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PlutoniumChemist t1_j4rcme7 wrote

Beta minus decay is the only mode of radioactive decay that increases atomic number.

These heavy radionuclides become more and more likely to decay by alpha decay or by spontaneous fission as their atomic numbers increase and less likely to decay by beta minus.

The half lives of these radionuclides also decrease as they get heavier, meaning they don't "survive" long enough to reliably capture neutrons before they decay into something else

By the time you get to Fm, there are no accessible beta minus decaying isotopes of Fm that can be created through neutron capture before the nuclide decays by some other mode.

This logic applies to nuclear reactors. Nuclear weapons have... a much higher neutron flux. This means the Fm isotopes could potentially capture a very large number of neutrons in a very short period of time in order to create an exotic beta minus decaying isotope of Fm before it decays by some other decay mode. Not sure if this was ever observed during the various nuclear weapons testing phases across the world