U-235 has a half-life of 0.7 billion years, while U-238 has a longer half-life of 4.5 billion years. So, they are still remaining after 4.6 billion years from the birth of the Earth. It is said that the percentage of U-235 in the natural uranium is 0.72 % and that any planets in the solar system have the same percentage of it. Nuclear power plants use enriched uranium in which the concentration of U-235 is increased to 3~5 % necessary for fission. A Japanese scientist, Kazuo Kuroda predicted that once the Earth had high percentage of U-235, the natural nuclear reactor existed. In 1972, a low percentage of U-235 was discovered at Oklo in Gabon, and the trace of the natural nuclear reactor of 2 billion years ago was revealed. The power of nature such as natural nuclear reactor of the Earth and the nuclear fusion energy of the Sun is beyond human capability.
