How did Earth generate and operate more than 15 Natural Nuclear Reactors?

Monday 15 Dec 2014, 13:30 → 15:00 Asia/Tokyo

RIBF Bldg. 2F Coference room (RIKEN Wako)

Title: How did Earth generate and operate more than 15 Natural Nuclear Reactors? Speaker: Prof. Benoit GALL, Strasbourg University, France Date: Monday, Dec. 15, 13:30 - Place: 2F Conference Room, RIBF Building Summary: Spontaneous ignition and operating of about 15 natural nuclear reactors 2 billions years ago in the Oklo site is still a scientific puzzle. Natural nuclear reactors were predicted by Pr. Kuroda, in 1956 and discovered in 1972 in Oklo mine site in Gabon. Perfect preservation of the site during a long geological period of time, makes it the only natural analogue site for a nuclear waste deposit in a geological medium over geological times. Geologists, geochemists and physicists studied this exceptional phenomenon since its discovery. Appearance of Oxygen in our atmosphere 2.3 to 2.0 billion years ago induced major changes in life and geochemical cycles on earth. The Franceville basin in Gabon kept impressive signs of this major changes with the natural nuclear reactors and the appearance of multicellular life. We will demonstrate in a first part how this period was essential in the genesis of the OKLO site, the history of our planet and life development. Startup of the reactors will be discussed in a second part. Up to now only start up of the biggest reactors could be explained. We recently succeeded in solving the mystery of small reactors start up and continues their study through the extension of a realistic numerical modeling of RZ9 reactor. This study finds its anchor points on relevant experimental parameters acting on geological conditions and neutron-transport physics such as ore content and homogeneity, initial neutronics poisons content, initial reactor size and efficiency of surrounding reflector rocs. The reactor inception will be discussed as well as operating and natural shut down.