'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. In fact, U-235 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
May 5,
1940 front-page ''New York Times'' article proclaiming the power of uranium-235.
The fission of one atom of U-235 generates 200
MeV = 3.2 × 10
-11 J, i.e. 18 TJ/
mol = 77 TJ/kg. However, approximately 5% of this energy is carried away by virtually undetectable neutrinos.
[1]
The
nuclear cross section for slow
thermal neutrons is about 1000
barns. For
fast neutrons it is in the order of 1 barn.
[1]
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, however U-235 devices remain a
nuclear proliferation concern due to the
simplicity of the design.
Uranium-235 has a
half-life of 700 million years.
See also
★
Depleted uranium
★
Uranium market
★
Nuclear reprocessing
★
United States Enrichment Corporation
★
Nuclear fuel cycle
★
Nuclear power
References
1. Some Physics of Uranium
★
Table of Nuclides
★
US DOE Handbook 1019/1-93
External links
★
Uranium | Radiation Protection Program | US EPA
★
NLM Hazardous Substances Databank - Uranium, Radioactive
العربية
中国
Français
Deutsch
Ελληνική
हिन्दी
Italiano
日本語
Português
Русский
Español
