U-235 (Uranium-235)


Full table
Name, symbol Uranium-235,235U
Neutrons 143
Protons 92
Nuclide data
Natural abundance 0.72%
Half-life 703,800,000 years
Parent isotopes 235Pa
Decay products 231Th
Isotope mass 235.0439299 u
Spin 7/2-
Excess energy 40914.062 ± 1.970 keV
Binding energy 1783870.285 ± 1.996 keV
Decay mode Decay energy
Alpha 4.679 MeV

Uranium-235 is an isotope of uranium that differs from the element’s other common isotope, uranium-238, by its ability to cause a rapidly expanding fission chain reaction, i.e., it is fissile. It is the only fissile isotope found in any economic quantity in nature. It was discovered in 1935 by Arthur Jeffrey Dempster.

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, and the mass of U-235 required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of U-235 if the neutrons from fission are moderated to lower their speed, since the probability for fission with slow neutrons is greater. A fission chain reaction produces intermediate mass fragments which are highly radioactive and produce further energy by their radioactive decay. Some of them produce neutrons, called delayed neutrons, which contribute to the fission chain reaction. In nuclear reactors, the reaction is slowed down by the addition of control rods which are made of elements such as boron, cadmium, and hafnium which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy released creates a nuclear explosion.

The fission of one atom of U-235 generates 202.5 MeV = 3.244 × 10−11 J, i.e. 19.54 TJ/mol = 83.14 TJ/kg.

Source Average energy released [MeV]
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 anti-neutrinos 8.8
Energy of delayed γ-rays 6.3
Sum 202.5
Energy released when those prompt neutrons which don’t (re)produce fission are captured 8.8
Energy converted into heat in an operating thermal nuclear reactor 202.5

The nuclear cross section for slow thermal neutrons is about 1000 barns. For fast neutrons it is on the order of 1 barn.[2]

Only around 0.72% of all natural uranium is uranium-235, the rest being mostly uranium-238. This concentration is insufficient for a self sustaining reaction in a light water reactor; enrichment, which just means separating out the uranium-238, must take place to get a usable concentration of uranium-235. Pressurised heavy water reactors, other heavy water reactors, and some graphite moderated reactors are known for using unenriched uranium. Uranium which has been processed to boost its uranium-235 proportion is known as enriched uranium, different applications require unique levels of enrichment.

The fissile uranium in nuclear weapons usually contains 85% or more of 235U known as weapon(s)-grade, though for a crude, inefficient weapon 20% is sufficient (called weapon(s)-usable); even less is sufficient, but then the critical mass required rapidly increases. However, judicious use of implosion and neutron reflectors can enable construction of a weapon from a quantity of uranium below the usual critical mass for its level of enrichment, though this would likely only be possible in a country which already had extensive experience in developing nuclear weapons. The Little Boy atomic bomb was fueled by enriched uranium. Most modern nuclear arsenals use plutonium as the fissile component,[3][4] however U-235 devices remain a nuclear proliferation concern due to the simplicity of this.

Uranium-235 is an
isotope of Uranium
Decay product of:
Decay chain
of Uranium-235
Decays to:

See Also:


2 comments to U-235 (Uranium-235)

  • M.A.Padmanabha Rao


    M.A. Padmanabha Rao,
    Discovery of Self-Sustained 235-U Fission Causing Sunlight by Padmanabha Rao Effect,
    IOSR Journal of Applied Physics (IOSR-JAP), Volume 4, Issue 2 (Jul. – Aug. 2013), PP 06-24, http://www.iosrjournals.org/iosr-jap/papers/Vol4-

    EXCERPTS OF THE PAPER: Sunlight phenomenon being one of the most complex phenomena in science evaded from previous researchers. Understanding the phenomenon needed advanced knowledge in the fields of nuclear physics, X-ray physics, and atomic spectroscopy. A surprise finding, optical emission detected from Rb XRF source in 1988 led to the discovery of a previously unknown atomic phenomenon causing Bharat radiation emission followed by optical emission from radioisotopes and XRF sources reported in 2010 [10]. The same phenomenon was found causing the Sunlight. However, it took nearly 25 years of research to reach the current level of understanding the Sunlight
    phenomenon reported here.

    (1) On the basis of fusion, many solar lines could not be identified previously and what causes these lines remained puzzling. Though 11 solar lines could be identified by other researchers, they became questionable. The significant breakthrough has come when it became possible now to identify as many as 153 lines on the basis of uranium fission taking place on Sun’s core surface. Surprisingly, the fission products released in Chernobyl reactor accident in 1986 also seem to be present in solar flares.
    (2) Explained what are Sun’s dark spots and their cause.
    (3) For the first time, it is shown what constitutes Dark Matter and showed existence of Dark Matter in Sun.
    (4) It is explained with unprecedented detail how Bharat Radiation from fission products (radioisotopes) causes Sunlight by an atomic phenomenon known as Padmanabha Rao Effect.

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