Actually, that's not all that far off from reality. Except that, in our solar system, nature has only one fusion reactor, which went critical roughly 4.5 billion years ago.
Nature also makes its own fission reactors on occasion, through natural processes concentrating fissionable materials. And through other natural processes which turn over portions of the land, some of those materials can become exposed or even distributed across large areas. One would expect this to happen very rarely, but over sufficiently long time scales it may be something which could be expected to happen nonetheless.