Two potential natural sources. First, every element heavier than iron came from inside a star, just like our sun is made up of stuff roaming about the universe from earlier stars that blew up. That is how baby stars happen — older, bigger ones blew up.
2nd, plutonium could have come from another one of those natural reactors in earth’s crust — like the one found near the eastern coast of Africa. Around Nigeria if I recall the map correctly.
I guess they don’t teach this stuff in high school physics anymore. Shame.
Nigeria is on the west coast of Africa.
Plutonium is an isotope with a short half-life. So, no, and your high school physics must have sucked.
Meet Oklo, the Earth’s Two-billion-year-old only Known Natural Nuclear Reactor