<P> Uranium miners have a higher incidence of cancer . An excess risk of lung cancer among Navajo uranium miners, for example, has been documented and linked to their occupation . The Radiation Exposure Compensation Act, a 1990 law in the USA, required $100,000 in "compassion payments" to uranium miners diagnosed with cancer or other respiratory ailments . </P> <P> During the Cold War between the Soviet Union and the United States, huge stockpiles of uranium were amassed and tens of thousands of nuclear weapons were created using enriched uranium and plutonium made from uranium . Since the break - up of the Soviet Union in 1991, an estimated 600 short tons (540 metric tons) of highly enriched weapons grade uranium (enough to make 40,000 nuclear warheads) have been stored in often inadequately guarded facilities in the Russian Federation and several other former Soviet states . Police in Asia, Europe, and South America on at least 16 occasions from 1993 to 2005 have intercepted shipments of smuggled bomb - grade uranium or plutonium, most of which was from ex-Soviet sources . From 1993 to 2005 the Material Protection, Control, and Accounting Program, operated by the federal government of the United States, spent approximately US $550 million to help safeguard uranium and plutonium stockpiles in Russia . This money was used for improvements and security enhancements at research and storage facilities . Scientific American reported in February 2006 that in some of the facilities security consisted of chain link fences which were in severe states of disrepair . According to an interview from the article, one facility had been storing samples of enriched (weapons grade) uranium in a broom closet before the improvement project; another had been keeping track of its stock of nuclear warheads using index cards kept in a shoe box . </P> <P> Along with all elements having atomic weights higher than that of iron, uranium is only naturally formed in supernovae . Primordial thorium and uranium are only produced in the r - process (rapid neutron capture), because the s - process (slow neutron capture) is too slow and cannot pass the gap of instability after bismuth . Besides the two extant primordial uranium isotopes, U and U, the r - process also produced significant quantities of U, which has a shorter half - life and has long since decayed completely to Th, which was itself enriched by the decay of Pu, accounting for the observed higher - than - expected abundance of thorium and lower - than - expected abundance of uranium . While the natural abundance of uranium has been supplemented by the decay of extinct Pu (half - life 0.375 million years) and Cm (half - life 16 million years), producing U and U respectively, this occurred to an almost negligible extent due to the shorter half - lives of these parents and their lower production than U and Pu, the parents of thorium: the Cm: U ratio at the formation of the Solar System was (7.0 ± 1.6) × 10 . </P> <P> Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water . Uranium is the 51st element in order of abundance in the Earth's crust . Uranium is also the highest - numbered element to be found naturally in significant quantities on Earth and is almost always found combined with other elements . The decay of uranium, thorium, and potassium - 40 in the Earth's mantle is thought to be the main source of heat that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics . </P>

Where did the uranium on earth come from