Well, that was interesting.  Started out reading my ecopy of The Science of Discworld, when a stray remark therein reminded me of the fossilized natural nuclear reactors someone found in the early '70s, and told me the name of the place was Oklo, Gabon in West Africa.  So, Google being handy, off I went to find this:

The Oklo Fossil Fission Reactors

Above: Oklo minesite 1997 [Photo by Minesite Engineer, Andreas Mittler].

This is one of the most fascinating stories in the relatively short history of Science and especially in the even shorter history of Nuclear Physics. In 1972 the very well preserved remains of several ancient natural nuclear reactors were discovered in the middle of the Oklo Uranium ore deposit. (more)

OK, after reading this article (go do likewise - they aren't kidding about the fascinating part), I was left wondering a) where on the planet was this location the billion years ago that the reactors were active, b) what life, if any, there might've been around at the time and c) if the author's eddress correctly implies that the Australians, like the British, refer to as a "thousand million" what the rest of us call a "billion".

More research.  1.2 billion years back a supercontinent dubbed "Rodinia" seems to have formed, with West Africa being the portion closest to the South Pole instead of the equator where you find it today. It came apart 350 or so million years later, bracketing the time I was looking for.  There wasn't any life on land to be discommoded, though there were multicellular lifeforms in the ocean.  Just a lot of rock deposits sitting there warming up their surroundings until the water baked off and the reaction stopped, then starting up again when they got more water.  Finally things changed too much and it stopped, leaving anything radioactive to decay peacefully, and proving that a billion years makes a heck of an effective radiation shield.  :)

Anybody got an answer to question c?