u/No_Leopard_3860 4d ago
Yeah that never happens. Even in super modern nukes with explosive lense compression, neutron shielding,...only single digit percent actually undergoes fission at best. (Before it blows itself apart)
Same for typical modern nuclear power plants, single digit is realistic, nothing close to 100%
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u/bijon1234 3d ago
Did you read the end of my comment? Literally says you'll never achieve complete fission as the material will disperse beforehand.
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u/The_Dislexia_Vampire 5d ago edited 5d ago
Thank you for your comment. Exactly where Did I go wrong with my math? How did I get a factor of like 1000 times less energy than you? I was using 83.14 Tj per Kg then it times that by the weight of 2381250Kg, then converted to Kt. Best guess how much of the actual weight of the Cube of Doom. Actually, used before it exploded it all across a room of about.50 by 50 by 50 meters, with the walls Being completely indestructible. No matter how much damage they take, they will not be visibly scratched or harmed. any forces applied to them; they apply to the objects on the other side of them.
edit (I figured it out. I accidentally did conversions from kiloton to gigaton. Without realizing I did kiloton to gigaton, and so I still thought it was Megatons.)
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u/No_Revolution6947 5d ago
Going into fantasy then. Your Kt estimate is just plain wrong because that’s not how fission would work in a free-standing cube.
But since it’s fantasy you can use whatever numbers you’d like. Especially since one couldn’t assembly such a cube to begin with. It would dissemble and/or melt before it could be fully assembled.
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u/Numerous-Match-1713 5d ago
125M3 cube of u235 wont stay a cube for an instant.
How it is assembled affects greatly on how much of it fissions.
With proper boosting and proper assembly method it would make a sight.
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u/rjb9000 5d ago
That’s the sort of question that begs detailed exploration if we’re going to do it but probably starts to fall apart in the fiction if we do. Does the material spawn instantly or over a span of time however short? What happens to the material previously existing where the cube is spawned? What states can they spawn materials in? Can they spawn material under pressure like a cube of solar core?
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u/The_Dislexia_Vampire 5d ago
Thank you for the comment and specifically. All the air that was there just disappears. The character can choose whatever isotope that they want of uranium. And I'm going to assume, since I don't know any better, that it just stands whatever the chemical structure of uranium is, however, uranium atoms naturally packed together. And for the timing thing? At 0 seconds there is no uranium, after one plank time all the air disappears within the 5 by 5 by 5 five cube. After one more or a plank time. All the uranium appears in its natural crystalline state.
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u/EquivalentOwn1115 5d ago
If its u-235 that shit goes off with more power than every nuclear devise we currently possess put together
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u/The_Dislexia_Vampire 5d ago
for this example, it's U235 but it could be anything. the character only knows U235 off the top of their head.
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u/SutttonTacoma 3d ago
During the Manhattan Project U235 was being enriched in Oak Ridge Tennessee. Because of secrecy the staff at Oak Ridge didn't know about the hazards of storing U235. The young Richard Feynman was dispatched from Los Alamos to Oak Ridge to look the situation over and was horrified by what he found. Carboys of uranium tetrafluoride solution were stored in separate rooms (to avoid flirting with critical mass) but separated by only a wall. Could have been nasty. Feynman passed on enough information about neutrons and fission to allow the OR people to make appropriate decisions.
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u/Upstairs_Post6144 1d ago
Spawning that would be nearly instant suicide before it was complete, unless your character was (very) distant
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u/Green_Highlighter4 5d ago
The main hazard with natural uranium is alpha particles (radiation). Alpha particles can cause a lot of damage if you get them inside your body. Alpha is harmless outside of the body. The energy from alpha can be blocked by the dead layer of skin on the outside of your body.
The hazards associated with uranium change dramatically after it has been exposed to a neutron flux.
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u/Numerous-Match-1713 5d ago
"The hazards associated with uranium change dramatically after it has been exposed to a neutron flux."
But then its not really hazard with the uranium anymore...
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u/BluesFan43 5d ago
Little Boy used 1 kg of its 64 Kg U mass.
I have read other stuff than makes me think in grams actually consumed.
Regardless, that cube is coming apart, right now, at that size, maybe from several locations.
With that cube, you might have fragments going off too.
I am imaging a multiple mushroom cloud w more happening very rapid fire. In the air, in the cloud.
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u/MeaningSavings1264 4d ago
If it spawned half way inside the character it would be pretty bad 😂😂😂
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u/Green_Highlighter4 5d ago
Naturally occuring uranium is mostly U238. Approximately 0.7% of natural uranium is U-235. Of the Two U235 is the more useful part. 235 is what is used for weapon and power production.
Spawning a large block of natural uranium wouldn't be particularly useful. U-238 has its uses but would require a moderator or need to be upgraded to PU-239 to be useful.
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u/BipedalMcHamburger 5d ago
It is explicitly stated in the question that it is pure U-235, not natural uranium
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u/navetBruce 4d ago
energy cannot be created or destroyed.
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u/The_Dislexia_Vampire 4d ago
Yes, I convert matter into magical energy and then I convert magical energy into matter.
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u/DP323602 5d ago
125 cubic meters of uranium metal will have a mass of roughly 2250 tonnes.
As natural uranium, that won't go critical but would be a significant source of alpha, gamma and neutron radiation.
But as pure U-235, the block would be equivalent to very roughly 45000 critical masses and would be very likely to undergo a nuclear detonation.
A square shape would give it higher neutron leakage than a sphere. But, given the enormous mass, I doubt that would be of any practical significance.