[granpa (OP)]

Uranium-238 (U-238) is common, but non-fissile.

Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235 it is non-fissile, which means it cannot sustain a chain reaction. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. 238U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable.


Question: if uranium atoms were 3 times smaller would U-238 still be non-fissile?

[alancalverd]

A few problems here. "3 times smaller" is meaningless. And nearly all the uranium atom is empty space, so however you define its size, what matters is the neutron capture crossection and subsequent stability of the nucleus, which isn't directly associated with its diameter, let alone that of the entire atom.

[evan_au]

The behavior of a nucleus when struck by a neutron depends on the structure of the nucleus.

The nucleus has a shell structure somewhat similar to the electron shell structure of an atom - with some significant differences:
- There are two types of particles (protons & neutrons) rather than one particle type (electrons)
- Protons & Neutrons have far more mass than an electron, so transitions release far more energy
- Protons & neutrons are held together by the strong nuclear force, while electrons are held in by electrostatic forces.
- Protons & neutrons have independent shells; in larger nucleii, neutrons occupy more shells of than do protons.
- What happens when a nucleus is struck by a neutron depends on the shell structure of the nucleus.

See: https://en.wikipedia.org/wiki/Magic_number_(physics)

[PmbPhy]

Question: if uranium atoms were 3 times smaller would U-238 still be non-fissile?

That can't be directly answered because for either the atom of the nucleus to be smaller then either the laws of nature would be different, some of the constants of nature would be different or a combination of those. What the others said also hold true.

[granpa (OP)]

Question: if uranium atoms were 3 times smaller would U-238 still be non-fissile?

That can't be directly answered because for either the atom of the nucleus to be smaller then either the laws of nature would be different, some of the constants of nature would be different or a combination of those. What the others said also hold true.

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

[PmbPhy]

Question: if uranium atoms were 3 times smaller would U-238 still be non-fissile?

That can't be directly answered because for either the atom of the nucleus to be smaller then either the laws of nature would be different, some of the constants of nature would be different or a combination of those. What the others said also hold true.

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

I have no idea why you posted that URL. When you post a URL its not enough to simply post it. You must state why you posted it. In this case why do you think that white dwarf stars has anything to do with what I posted?

[Kryptid]

What exactly do you mean when you say "3 times smaller"? Three times smaller in mass? Three times smaller in covalent bond radius?

[evan_au]

white dwarf

I think you are hinting at the fact that under sufficient pressure and temperature (like in a white dwarf star), it is possible to pack atomic nuclei more closely than if they were in separate atoms. In this sense, I guess you could say that the "atom is smaller in volume"; but because this material is a plasma, atoms as such don't really exist.

However, whether a nucleus is fissile depends on the characteristics of the nucleus, and these characteristics are unchanged, even in the core of a white dwarf star. White dwarfs do not fuse elements beyond iron, so there would be no uranium nuclei formed in a white dwarf.

atoms were 3 times smaller

There is a way that a neutral hydrogen atom can become significantly smaller, and that is if its electron is replaced by a muon. Because the muon is 207 times more massive than an electron, the radius of its orbit is 207 times smaller, and the volume of this exotic Hydrogen atom is 8 million times smaller.

This is sufficient to cause nuclear fusion in Deuterium atoms.

However, it doesn't affect larger atoms (like Helium or Uranium), because any remaining electrons have their usual radius, so the atom retains its usual radius.

See: https://en.wikipedia.org/wiki/Muon-catalyzed_fusion

[PmbPhy]

white dwarf

I think you are hinting at the fact that under sufficient pressure and temperature (like in a white dwarf star), it is possible to pack atomic nuclei more closely than if they were in separate atoms.

If that was the case then perhaps he was ignoring the comments made above regarding the fact that atomic radii have no relevance regarding the question asked in the OP. The radii of the nucleus is certainly unaffected by such pressure unless its a neutron star where all the nuclei are pushed together effectively making it one huge nucleus. Yikes!