[Zathras]

U235 is probably the most unique element-Isotope in the entire Periodic Table.
A little known fact is more people have been killed from U235 Prompt Critical events than any other element-isotope.
Just standing near a sub critical mass can cause it to go Prompt.

And yet without, the entire world of nuclear science would vanish.
I still think that Seaborg’s original creation of Pu239 was an attempt to create more U235.

It DOES work but Pu239 -> U235 decay has 1/2 life of 24,000 years

[Hamiltonian]

And another question is whether this accident actually did involve depleted UF6.

[103198]

U235 isotope half life is 703.8 million years. That's the reason .72% of natural Uranium is U235 while the vast majority is U238. Hence the need for enrichment plants (i.e., gaseous diffusion, centrifugal, laser, etc.) for reactor full and weapon uses. The bombardment of U238 with a neutron is what creates Pu239 via a Neptunium decay.