Uranium-235

Uranium 235

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Uranium-235

Uranium-235 is an isotope of uranium

Isotopes of uranium

Uranium is a naturally occurring element with no stable isotopes. In other words, all uranium is radioactive and hence vanishing by radioactive decay, yet it is also found in great quantity in the earth's crust....
that differs from the element's other common isotope, uranium-238

Uranium-238

Uranium-238 , is the most common Isotopes of uranium of uranium found in nature. When hit by a neutron, it becomes uranium-239 , an unstable isotope which radioactive decay into neptunium-239 , which then itself decays, with a half-life of 2.355 days, into plutonium-239 ....
, by its ability to cause a rapidly expanding fission

Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the atomic nucleus of an atom splits into smaller parts, often producing free neutrons and lighter atomic nucleus, which may eventually produce photons ....
chain reaction

Chain reaction

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
, i.e., it is fissile

Fissile

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate....
. It is the only fissile isotope found in any economic quantity in nature. It was discovered in 1935 by Arthur Jeffrey Dempster

Arthur Jeffrey Dempster

Arthur Jeffrey Dempster was a Canada-United States physicist best known for his work in mass spectrometry and his discovery of the uranium isotope uranium-235....
.

If at least one neutron

Neutron

The neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutrons are usually found in atomic nucleus....
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

Critical

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Encyclopedia

Uranium-235 is an isotope of uranium

Isotopes of uranium

Uranium-238

Nuclear fission

Chain reaction

Fissile

Arthur Jeffrey Dempster

Arthur Jeffrey Dempster was a Canada-United States physicist best known for his work in mass spectrometry and his discovery of the uranium isotope uranium-235....
.

If at least one neutron

Neutron

Critical

Critical may denote:*pertaining to a critic*pertaining to a critique*pertaining to a crisisMore specifically:...
, 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

Neutron moderator

In nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
to lower their speed, since the probability for fission with slow neutrons is greater. A fission chain reaction produces intermediate mass fragments

Fission product

Fission products are the atomic fragments left after a large nucleus Nuclear fission. Typically, a large nucleus like Uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos....
which are highly radioactive and produce further energy by their radioactive decay

Radioactive decay

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide....
. Some of them produce neutrons, called delayed neutrons, which contribute to the fission chain reaction. In nuclear reactor

Nuclear reactor

A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate, as opposed to a nuclear bomb, in which the chain reaction occurs in a fraction of a second and is uncontrolled causing an explosion....
s, the reaction is slowed down by the addition of control rods which are made of element

Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
s such as boron

Boron

Boron is a chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite....
, cadmium

Cadmium

Cadmium is a chemical element with the symbol Cd and atomic number 48. A relatively abundant , soft, bluish-white, transition metal, cadmium is known to cause cancer and occurs with zinc ores....
, and hafnium

Hafnium

Hafnium is a chemical element with the element symbol Hf and atomic number 72. A lustre , silvery gray, tetravalence, transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals....
which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of energy

Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
released creates a nuclear explosion

Nuclear explosion

A nuclear explosion occurs as a result of the rapid release of energy from an intentionally high-speed nuclear reaction. The driving reaction may be nuclear fission, nuclear fusion or a multistage cascading combination of the two, though to date all fusion based weapons have used a fission device to initiate fusion, and a pure fusion weapon...
.

The fission of one atom of U-235 generates 202.5 MeV

Electronvolt

In physics, the electron volt is a unit of energy. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an Electrostatics potential difference of one volt....
= 3.244 × 10-11 J, i.e. 19.54 TJ/mol

Mole (unit)

The mole is a Units of measurement of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity....
= 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 ?-rays	7.0
Energy from decaying fission products
Energy of ß--particles	6.5
Energy of anti-neutrinos	8.8
Energy of delayed ?-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

Nuclear cross section

The nuclear cross section of a nucleus is used to characterize the probability that a nuclear reaction will occur. The concept of a nuclear cross section is somewhat difficult to conceptualize but can be quantified physically in terms of "characteristic area" where a larger area means a larger probability of interaction....
for slow thermal neutrons is about 1000 barn

Barn (unit)

A barn is a unit of area. While the barn is not an SI unit, it is accepted for use with the SI. Originally used in nuclear physics for expressing the cross section area of nuclei and nuclear reactions, today it is used in all fields of particle physics to express the cross sections of any scattering process....
s. For fast neutrons it is on the order of 1 barn.

Only around 0.72% of all natural uranium

Natural uranium

Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight....
is uranium-235, the rest being mostly uranium-238

Uranium-238

Uranium-238 , is the most common Isotopes of uranium of uranium found in nature. When hit by a neutron, it becomes uranium-239 , an unstable isotope which radioactive decay into neptunium-239 , which then itself decays, with a half-life of 2.355 days, into plutonium-239 ....
. This concentration is insufficient for a self sustaining reaction in a light water reactor

Light water reactor

The light water reactor or LWR is a type of thermal reactor, a reactor that uses a neutron moderator to reduce the speed of neutrons to low velocity thermal neutrons....
; enrichment, which just means separating out the uranium-238, must take place to get a usable concentration of uranium-235. Pressurised Heavy Water Reactor

Pressurised heavy water reactor

A pressurised heavy water reactor is a Nuclear power plant, commonly using unenriched natural uranium as its fuel, that uses heavy water as its coolant and Neutron moderator....
s, other heavy water reactor

Heavy water reactor

Heavy water reactors use heavy water as a neutron moderator. Heavy water is deuterium oxide, D2O. Neutrons in a nuclear reactor that uses uranium must be slowed down so that they are more likely to split other atoms and get more neutrons released to split other atoms....
s, 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

Enriched uranium

Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation....
, different applications require unique levels of enrichment.

The fissile uranium in nuclear weapon

Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission or a combination of fission and nuclear fusion....
s 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 reflector

Neutron reflector

A neutron reflector is any material that reflects neutrons. Usually, this term refers to the elastic scattering rather than to a specular reflection....
s 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

Little Boy

Little Boy was the codename of the atomic bomb that was dropped on Hiroshima, Japan, on August 6, 1945 by the B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets in the 393d Bomb Squadron of the United States Army Air Forces....
atomic bomb was fueled by enriched uranium. Most modern nuclear arsenals use plutonium

Plutonium

Plutonium is a rare transuranic radioactive chemical element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when plutonium oxide....
as the fissile component, however U-235 devices remain a nuclear proliferation

Nuclear proliferation

Nuclear proliferation is a term now used to describe the spread of nuclear weapons, fissile material, and weapons-applicable nuclear technology and information, to nations which are not recognized as "nuclear weapon States" by the Treaty on the Nonproliferation of Nuclear Weapons, also known as the Nuclear Nonproliferation Treaty or NPT....
concern due to the simplicity of this.

Uranium-235 has a half-life

Half-life

The half-life of a quantity whose value decreases with time is the interval required for the quantity to decay to half of its initial value. The concept originated in describing how long it takes atoms to undergo radioactive decay but also applies in a wide variety of other situations....
of 700 million years.

Uranium-235

See also

External links