Does the article really mean that? " r-process can produce atoms with an atomic mass of at LEAST 260 before they fission."

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

and

https://www.researchgate.net/publication/279166139_Charge_sp...

Moreover, the nuclides with atomic mass close to 260, i.e. isotopes of einsteinium and fermium, would be produced in relatively small quantities in comparison with lighter elements.

Exceeding 260 is highly improbable because such nuclei fission spontaneously extremely quickly, in milliseconds or microseconds, so the neutron flux would need to be much more intense than in nuclear explosions in order to produce a non-negligible equilibrium concentration.

Most elements that are heavier than plutonium would decay before the materials containing them could aggregate into a planet (which may take at least a few million years). When the planets of the Solar System have formed, they probably still contained significant quantities of plutonium and neptunium, but then they have decayed quickly, leaving uranium as the heaviest surviving primordial element.

While there might exist an island of stability for super-heavy nuclei of elements beyond any of those that have been synthesized artificially by ion collisions, for now there is no known natural process that could produce them in measurable quantities. By "stability" it is meant that the super-heavy elements might have lifetimes measured in years instead of milliseconds, not that they could be as stable as the already not very stable uranium or plutonium.

I find it interesting that the .003% difference in U-235 was a large enough deviation to attract attention to Oklo. Thanks for the link

Look what effort we went into the last two decades trying to stop Iran from developing their uranium enrichment infrastructure.

Then back in WW2 we did a complete covert op to sabotage the heavy water (used in enrichment) generators in the mountains of Norway just to stop the Germans or Stalin from getting access to weapons grade material.

Hell, they only even had some tons set aside by accident to enrich in the first place.

Anyways, finding already fissled uranium material probably set off many alarm bells at the Pentagon and the Atomic Energy Commission in 1972.

Isn't it the case it is currently not possible to create any neutron heavy isotopes and that causes the half-lives to be on the low end?

All of these isotopes are neutron-heavy. Lawrencium 261 has 103 protons and 158 neutrons, for example.

In fact, the only stable isotopes with fewer neutrons than protons are helium 3 and ordinary hydrogen (protium).

It isn't crazy to postulate, as a layman, that if we synthesized Lr-276, say, it might have a longer half-life.

(Not that we expect that pattern to continue indefinitely, but still, have we discovered the most stable isotope of lawrencium yet?)