 Uranium metal highly enriched in uranium-235 | |
| General | |
|---|---|
| Symbol | 235U |
| Names | uranium-235, 235U, U-235 |
| Protons(Z) | 92 |
| Neutrons(N) | 143 |
| Nuclide data | |
| Natural abundance | 0.72% |
| Half-life(t1/2) | 703800000 years |
| Isotope mass | 235.0439299Da |
| Spin | 7/2− |
| Excess energy | 40914.062±1.970keV |
| Binding energy | 1783870.285±1.996 keV |
| Parent isotopes | 235Pa 235Np 239Pu |
| Decay products | 231Th |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Alpha | 4.679 |
| Isotopes of uranium Complete table of nuclides | |
Uranium-235 (235
U
orU-235) is anisotope of uranium making up about 0.72% ofnatural uranium. Unlike the predominant isotopeuranium-238, it isfissile, i.e., it can sustain anuclear chain reaction. It is the only fissile isotope that exists in nature as aprimordial nuclide.
Uranium-235 has ahalf-life of 703.8 million years. It was discovered in 1935 byArthur Jeffrey Dempster. Itsfission cross section for slowthermal neutrons is about584.3±1barns.[1] Forfast neutrons it is on the order of 1 barn.[2]Mostneutron absorptions induce fission, though a minority (about 15%) result in the formation ofuranium-236.[3][4]
The fission of one atom of uranium-235 releases202.5 MeV (3.24×10−11 J) inside the reactor. That corresponds to 19.54 TJ/mol, or 83.14 TJ/kg.[5] Another 8.8 MeV escapes the reactor as anti-neutrinos. When235
92U nuclei are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown in the adjacent image):
Heavy water reactors and somegraphite moderated reactors can use natural uranium, butlight water reactors must uselow enriched uranium because of the higherneutron absorption of light water.Uranium enrichment removes some of the uranium-238 and increases the proportion of uranium-235.Highly enriched uranium (HEU), which contains an even greater proportion of uranium-235, is sometimes used in the reactors ofnuclear submarines,research reactors andnuclear weapons.
If at least oneneutron from uranium-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction continues to sustain itself, it is said to becritical, and the mass of235U required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of235U if the neutrons from fission aremoderated to lower their speed, since the probability for fission withslow neutrons is greater. A fission chain reaction produces intermediatemass fragments which are highlyradioactive and produce further energy by theirradioactive decay. Some of them produce neutrons, calleddelayed neutrons, which contribute to the fission chain reaction. The power output ofnuclear reactors is adjusted by the location ofcontrol rods containing elements that strongly absorb neutrons, e.g.,boron,cadmium, orhafnium, in the reactor core. Innuclear bombs, the reaction is uncontrolled and the large amount ofenergy released creates anuclear explosion.
TheLittle Boygun-type atomic bomb dropped on Hiroshima on August 6, 1945, was made of highly enriched uranium with a largetamper. The nominal spherical critical mass for an untampered235U nuclear weapon is 56 kilograms (123 lb),[6] which would form a sphere 17.32 centimetres (6.82 in) in diameter. The material must be 85% or more of235U and is known asweapons grade uranium, though for a crude and inefficient weapon 20% enrichment is sufficient (calledweapon(s)-usable). Even lower enrichment can be used, but this results in the requiredcritical mass rapidly increasing. Use of a large tamper,implosion geometries, trigger tubes,polonium triggers,tritium enhancement, andneutron reflectors 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 designs useplutonium-239 as the fissile component of the primary stage;[7][8] however, HEU (highly enriched uranium, in this case uranium that is 20% or more235U) is frequently used in the secondary stage as an ignitor for the fusion fuel.
| Source | Average energy released [MeV][5] |
|---|---|
| Instantaneously released energy | |
| Kinetic energy of fission fragments | 169.1 |
| Kinetic energy of prompt neutrons | 4.8 |
| Energy carried by prompt γ-rays | 7.0 |
| Energy from decaying fission products | |
| Energy of β− particles | 6.5 |
| Energy of delayed γ-rays | 6.3 |
| Energy released when those prompt neutrons which do not (re)produce fission are captured | 8.8 |
| Total energy converted into heat in an operating thermal nuclear reactor | 202.5 MeV |
| Energy of anti-neutrinos | 8.8 |
| Sum | 211.3 MeV |
Knowledge of current and theoretical production ratios of uranium-235 to uranium-238 allowsradiometric dating, the time since modern uranium nuclei were formed instellar nucleosynthesis.
The 1957B2FH landmark paper in astrophysics explained ther-process by which both nuclei form. The authors predicted their relative abundances, and those of their rapidly alpha-chain decayingparent nuclides. Thus they predicted 1.64 as the235U/238U ratio contributed to theinterstellar medium by r-process events (supernovae and subsequently discoveredkilonovae). This takes billions of years to diminish to their present value of 0.0072 (seenatural uranium). They investigate scenarios for historical contribution to thesolar nebula, before contribution is cut off at the Sun's formation 4.5 billion years ago. The scenarios are: a single supernova, a finite continuous uniform series of supernovae representing the lifetime of theMilky Way, and an infinite series representing thesteady-state universe. From the second scenario, they estimated an age of the Milky Way at around 10 billion years, compared to a modern value of 13.61 billion years. Significantly, at this point the oldest known objects werestellar clusters at 6.5 billion years old.[9]
This technique is identical touranium–thorium dating anduranium-234-uranium-238 dating.
| Lighter: uranium-234 | Uranium-235 is an isotope ofuranium | Heavier: uranium-236 |
| Decay product of: protactinium-235 neptunium-235 plutonium-239 | Decay chain of uranium-235 | Decays to: thorium-231 |