Charlie Stross, an author and someone I've been acquainted with online for almost two decades now, links to a paper (PDF) presented at a conference about a theoretical natural nuclear reactor on Mars going boomski. If my math is not too far off, its on the order of 10 times the megatonnage of Chicxulub's impact. Unfortunately, it has a three strikes against it.
The first is that that's enormous and seems unlikely to have possibly reached that concentration of Thorium and Uranium on Mars. That can be confirmed or debunked though. That's the nice thing about science.
The second, also science driven, is that there ought to be a blast crater, even if its a surface kaboomski, its going to be blowing a massive hole. It does not appear to be present. Chicxulub left something after 65+/- million years. Even the Manicouagan Crater is still painfully observable and that is 214+/- million years old. The Vredefort crater is OLDER than the proposed Martian explosion at 2 billion years and still a observable. These are all on a VERY wet, geologically active world
The really big strike against it is the author. Ouch.
Alas. Nifty idea is probably pure junk.