The discussion revolves around the concept of spontaneous fission and its relation to isotope decay, specifically whether there are isotopes that decay solely through spontaneous fission and the conditions under which this occurs. It touches on theoretical aspects, historical occurrences, and specific examples related to natural fission reactors.
Participants generally agree on the existence of nuclides that may decay solely through spontaneous fission, but there is no consensus on the observation of such decays. The discussion about Oklo reflects a shared understanding of its historical significance, though details about its current state and implications may vary.
The discussion does not resolve the uncertainties surrounding the conditions necessary for spontaneous fission or the specific characteristics of the nuclides mentioned. There are also assumptions regarding the stability and decay modes of isotopes that remain unexamined.
mfb said:There are a few nuclides where spontaneous fission is the only possible decay mode (apart from proton decay maybe), but it has never been observed and the expected lifetime is so long that it is unlikely that we will ever see such a decay.
As usual, Wikipedia has a list.
The natural reactors at Oklo were active about 1.7 billion years ago. At that time, the percentage of U-235 in uranium deposits was several times the current fraction of 0.7% found in natural uranium. Since that time, the percentage of U-235 has declined, because its half-life is roughly 700 million years or so.mfb said:Oklo is the most prominent place. It happened in the past, when the fraction of fissile uranium was higher. Today it is too low.