From the CRC handbook of Chemistry and Physics, CRC Press, Cleveland,  55th ed., 1974-75 "The Elements" by C.R. Hammond. page B-35,36
URANIUM (Planet Uranus), U; at. wt. 238.029; at. no. 92; m.p. 1132.3 +/- 0.8 C; b.p. 2818 C; sp.gr. ~18.95; valence 2,3,4,5, or 6.   Yellow-colored glass, containing more than 1% uranium oxide and dating  back to 79 A.D., has been found near Naples, Italy.  Klaproth recognized an unknown element in pitchblende and attempted to isolate the metal in  1789.  The metal apparently was first isolated in 1841 by Peligot, who reduced the anhydrous chloride with potassium.  Uranium is not as rare as  it was once thought.  It is now considered to be more plentiful than mercury, antimony, silver, or cadmium, and is about as abundant as  molybdenum or arsenic.  It occurs in numerous minerals, such as pitchblende, uraninite, carnotite, autunite, uranophane, davidite, and tobernite.  It is also found in phosphate rock, lignite, monazite sands, and can be  recovered commercially from these sources.  The A.E.C. purchases uranium in the form of acceptable U3O8 concentrates.  This incentive program has greatly increased the known uranium reserves.  Uranium can be prepared by reducinguranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures.  The metal can also be produced by electolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2-NaCl.  High-purity uranium can be prepared by thermal decomposition of uranium halides on a hot filament.  Uranium exhibits three crystallographic modifications as follows:  	      667 C         772 C 	alpha ----->  beta  ----->  gamma  Uranium is a heavy, silvery-white metal , which is pyrophoric when finely divided.  It is a little softer than steel, and is attacked by cold water in a finely divided state.  It is malleable, ductile, and slightly  paramagnetic.  In air, the metal becomes coated with a layer of oxide.   Acids dissolve the metal, but it is unaffected by alkalis.  Uranium has  fourteen isotopes, all of which are ardioactive.  Naturally-occurring  uranium nominally contains 99.2830% by weight U(238), 0.7110 % U(235), and 0.0054% U(234).  Studies show that the percentage weight of U(235) in natural unranium varies by as much as 0.1&, depending on the source. The A.E.C. has adopted the value of 0.711 as being their "official" percentage of U(235) in natural uranium.  Natural uranium is sufficently radioactive to expose a photographic plate in an hour or so.  Much of the internal heat of the earth is thought to be attributable to the presence of uranium and thorium.  U(238), with a half-life of 4.51X10^9 years, has been used to estimate the age of igneous rocks.  The origin of uranium, the highest member of the natural-occurring elements - except perhaps for traces of neptunium or plutonium, is not clearly understood, although it may be presumed that uranium is a decay product of elements of higher atomic weight, which may have once been present on earth or  elsewhere in the universe.  These original elements may have been created  as a result of a primordial "creation", known as "the big bang," in a  supernovae, or in some other nuclear fuel.  U(238) can be converted into fissionable plutonium by the following reactions:                238          239    beta -      239    beta -     239              U   (n,gamma)U      -------->  Np     -------->  Pu   .  This nuclear conversion can be brought about in "breeder" reactors where it is possible to produce more new fissionable material than the fissionable material used in maintaining the chain reaction.  U(235) is of even greater importance, for it is the key to the utilization of uranium.  U(235), while occurring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made to occur in a reactor constructed from natural uranium and a suitable  moderator, such as heavy water or graphite, alone.  U(235) can be  concentrated by gaseous diffusion and other physical processes if desired, and used directly as a nuclear fuel., instead of natural uranium, or used as an explosive.  Natural uranium, slightly with U(235) by a small  percentage is used to fuel nuclear power reactors fro the generation of  electricity.  Natural thorium can be irradiated with neutrons as follows to produce the important isotope U(233).                 232           233   beta -      233    beta -    233              Th   (n,gamma)Th     -------->  Pa     -------->  U   .  While thorium itself is not fissionable, U(233) is, and in this way may  be used as a nuclear fuel.  One pound of completely fissioned uranium has the fuel value of over 1500 tons of coal.  The uses of nuclear fuels to  generate electrical power, to make isotopes for peaceful purposes, and to make explosives are well known.  THe estimated world-wide capacity of  nuclear power reactors in 1970 amounted to about 25 million kW and is  expected to grow to between 200 and 250 million kW by 1980.  Uranium in  the U.S.A. is controlled by the Atomic Energy Commission.  New uses are being found for "depleted" uranium (i.e. uranium with the percentage of U(238) lowered to about 0.2%).  It has found use in inertial guidence devices, gyro compasses, counterweights for aircraft control surfaces, as ballast for missile reentry vehicles, and as a shielding material.   Uranium metal is used for x-ray targets for production of high-energy x-rays; the nitrate has been used as photographic toner; and the acetate is used in analytical chemistry.  Crystals of uranium nitrate are  triboluminescent.  Uranium salts have also been used for producing yellow "vaseline" glass and glazes.  Uranium and its compounds are highly toxic, both from a chemical and radiological standpoint.  Finely divided uranium metal, being pyrophoric, presents a fire hazard.  The maximum recommended allowable concentration of soluble uranium compounds in air (based on  chemical toxicity) is 0.05 mg/cu meter; for insoluble  compounds the concentration is set at 0.25 mg/cu meter of air.  The permissible body level of natural uranium (based on radiotoxicity) is 0.2 microcurie for  soluble compounds; for insoluble compounds the level is 0.009 microcurie, or in air 1.7X10^-11 microcurie per milliliter.