A. Notable characteristics

Date: 2015-10-07; view: 711.

When refined, uranium is a silvery white, weakly radioactive metal, which is slightly softer than steel. It is malleable, ductile, and slightly paramagnetic. Uranium metal has very high density, 65% more dense than lead, but slightly less dense than gold. When finely divided, it can react with cold water; in air, uranium metal becomes coated with a layer of uranium oxide. Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry.

Uranium metal has three allotropic forms:

Natural uranium metal contains about 0.71% U-235, 99.28% U-238, and about 0.0054% U-234. In order to produce enriched uranium, the process of isotope separation removes a substantial portion of the U-235 for use in nuclear power, weapons, or other uses. The remainder, depleted uranium, contains only 0.2% to 0.4% U-235. Because natural uranium begins with such a low percentage of U-235, the enrichment process produces large quantities of depleted uranium. For example, producing 1 kg of 5% enriched uranium requires 11.8 kg of natural uranium, and leaves about 10.8 kg of depleted uranium with only 0.3% U-235 remaining.

Its two principal isotopes are ²³⁵U and ²³⁸U. Naturally-occurring uranium also contains a small amount of the ²³⁴U isotope, which is a decay product of ²³⁸U. The isotope ²³⁵U or enriched uranium is important for both nuclear reactors and nuclear weapons because it is the only isotope existing in nature to any appreciable extent that is fissile, that is, fissionable by thermal neutrons. The isotope ²³⁸U is also important because it absorbs neutrons to produce a radioactive isotope that subsequently decays to the isotope ²³⁹Pu (plutonium), which also is fissile.

The artificial ²³³U isotope is also fissile and is made from thorium-232 by neutron bombardment.

Uranium was the first element that was found to be fissile. Upon bombardment with slow neutrons, its ²³⁵U isotope becomes the very short lived ²³⁶U which immediately divides into two smaller nuclei, releasing nuclear binding energy and more neutrons. If these neutrons are absorbed by other ²³⁵U nuclei, a nuclear chain reaction occurs and, if there is nothing to absorb some neutrons and slow the reaction, the reaction is explosive. The first atomic bomb worked by this principle (nuclear fission). A more accurate name for both this and the hydrogen bomb (nuclear fusion) would be "nuclear bomb" or "nuclear weapon", because only the nuclei participate.