An induced fission reaction. A
neutron is absorbed by a
uranium-235 nucleus, turning it briefly into an excited uranium-236 nucleus, with the excitation energy provided by the kinetic energy of the neutron plus the forces that bind the neutron. The uranium-236, in turn, splits into fast-moving lighter elements (fission products) and releases three free neutrons. At the same time, one or more "prompt
gamma rays" (not shown) are produced, as well.
In
nuclear physics and
nuclear chemistry,
nuclear fission is either a
nuclear reaction or a
radioactive decay process in which the
nucleus of an atom splits into smaller parts (lighter
nuclei). The fission process often produces free
neutrons and
photons (in the form of
gamma rays), and releases a very large amount of
energy even by the energetic standards of radioactive decay.
Nuclear fission of heavy elements was discovered on December 17, 1938 by
Otto Hahn and his assistant
Fritz Strassmann, and explained theoretically in January 1939 by
Lise Meitner and her nephew
Otto Robert Frisch. Frisch named the process by analogy with
biological fission of living cells. It is an
exothermic reaction which can release large amounts of
energy both as
electromagnetic radiation and as
kinetic energy of the fragments (
heating the bulk material where fission takes place).
In order for fission to produce energy, the total
binding energy of the resulting elements must be greater than that of the starting element.
Fission is a form of
nuclear transmutation because the resulting fragments are not the same
element as the original atom. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common
fissile isotopes.
[1][2] Most fissions are binary fissions (producing two charged fragments), but occasionally (2 to 4 times per 1000 events),
three positively charged fragments are produced, in a
ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus.
Fission as encountered in the modern world is usually a deliberately produced man-made
nuclear reaction induced by a neutron. It is less commonly encountered as a natural form of spontaneous
radioactive decay (not requiring a neutron), occurring especially in very high-mass-number isotopes.
The unpredictable composition of the products (which vary in a broad probabilistic and somewhat chaotic manner) distinguishes fission from purely quantum-tunnelling processes such as
proton emission,
alpha decay and
cluster decay, which give the same products each time. Nuclear fission produces energy for
nuclear power and drives the explosion of
nuclear weapons. Both uses are possible because certain substances called
nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes possible a self-sustaining
nuclear chain reaction that releases energy at a controlled rate in a
nuclear reactor or at a very rapid uncontrolled rate in a
nuclear weapon.
The amount of
free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as
gasoline, making nuclear fission a very dense source of energy. The products of nuclear fission, however, are on average far more
radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a
nuclear waste problem. Concerns over
nuclear waste accumulation and over the
destructive potential of
nuclear weapons may counterbalance the desirable qualities of
fission as an energy source, and give rise to ongoing
political debate over
nuclear power.
Mechanism
A visual representation of an induced nuclear fission event where a slow-moving neutron is absorbed by the nucleus of a uranium-235 atom, which fissions into two fast-moving lighter elements (fission products) and additional neutrons. Most of the energy released is in the form of the kinetic velocities of the fission products and the neutrons.
Nuclear fission can occur without
neutron bombardment, as a type of
radioactive decay. This type of fission (called
spontaneous fission) is rare except in a few heavy isotopes. In engineered nuclear devices, essentially all nuclear fission occurs as a "
nuclear reaction" — a bombardment-driven process that results from the collision of two subatomic particles. In nuclear reactions, a subatomic particle collides with an atomic nucleus and causes changes to it. Nuclear reactions are thus driven by the mechanics of bombardment, not by the relatively constant
exponential decay and
half-life characteristic of spontaneous radioactive processes.
Many types of
nuclear reactions are currently known. Nuclear fission differs importantly from other types of nuclear reactions, in that it can be amplified and sometimes controlled via a
nuclear chain reaction (one type of general
chain reaction). In such a reaction, free
neutrons released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions.
The
chemical element isotopes that can sustain a fission chain reaction are called
nuclear fuels, and are said to be
fissile. The most common nuclear fuels are
235U (the isotope of
uranium with an
atomic mass of 235 and of use in nuclear reactors) and
239Pu (the isotope of
plutonium with an
atomic mass of 239). These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135
u (
fission products). Most nuclear fuels undergo
spontaneous fission only very slowly, decaying instead mainly via an
alpha/
beta decay chain over periods of
millennia to
eons.
In a
nuclear reactor or nuclear weapon, the overwhelming majority of fission events are induced by bombardment with another particle, a neutron, which is itself produced by prior fission events.
Nuclear fissions in fissile fuels are the result of the nuclear excitation energy produced when a fissile nucleus captures a neutron. This energy, resulting from the neutron capture, is a result of the attractive
nuclear force acting between the neutron and nucleus. It is enough to deform the nucleus into a double-lobed "drop," to the point that nuclear fragments exceed the distances at which the nuclear force can hold two groups of charged nucleons together, and when this happens, the two fragments complete their separation and then are driven further apart by their mutually repulsive charges, in a process which becomes irreversible with greater and greater distance. A similar process occurs in
fissionable isotopes (such as uranium-238), but in order to fission, these isotopes require additional energy provided by
fast neutrons (such as those produced by
nuclear fusion in
thermonuclear weapons).
The
liquid drop model of the
atomic nucleus predicts equal-sized fission products as an outcome of nuclear deformation. The more sophisticated
nuclear shell model is needed to mechanistically explain the route to the more energetically favorable outcome, in which one fission product is slightly smaller than the other. A theory of the fission based on shell model has been formulated by
Maria Goeppert Mayer.
The most common fission process is binary fission, and it produces the fission products noted above, at 95±15 and 135±15
u. However, the binary process happens merely because it is the most probable. In anywhere from 2 to 4 fissions per 1000 in a nuclear reactor, a process called
ternary fission produces three positively charged fragments (plus neutrons) and the smallest of these may range from so small a charge and mass as a proton (Z=1), to as large a fragment as
argon (Z=18). The most common small fragments, however, are composed of 90% helium-4 nuclei with more energy than alpha particles from alpha decay (so-called "long range alphas" at ~ 16 MeV), plus helium-6 nuclei, and tritons (the nuclei of
tritium). The ternary process is less common, but still ends up producing significant helium-4 and tritium gas buildup in the fuel rods of modern nuclear reactors.
[3]
__________________________________________
Nuclear fusion
In
nuclear physics,
nuclear fusion is a
nuclear reaction in which two or more
atomic nuclei collide at a very high speed and join to form a new type of atomic nucleus. During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to
photons (
energy). Fusion is the process that powers active or "
main sequence"
stars.
The fusion of two nuclei with lower masses than
iron (which, along with
nickel, has the largest
binding energy per
nucleon) generally releases energy, while the fusion of nuclei heavier than iron
absorbs energy. The opposite is true for the reverse process,
nuclear fission. This means that fusion generally occurs for lighter elements only, and likewise, that fission normally occurs only for heavier elements. There are extreme
astrophysical events that can lead to short periods of fusion with heavier nuclei. This is the process that gives rise to
nucleosynthesis, the creation of the heavy elements during events such as
supernovae. Following the discovery of
quantum tunneling by
Friedrich Hund, in 1929
Robert Atkinson and
Fritz Houtermans used the measured masses of light elements to predict that large amounts of energy could be released by fusing small nuclei. Building upon the
nuclear transmutation experiments by
Ernest Rutherford, carried out several years earlier, the laboratory fusion of
hydrogen isotopes was first accomplished by
Mark Oliphant in 1932. During the remainder of that decade the steps of the main cycle of nuclear fusion in stars were worked out by
Hans Bethe.
Research into fusion for military purposes began in the early 1940s as part of the
Manhattan Project. Fusion was accomplished in 1951 with the
Greenhouse Item nuclear test. Nuclear fusion on a large scale in an explosion was first carried out on November 1, 1952, in the
Ivy Mike hydrogen bomb test.
Research into developing controlled
thermonuclear fusion for civil purposes also began in earnest in the 1950s, and it continues to this day. Two projects, the
National Ignition Facility and
ITER, have the goal of high
gains, that is, producing more energy than required to ignite the reaction, after 60 years of design improvements developed from previous experiments.
[citation needed] While these
ICF and
Tokamak designs became popular in recent times, experiments with
Stellarators are gaining international scientific attention again, like
Wendelstein 7-X in
Greifswald, Germany.
fusion energy.
The reaction
cross section σ is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, e.g. a thermal distribution, then it is useful to perform an average over the distributions of the product of cross section and velocity. This average is called the 'reactivity', denoted <σv>. The reaction rate (fusions per volume per time) is <σv> times the product of the reactant number densities:
If a species of nuclei is reacting with itself, such as the DD reaction, then the product
must be replaced by
.
increases from virtually zero at room temperatures up to meaningful magnitudes at temperatures of
10–
100 keV. At these temperatures, well above typical
ionization energies (13.6 eV in the hydrogen case), the fusion reactants exist in a
plasma state.
The significance of
as a function of temperature in a device with a particular energy
confinement time is found by considering the
Lawson criterion. This is an extremely challenging barrier to overcome on Earth, which explains why fusion research has taken many years to reach the current high state of technical prowess.
[10]
Methods for achieving fusion
Thermonuclear fusion
If the matter is sufficiently heated (hence being
plasma), the fusion reaction may occur due to collisions with extreme thermal kinetic energies of the particles. In the form of thermonuclear weapons, thermonuclear fusion is the only fusion technique so far to yield undeniably large amounts of useful
fusion energy.
[citation needed] Usable amounts of thermonuclear fusion energy released in a controlled manner have yet to be achieved.
Inertial confinement fusion
Inertial confinement fusion (
ICF) is a type of
fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of
deuterium and
tritium.
Beam-beam or beam-target fusion
If the energy to initiate the reaction comes from
accelerating one of the nuclei, the process is called
beam-target fusion; if both nuclei are accelerated, it is
beam-beam fusion.
Accelerator-based light-ion fusion is a technique using particle accelerators to achieve particle kinetic energies sufficient to induce light-ion fusion reactions. Accelerating light ions is relatively easy, and can be done in an efficient manner—all it takes is a vacuum tube, a pair of electrodes, and a high-voltage transformer; fusion can be observed with as little as 10 kV between electrodes. The key problem with accelerator-based fusion (and with cold targets in general) is that fusion cross sections are many orders of magnitude lower than Coulomb interaction cross sections. Therefore the vast majority of ions end up expending their energy on
bremsstrahlung and ionization of atoms of the target. Devices referred to as sealed-tube
neutron generators are particularly relevant to this discussion. These small devices are miniature particle accelerators filled with deuterium and tritium gas in an arrangement that allows ions of these nuclei to be accelerated against hydride targets, also containing deuterium and tritium, where fusion takes place. Hundreds of neutron generators are produced annually for use in the petroleum industry where they are used in measurement equipment for locating and mapping oil reserves.
Muon-catalyzed fusion
Muon-catalyzed fusion is a well-established and reproducible fusion process that occurs at ordinary temperatures. It was studied in detail by
Steven Jones in the early 1980s. Net energy production from this reaction cannot occur because of the high energy required to create
muons, their short 2.2 µs
half-life, and the high chance that a muon will bind to the new
alpha particle and thus stop catalyzing fusion.
[11]
Other principles
Some other confinement principles have been investigated, some of them have been confirmed to run nuclear fusion while having lesser expectation of eventually being able to produce net power, others have not yet been shown to produce fusion.
Sonofusion or
bubble fusion, a controversial variation on the
sonoluminescence theme, suggests that acoustic shock waves, creating temporary bubbles (cavitation) that expand and collapse shortly after creation, can produce temperatures and pressures sufficient for nuclear fusion.
[12]
The
Farnsworth–Hirsch fusor is a tabletop device in which fusion occurs. This fusion comes from high effective temperatures produced by electrostatic acceleration of ions.
The
Polywell is a non-thermodynamic equilibrium machine that uses electrostatic confinement to accelerate ions into a center where they fuse together.
Antimatter-initialized fusion uses small amounts of
antimatter to trigger a tiny fusion explosion. This has been studied primarily in the context of making
nuclear pulse propulsion, and
pure fusion bombs feasible. This is not near becoming a practical power source, due to the cost of manufacturing antimatter alone.
Pyroelectric fusion was reported in April 2005 by a team at
UCLA. The scientists used a
pyroelectric crystal heated from −34 to 7 °C (−29 to 45 °F), combined with a
tungsten needle to produce an
electric field of about 25 gigavolts per meter to ionize and accelerate
deuterium nuclei into an
erbium deuteride target. At the estimated energy levels,
[13] the
D-D fusion reaction may occur, producing
helium-3 and a 2.45 MeV
neutron. Although it makes a useful neutron generator, the apparatus is not intended for power generation since it requires far more energy than it produces.
[14][15][16][17]
Hybrid nuclear fusion-fission (hybrid nuclear power) is a proposed means of generating
power by use of a combination of nuclear fusion and
fission processes. The concept dates to the 1950s, and was briefly advocated by
Hans Bethe during the 1970s, but largely remained unexplored until a revival of interest in 2009, due to the delays in the realization of pure fusion.
[18] Project PACER, carried out at
Los Alamos National Laboratory (LANL) in the mid-1970s, explored the possibility of a fusion power system that would involve exploding small
hydrogen bombs (fusion bombs) inside an underground cavity. As an energy source, the system is the only fusion power system that could be demonstrated to work using existing technology. However it would also require a large, continuous supply of nuclear bombs, making the economics of such a system rather questionable.
