That showed up in Scientific American back in the 1970's. Quite an interesting occurrence. It would require a young planet though, to get a high enough concentration of U-235 to work, or a very odd formation rate for Uranium.
You could also get that on a planet with a high concentration of deuterium in the water, as natural uranium can be effectively moderated by heavy water. It does make for some interesting possibilities. Radioactive geysers anyone?
http://www.scientificamerican.com/article/ancient-nuclear-reactor/