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Infobox isotope|| image = HEUraniumC.jpg| image_caption = Uranium metal highly enriched in Uranium-235| background = #fc6| text_color =| isotope_name = Uranium-235| isotope_filename =| alternate_names =| mass_number = 235| symbol =U| num_neutrons = 143| num_protons = 92| abundance =0.72%| halflife = 703,800,000 years| error_halflife =| decay_product = Thorium-231| decay_mass = 231| decay_symbol =Th| parent = Protactinium-235| parent_mass = 235| parent_symbol =Pa| parent_decay =| parent2 = Neptunium-235| parent2_mass = 235| parent2_symbol = Np| parent2_decay =| parent3 = Plutonium-239| parent3_mass = 239| parent3_symbol = Pu| parent3_decay =| mass = 235.0439299| spin = 7/2-| excess_energy = 40914.062 ± 1.970| error1 =| binding_energy = 1783870.285 ± 1.996| error2 =| decay_mode1 = Alpha| decay_energy1 = 4.679| decay_mode2 =| decay_energy2 =| decay_mode3 =| decay_energy3 =| decay_mode4 =| decay_energy4 = Uranium-235 is an Isotopes of uranium|isotope of uranium making up about 0.72% of natural uranium . Unlike the predominant isotope uranium-238 it is fissile , i.e., it can sustain nuclear fission|fission chain reaction . It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature.
Uranium-235 has a half-life of 700 million years. It was discovered in 1935 by Arthur Jeffrey Dempster . Its nuclear cross section for slow thermal neutron s is about 1000 Barn (unit)|barn s. For fast neutron s it is on the order of 1& nbsp;barn. http://www.uic.com.au/uicphys.htm Some Physics of Uranium Most but not all neutron absorption s result in fission; a minority result in neutron capture forming uranium-236 . The fission of one atom of U-235 generates 202.5 Electronvolt|MeV = 3.244 & times; 10-11 J, i.e. 19.54 TJ/ mole (unit)|mol = 83.14 TJ/kg. http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_1.html Nuclear fission and fusion, and neutron interactions, National Physical Laboratory. Heavy water reactor s, and some graphite moderated reactor s can use unenriched uranium, but light water reactor s must use low enriched uranium because of light water's neutron absorption . Uranium enrichment removes some of the uranium-238 and increases the proportion of uranium-235. In nuclear weapon design , highly enriched uranium containing 40% or greater U-235 is sometimes used in the secondary stage in place of natural or depleted uranium. Primary stages today most commonly use plutonium but when uranium is used, it is even more highly enriched in U-235.
If at least one neutron from U-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction will sustain itself, it is said to be critical mass|critical , and the mass of U-235 required to produce the critical condition is said to be a Critical mass (nuclear)|critical mass . A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are neutron moderator|moderated to lower their speed, since the probability for fission with Thermal neutron|slow neutrons is greater. A fission chain reaction produces intermediate Fission product|mass fragments which are highly radioactive and produce further energy by their radioactive decay . Some of them produce neutrons, called delayed neutron s, which contribute to the fission chain reaction. In nuclear reactor s, the reaction is slowed down by the addition of control rods which are made of chemical element|element s such as boron , cadmium , and hafnium which can absorb a large number of neutrons. In nuclear bomb s, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion .
The Little Boy gun type atomic bomb dropped on Hiroshima on August 6, 1945 was fueled by highly enriched uranium with a large Nuclear weapon design #Pure fission weapons|tamper . The nominal spherical critical mass for an untampered 235U nuclear weapon is 56& nbsp;kg, http://www.fas.org/nuke/intro/nuke/design.htm FAS Nuclear Weapons Design FAQ, accessed 2010-9-2; There are numerous other references on the net, and with modern computers, this is fairly easy to calculate, so secrecy cannot aid security. a sphere 17.32& nbsp;cm (6.8") in diameter. The required material must be 85 percent or more of 235U and is known as weapons grade uranium, though for a crude, inefficient weapon 20 percent is sufficient (called weapon(s)-usable ). Even lower enrichment can be used, but then the required Critical mass (nuclear)|critical mass rapidly increases. Use of a large tamper, Implosion-type nuclear weapon|implosion geometries, trigger tubes, polonium triggers, Tritium enhancement, and neutron reflector s can enable a more compact, economical weapon using one-fourth or less of the nominal critical mass, though this would likely only be possible in a country that already had extensive experience in engineering nuclear weapons. Most modern nuclear weapon design s use plutonium as the fissile component of the primary stagecite book|title=Nuclear Weapon Design |editor=FAS contributors |publisher=Federation of American Scientists |url= http://www.fas.org/nuke/intro/nuke/design.htm cite book |title=Miner |year=1968 |pages=541 however HEU is often used in the secondary stage.
Source !! Average energy released MeV
Instantaneously released energy
Kinetic energy of fission fragments
Kinetic energy of prompt neutrons
Energy carried by prompt ?-rays
Energy from decaying fission products
Energy of ß--particles
Energy of delayed ?-rays
Energy released when those prompt neutrons which don't (re)produce fission are captured
Energy converted into heat in an operating thermal nuclear reactor
DOE Fundamentals handbook: Nuclear Physics and Reactor theory http://www.hss.energy.gov/nuclearsafety/ns/techstds/standard/hdbk1019/h1019v1.pdf Vol. 1, http://www.hss.energy.gov/nuclearsafety/ns/techstds/standard/hdbk1019/h1019v2.pdf Vol. 2.
A piece of U-235 (uranium-235, a rare form of uranium) the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. (Contemporary's Science)
External links
http://www.epa.gov/rpdweb00/radionuclides/uranium.html Uranium | Radiation Protection Program | US EPA
http://books.google.com/books? id=ftkDAAAAMBAJ& pg=PA1& dq=Popular+Science+1930+plane+%22Popular+Mechanics%22& hl=en& ei=4m2FTsGiHYfe0QHOutj2Dw& sa=X& oi=book_result& ct=result& resnum=10& ved=0CFEQ6AEwCTge#v=onepage& q& f=true "The Miracle of U-235" Popular Mechanics , January 1941 one of the earliest articles on U-235 for the general public
Category:Actinides Category:Isotopes of uranium Category:Fissile materials Category:Special nuclear materials
