The cycle of the natural nuclear reactor

In Gabon, West Africa, has been uncovered. In 1972, a site with the necessary conditions for selfsustained fission was found at the Oklo mine in Gabon: A 2-billion-year-old uranium deposit some 5–10 meters thick and 600–900 meters wide was bathed by an ancient river. That natural reactor is estimated to have operated at an average power of 100 kW over its 150 000-year lifetime. By examining in detail the reactor’s krypton and xenon isotopes in grains of aluminous hydroxy phosphate, physicists at Washington University in St. Louis have now discovered the reactor’s operating cycle: 30 minutes on followed by 2.5 hours off. While on, the reactor’s heat boiled the nearby water until there wasn’t enough to slow the neutrons adequately, whereupon the reactor turned off until it cooled enough for the steam to condense. The researchers also note that the Al phosphate’s ability to capture fission products may be useful in man-made reactors and nuclear waste storage. (A. P. Meshik et al., Phys. Rev. Lett. 93 , 182302, 2004 http://dx.doi.org/10.1103/PhysRevLett.93.182302 .)