Uranium has the highest atomic weight of all the naturally occurring elements. In nature, most uranium atoms exist as U-238 (99.3%), U-235 (0.7 %) and a very small amount of U-234. Uranium decays slowly (1/2 life of 4.47 billion years for U-238 and 704 million years for U-235) by emitting an alpha particle; Geiger counters easily detect uranium’s beta and gamma decay products.
Uranium has been used since antiquity to color glass yellow (hence the yellow color of radiation signage). A more modern use of Uranium stems from U-235‘s ability to fission by neutron capture with a large amount of energy release. This ability to fission was confirmed by Columbia University scientists in the late 1930’s using the Pupin Cyclotron, a discovery that was instrumental in developing the atomic bomb and harnessing nuclear energy.
In order for most nuclear reactors to function, the percentage of U-235 must be raised from 0.7% to at least 3%, a process known as enrichment (for weapon grade uranium, U-235 must be enriched above 95%). Several methods are used to accomplish this; an early such method, gaseous diffusion, was also developed in the late 1930’s at Columbia University. In the early 1970’s Columbia University built a small research nuclear reactor which for political and economic reasons was never fueled or operated. In the late 1990’s Columbia University returned 5 tons of natural uranium (unrelated to the above mentioned nuclear reactor) to the Department of Energy.
Currently uranium (as uranyl acetate) is used at Morningside campus for its density rather than its radioactivityfor preparing samples for electron microscopes. Its use causes a dilemma for some colleges because purchasing the compound does not require a license; however it must be disposed as radioactive waste, which does require a license.