In what sense did the Oklo reactor "trap" its own nuclear waste?
According to Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo/Okelobondo Area in Gabon, the Xe is trapped in crystalline cage-like structures formed of the aluminum phosphate that are similar to zeolites.

The reference also explains that the aluminum phosphate structures grow quickly under hydrothermal conditions of 270-300 degrees C.