| 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 |
Uranium-235 is an
isotope of uraniumUranium 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. The natural isotopes are uranium-234, uranium-235, and uranium-238, with the average...
making up about 0.72% of
natural uraniumNatural 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. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...
. Unlike the predominant isotope
uranium-238Uranium-238 is the most common isotope of uranium found in nature. When hit by a neutron, it eventually becomes plutonium-239 ....
it is
fissileIn 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...
, i.e. it can sustain
fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts, often producing free neutrons and lighter nuclei, which may eventually produce photons...
chain reactionA 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...
. It is the only fissile isotope that is a
primordial nuclideIn geochemistry and geonuclear physics, primordial nuclides or primordial isotopes are nuclides found on the earth that have existed in their current form since before Earth was formed, according to stellar evolution theory. Due to the age of the Earth of , this means that the half-life of the...
or found in significant quantity in nature.
Uranium-235 has a
half-lifeHalf-life is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....
of 700 million years and was discovered in 1935 by
Arthur Jeffrey DempsterArthur Jeffrey Dempster was a Canadian-American physicist best known for his work in mass spectrometry and his discovery of the uranium isotope
235U.-Biography:...
.
Its
nuclear cross sectionThe 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...
for slow thermal neutrons is about 1000
barnA barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process...
s. For fast neutrons it is on the order of 1 barn.
Most but not all neutron absorptions result in fission; a minority result in
neutron captureNeutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than charged particles which are repelled by electrostatic...
forming
uranium-236Uranium-236 is an isotope of Uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.-Creation and...
.
The fission of one atom of U-235 generates 202.5
MeVIn 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 electrostatic potential difference of one volt...
= 3.244 × 10
−11 J, i.e. 19.54 TJ/
molThe mole is a unit of amount of substance: it is an SI base unit, and one of the few units used to measure this physical quantity. The name "mole" was coined in German by Wilhelm Ostwald in 1893, although the related concept of equivalent mass had been in use at least a century earlier...
= 83.14 TJ/kg.
Heavy water reactorHeavy 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 can use unenriched uranium, but
light water reactorThe light water reactor or LWR is a type of thermal reactor that uses light water as a coolant and neutron moderator as opposed to heavy water as a coolant/moderator...
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 designNuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three basic design types...
, 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
plutoniumPlutonium is a rare transuranic radioactive element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen and...
but when uranium is used, it is even more highly enriched in U-235.
If at least one
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
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
criticalCritical may denote:*pertaining to a critic*pertaining to a critique*pertaining to a crisisMore specifically:-Psychology and education:*critical pedagogy - helping students achieve critical consciousness...
, 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
moderatedIn 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 fragmentsNuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of 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 decayRadioactive decay is the process in which an unstable atomic nucleus spontaneously 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, named the daughter...
. Some of them produce neutrons, called delayed neutrons, which contribute to the fission chain reaction. In
nuclear reactorA nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate.The most significant use of nuclear reactors is as an energy source for the generation of electrical power and for the power in some ships...
s, the reaction is slowed down by the addition of control rods which are made of
elementA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
s such as
boronBoron is the 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....
,
cadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. The soft, bluish-white transition metal is chemically similar to the two other metals in group 12, zinc and mercury. Similar to zinc it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, and
hafniumHafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
which can absorb a large number of neutrons. In nuclear bombs, the reaction is uncontrolled and the large amount of
energyIn physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law...
released creates a
nuclear explosionA 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...
.
The fissile uranium in
nuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion...
s, containing 85% or more of
235U is known as weapons 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, use of
implosionImplosion is a process in which objects are destroyed by collapsing on themselves. The opposite of explosion, implosion concentrates matter and energy...
and
neutron reflectorA neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, lead, steel, tungsten carbide, or other materials...
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 BoyLittle Boy was the codename of the atomic bomb dropped on Hiroshima on August 6, 1945 by the B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets of the 393d Bombardment Squadron, Heavy, of the United States Army Air Forces. It was the first atomic bomb to be used as a weapon...
atomic bomb was fueled by highly enriched uranium. Most modern
nuclear weapon designNuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three basic design types...
s use
plutoniumPlutonium is a rare transuranic radioactive element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen and...
as the fissile component of the primary stage however HEU is often used in the secondary stage.
See also
- 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. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...
- Thorium
Thorium is a chemical element with the symbol Th and atomic number 90. It is a naturally occurring, slightly radioactive metal. Thorium is estimated to be about three to four times more abundant than uranium in the earth's crust...
- Nuclear fuel cycle
The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in...
- Thorium fuel cycle
The thorium fuel cycle is a nuclear fuel cycle that uses the naturally abundant isotope of thorium, 232Th, as fertile material, and the artificial uranium isotope, 233U, as fissile fuel for a nuclear reactor...
- Nuclear power
Nuclear power is power produced from controlled nuclear reactions. Commercial plants in use to date use nuclear fission reactions....
- Nuclear reprocessing
Nuclear reprocessing separates components of spent nuclear fuel.Reprocessing serves multiple purposes, whose relative importance has changed over time:*Producing plutonium for nuclear weapons...
- AREVA
AREVA is a French public multinational industrial conglomerate that is mainly known for nuclear power; it also has interests in other energy projects. It was created on 3 September 2001, by the merger of Framatome and Cogema...
- United States Enrichment Corporation
The United States Enrichment Corporation, a subsidiary of USEC Inc. , is a corporation that contracts with the United States Department of Energy to produce enriched uranium for use in nuclear power plants.-History:...
- Uranium market
The uranium market, like all commodity markets, has a history of volatility, moving not only with the standard forces of supply and demand, but also to whims of geopolitics. It has also evolved particularities of its own in response to the unique nature and use of this material.The only significant...
External links