"More than two tons of this plutonium isotope were generated within the Oklo deposit. Although almost all this material, which has a 24,000-year halflife, has since disappeared (primarily through natural radioactive decay), some of the plutonium itself underwent fission, as attested by the presence of its characteristic fission products. The abundance of those lighter elements allowed scientists to deduce that fission reactions must have gone on for hundreds of thousands of years. From the amount of uranium 235 consumed, they calculated the total energy released, 15,000 megawatt-years, and from this and other evidence were able to work out the average power output, which was probably less than 100 kilowatts—say, enough to run a few dozen toasters." - Steven Perez from Bookmarklet

Great Scientific American article. These reactors seemed to have even more passive safety systems than any of the Gen III+ reactors currently being built worldwide! - Philippe