There are some, very theoretical, decay pathways to form more stable Krypton on an existing world, mainly by action of neutron radiation on Rubidium, or electron capture of Strontium, and the percentages are low. Failing that your Krypton rich atmosphere has to start with a high abundance of Krypton produced by stellar action, period.

Furthermore because the unstable Krypton-81 is a cosmogenic nuclide derived from stable Krypton-80 by the action of cosmic rays the abundance of Krypton in the atmosphere is actually going to shrink over geological time sorry.

There are some, very theoretical, decay pathways to form more stable Krypton on an existing world, mainly by action of neutron radiation on Rubidium, or electron capture of Strontium (i.e. not under natural conditions you might expect to find on planet), and the percentages are low. Failing that your Krypton rich atmosphere has to start with a high abundance of Krypton produced by stellar action, period.

Furthermore because the unstable Krypton-81 is a cosmogenic nuclide derived from stable Krypton-80 by the action of cosmic rays the abundance of Krypton in the atmosphere is actually going to shrink over geological time, sorry.