Nuclear reactor physics
Modern deposits of uranium contain only up to ~0.7% U (and ~99.3% U), which is not enough to sustain a chain reaction moderated by ordinary water. But U has a much shorter half-life (700 million years) than U (4.5 billion years), so in the distant past the percentage of U was much higher. About two billion years ago, a water-saturated uranium deposit (in what is now the Oklo mine in Gabon, West Africa) underwent a naturally occurring chain reaction that was moderated by groundwater and, presumably, controlled by the negative void coefficient as the water boiled from the heat of the reaction. Uranium from the Oklo mine is about 50% depleted compared to other locations: it is only about 0.3% to 0.7% U; and the ore contains traces of stable daughters of long-decayed fission products.Fermi age theory
Astrological transit
Uranus takes about 84 years to complete a full return. However, one of its most important influences is felt when it reaches the halfway stage in its journey, at around age 42. The Uranus half-return is the origin of what for many people is the 'mid-life crisis'. Women, especially those with children growing up and leaving home, may feel that life has passed them by. A lot of women also begin the pre-menopause stage at this time, and many men experience the 'male menopause'. Permanent and successful long-term relationships may be unsettled or disrupted by casual flirtations and affairs. The Uranus half-return can be stressful and tense, but also energising. It is the ideal time for people to break out of routine lifestyles that they have been stuck in for years, and develop new interests.