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Nuclear fission

2007 Schools Wikipedia Selection. Related subjects: General Physics

   An induced nuclear fission event. A thermal (slow-moving) neutron is
   absorbed by the nucleus of a uranium-235 atom, which in turn splits
   into fast-moving lighter elements (fission products) and free neutrons.
   The particular elements and number of neutrons produced by each single
   fission event are random.
   Enlarge
   An induced nuclear fission event. A thermal (slow-moving) neutron is
   absorbed by the nucleus of a uranium-235 atom, which in turn splits
   into fast-moving lighter elements (fission products) and free neutrons.
   The particular elements and number of neutrons produced by each single
   fission event are random.

   Nuclear fission—also known as atomic fission—is a process in nuclear
   physics and nuclear chemistry in which the nucleus of an atom splits
   into two or more smaller nuclei as fission products, and usually some
   by-product particles. Hence, fission is a form of elemental
   transmutation. The by-products include free neutrons, photons usually
   in the form gamma rays, and other nuclear fragments such as beta
   particles and alpha particles. Fission of heavy elements is an
   exothermic reaction and can release substantial amounts of useful
   energy both as gamma rays and as kinetic energy of the fragments
   (heating the bulk material where fission takes place).

   Nuclear fission produces energy for nuclear power and to drive
   explosion of nuclear weapons. Fission is useful as a power source
   because some materials, called nuclear fuels, both generate neutrons as
   part of the fission process and also undergo triggered fission when
   impacted by a free neutron. Nuclear fuels can be part of a
   self-sustaining 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 tempting source of
   energy; however, the waste products of nuclear fission are highly
   radioactive and remain so for millennia, giving rise to a nuclear waste
   problem. Concerns over nuclear waste accumulation and over the immense
   destructive potential of nuclear weapons counterbalance the desirable
   qualities of fission as an energy source, and give rise to intense
   ongoing political debate over nuclear power.

Physical overview

   Nuclear fission differs from other forms of radioactive decay in that
   it can be harnessed and controlled via a chain reaction: free neutrons
   released by each fission event can trigger yet more events, which in
   turn release more neutrons and cause more fissions. Chemical 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 ^239Pu (the isotope
   of plutonium with an atomic mass of 239). These fuels break apart into
   a range of chemical elements with atomic masses near 100 (fission
   products). Most nuclear fuels undergo spontaneous fission only very
   slowly, decaying mainly via an alpha/ beta decay chain over periods of
   millennia to eons. In a nuclear reactor or nuclear weapon, most fission
   events are induced by bombardment with another particle such as a
   neutron.

   Typical fission events release several hundred MeV of energy for each
   fission event, which is why nuclear fission is used as an energy
   source. By contrast, most chemical oxidation reactions (such as burning
   coal or TNT) release at most a few tens of eV per event, so nuclear
   fuel contains at least ten million times more usable energy than does
   chemical fuel. The energy of nuclear fission is released as kinetic
   energy of the fission products and fragments, and as electromagnetic
   radiation in the form of gamma rays; in a nuclear reactor, the energy
   is converted to heat as the particles and gamma rays collide with the
   atoms that make up the reactor and its working fluid, usually water or
   occasionally heavy water.

   Nuclear fission of heavy elements produces energy because the specific
   binding energy (binding energy per mass) of intermediate-mass nuclei
   with atomic numbers and atomic masses close to ^61Ni and ^56Fe is
   greater than the specific binding energy of very heavy nuclei, so that
   energy is released when heavy nuclei are broken apart.

   The total mass of the fission products (Mp) from a single reaction,
   after their kinetic energy has been dissipated, is less than the mass
   of the original fuel nucleus. The excess mass Δm is associated with the
   released energy which carries it away, according to Einstein's relation
   E=mc², where the mass is Δm. In comparison, the specific binding
   energies of many lighter elements [elements 1 (hydrogen) through
   approximately 12 (magnesium)] are also significantly less than that of
   intermediate-mass nuclei, so if the lighter elements undergo nuclear
   fusion (the counterpart to nuclear fission), this process also releases
   heat energy (is "exothermic").

   E=M_{U^{235}}~c^2- M_P~c^2

   The variation in specific binding energy with atomic number is due to
   the interplay of the two fundamental forces acting on the component
   nucleons (protons and neutrons) that make up the nucleus. Nuclei are
   bound by an attractive strong nuclear force between nucleons, which
   overcomes the electrostatic repulsion between protons. However, the
   strong nuclear force acts only over extremely short ranges, since it
   follows a Yukawa potential. For this reason large nuclei are less
   tightly bound per unit mass than small nuclei, and breaking a very
   large nucleus into two or more intermediate-sized nuclei releases
   energy. In practice, as noted, most of this energy appears as kinetic
   energy as the two smaller nuclei mutually repel and fly away from each
   other at high speed.

   In nuclear fission events the nuclei may break into any combination of
   lighter nuclei, but the most common event is not fission to equal mass
   nuclei of about mass 120; the most common event (depending on isotope
   and process) is a slightly unequal fission in which one daughter
   nucleus has a mass of about 90 to 100 u and the other the remaining 130
   to 140 u . Unequal fissions are energetically more favorable because
   this allows one product to be closer to the energetic minimum near mass
   60 u (only a quarter of the average fissionable mass), while the other
   nucleus with mass 135 u is still not far out of the range of the most
   tightly bound nuclei (another statement of this, is that the atomic
   binding energy curve is slightly steeper to the left of mass 120 u than
   to the right of it).

   Because of the short range of the strong binding force, large nuclei
   must contain proportionally more neutrons than do light elements, which
   are most stable with a 1-1 ratio of protons and neutrons. Extra
   neutrons stabilize heavy elements because they add to strong-force
   binding without adding to proton-proton repulsion. Fission products
   have, on average, about the same ratio of neutrons and protons as their
   parent nucleus, and are therefore usually unstable because they have
   proportionally too many neutrons compared to stable isotopes of similar
   mass. This is the fundamental cause of the problem of radioactive high
   level waste from nuclear reactors. Fission products tend to be beta
   emitters, emitting fast-moving electrons to conserve electric charge as
   excess neutrons convert to protons inside the nucleus of the fission
   product atoms.

   The most common nuclear fuels, ^235U and ^239Pu, are not major
   radiologic hazards by themselves: ^235U has a half-life of
   approximately 700 million years, and although ^239Pu has a half-life of
   only about 24,000 years, it is a pure alpha particle emitter and hence
   not particularly dangerous unless ingested. Once a fuel element has
   been used, the remaining fuel material is intimately mixed with highly
   radioactive fission products that emit energetic beta particles and
   gamma rays. Some fission products have half-lives as short as seconds;
   others have half-lives of tens of thousands of years, requiring
   long-term storage in facilities such as Yucca mountain until the
   fission products decay into non-radioactive stable isotopes.

Spontaneous and induced fission; chain reactions

   Many heavy elements, such as uranium, thorium, and plutonium, undergo
   both spontaneous fission, a form of radioactive decay and induced
   fission, a form of nuclear reaction. Elemental isotopes that undergo
   induced fission when struck by a free neutron are called fissionable;
   isotopes that undergo fission when struck by a thermal, slow moving
   neutron are also called fissile. A few particularly fissile and readily
   obtainable isotopes (notably ^235U and ^239Pu) are called nuclear fuels
   because they can sustain a chain reaction and can be obtained in large
   enough quantities to be useful.

   All fissionable and fissile isotopes undergo a small amount of
   spontaneous fission which releases a few free neutrons into any sample
   of nuclear fuel. The neutrons typically escape rapidly from the fuel
   and become a free neutron, with a half-life of about 15 minutes before
   they decay to protons and beta rays. The neutrons usually impact and
   are absorbed by other nuclei in the vicinity before this happens.
   However, some neutrons will impact fuel nuclei and induce further
   fissions, releasing yet more neutrons. If enough nuclear fuel is
   assembled into one place, or if the escaping neutrons are sufficiently
   contained, then these freshly generated neutrons outnumber the neutrons
   that escape from the assembly, and a sustained nuclear chain reaction
   will take place.

   An assembly that supports a sustained nuclear chain reaction is called
   a critical assembly or, if the assembly is almost entirely made of a
   nuclear fuel, a critical mass. The word "critical" refers to a cusp in
   the behaviour of the differential equation that governs the number of
   free neutrons present in the fuel: if less than a critical mass is
   present, then the amount of neutrons is determined by radioactive
   decay, but if a critical mass or more is present, then the amount of
   neutrons is controlled instead by the physics of the chain reaction.
   The actual mass of a critical mass of nuclear fuel depends strongly on
   the geometry and surrounding materials.

   Not all fissionable isotopes can sustain a chain reaction. For example,
   ^238U, the most abundant form of uranium, is fissionable but not
   fissile: it undergoes induced fission when impacted by an energetic
   neutron with over 1 MeV of kinetic energy. But too few of the neutrons
   produced by ^238U fission are energetic enough to induce further
   fissions in ^238U, so no chain reaction is possible with this isotope.
   Instead, bombarding ^238U with slow neutrons causes it to absorb them
   (becoming ^239U) and decay by beta emission to ^239Pu; that process is
   used to manufacture ^239Pu in breeder reactors, but does not contribute
   to a neutron chain reaction.

   Fissionable, non-fissile isotopes can be used as fission energy source
   even without a chain reaction. Bombarding ^238U with fast neutrons
   induces fissions, releasing energy as long as the external neutron
   source is present. That effect is used to augment the energy released
   by modern thermonuclear weapons, by jacketing the weapon with ^238U to
   react with neutrons released by nuclear fusion at the centre of the
   device.

Fission reactors

   Critical fission reactors are the most common type of nuclear reactor.
   In a critical fission reactor, neutrons produced by fission of fuel
   atoms are used to induce yet more fissions, to sustain a controllable
   amount of energy release. Devices that produce engineered but
   non-self-sustaining fission reactions are subcritical fission reactors.
   Such devices use radioactive decay or particle accelerators to trigger
   fissions.

   Critical fission reactors are built for three primary purposes, which
   typically involve different engineering trade-offs to take advantage of
   either the heat or the neutrons produced by the fission chain reaction:
     * power reactors are intended to produce heat for nuclear power,
       either as part of a generating station or a local power system such
       as a nuclear submarine.
     * research reactors are intended to produce neutrons and/or activate
       radioactive sources for scientific, medical, engineering, or other
       research purposes.
     * breeder reactors are intended to produce nuclear fuels in bulk from
       more abundant isotopes. The most common type makes ^239Pu (a
       nuclear fuel) from the naturally very abundant ^238U (not a nuclear
       fuel).

   While, in principle, all fission reactors can act in all three
   capacities, in practice the tasks lead to conflicting engineering goals
   and most reactors have been built with only one of the above tasks in
   mind. (There are several early counter-examples, such as the Hanford N
   reactor, now decommissioned). Power reactors generally convert the
   kinetic energy of fission products into heat, which is used to heat a
   working fluid and drive a heat engine that generates mechanical or
   electrical power. The working fluid is usually water with a steam
   turbine, but some designs use other materials such as gaseous helium.
   Research reactors produce neutrons that are used in various ways, with
   the heat of fission being treated as an unavoidable waste product.
   Breeder reactors are a specialized form of research reactor, with the
   caveat that the sample being irradiated is usually the fuel itself, a
   mixture of ^238U and ^235U.

   For a more detailed description of the physics and operating principles
   of critical fission reactors, see nuclear reactor physics. For a
   description of their social, political, and environmental aspects, see
   nuclear reactor.

Fission bombs

   One class of nuclear weapon, a fission bomb (not to be confused with
   the fusion bomb), otherwise known as an atomic bomb or atom bomb, is a
   fission reactor designed to liberate as much energy as possible as
   rapidly as possible, before the released energy causes the reactor to
   explode (and the chain reaction to stop). Development of nuclear
   weapons was the motivation behind early research into nuclear fission:
   the Manhattan Project of the U.S. military during World War II carried
   out most of the early scientific work on fission chain reactions,
   culminating in the Little Boy and Fat Man bombs that were exploded over
   Hiroshima and Nagasaki, Japan in August of 1945.

   Even the first fission bombs were thousands of times more explosive
   than a comparable mass of chemical explosive. For example, Little Boy
   weighed a total of about four tons (of which 60 kg was nuclear fuel),
   and yielded an explosion equivalent to about 15,000 tons of TNT,
   destroying a large part of the city of Hiroshima. Modern nuclear
   weapons (which include a thermonuclear fusion as well as one or more
   fission stages) are literally hundreds of times more energetic for
   their weight than the first pure fission atomic bombs, so that a modern
   single missile warhead bomb weighing less than 1/8th as much as Little
   Boy (see for example W88) has a yield of 475,000 tons of TNT, and could
   bring destruction to 10 times the city area.

   While the fundamental physics of the fission chain reaction in a
   nuclear weapon is similar to the physics of a controlled nuclear
   reactor, the two types of device must be engineered quite differently
   (see nuclear reactor physics). It would be extremely difficult to
   convert a nuclear reactor to cause a true nuclear explosion (though
   fuel meltdowns and steam explosions have occurred), and similarly
   difficult to extract useful power from a nuclear explosive (though at
   least one rocket propulsion system, Project Orion, was intended to work
   by exploding fission bombs behind a massively padded vehicle!).

   The strategic importance of nuclear weapons is a major reason why the
   technology of nuclear fission is politically sensitive. Viable fission
   bomb designs are within the capabilities of bright undergraduates (see
   John Aristotle Phillips), but nuclear fuel to realize the designs is
   thought to be difficult to obtain (see uranium enrichment and nuclear
   fuel cycle).

History

   The results of the bombardment of uranium by neutrons had proved
   interesting and puzzling. First studied by Enrico Fermi and his
   colleagues in 1934, they were not properly interpreted until several
   years later.

   On January 16, 1939, Niels Bohr of Copenhagen, Denmark, arrived in the
   United States to spend several months in Princeton, New Jersey, and was
   particularly anxious to discuss some abstract problems with Albert
   Einstein. (Four years later Bohr was to escape to Sweden from
   Nazi-occupied Denmark in a small boat, along with thousands of other
   Danish Jews, in large scale operation.) Just before Bohr left Denmark,
   two of his colleagues, Otto Robert Frisch and Lise Meitner (both
   refugees from Germany), had told him their guess that the absorption of
   a neutron by a uranium nucleus sometimes caused that nucleus to split
   into approximately equal parts with the release of enormous quantities
   of energy, a process that Frisch dubbed "nuclear fission" ( fission, as
   previously used up to this point, was a term which was borrowed from
   biology, where it was and is used to describe the splitting of one
   living cell into two). In 1939, Frisch and Meitner submitted their
   article "Disintegration of uranium by neutrons: a new type of nuclear
   reaction" to the scientific journal Nature.

   The occasion for this hypothesis was the basic and historically most
   momentous discovery of Otto Hahn and Fritz Strassmann in Germany
   (published in their first famous article in Naturwissenschaften,
   January 6, 1939) which proved that an isotope of barium was produced by
   neutron bombardment of uranium. Bohr had promised to keep the
   Meitner/Frisch interpretation secret until their paper was published to
   preserve priority, but on the boat he discussed it with Léon Rosenfeld,
   but forgot to tell him to keep it secret. Rosenfeld immediately upon
   arrival told everyone at Princeton University, and from them the news
   spread by word of mouth to neighboring physicists including Enrico
   Fermi at Columbia University. As a result of conversations among Fermi,
   John R. Dunning, and G. B. Pegram, a search was undertaken at Columbia
   for the heavy pulses of ionization that would be expected from the
   flying fragments of the uranium nucleus. On January 26, 1939, there was
   a conference on theoretical physics at Washington, D.C., sponsored
   jointly by the George Washington University and the Carnegie
   Institution of Washington.

   Fermi left New York to attend this meeting before the Columbia fission
   experiments had been tried. At the meeting Bohr and Fermi discussed the
   problem of fission, and in particular Fermi mentioned the possibility
   that neutrons might be emitted during the process. Although this was
   only a guess, its implication of the possibility of a nuclear chain
   reaction was obvious. " Chain reactions" at that time were a known
   phenomenon in chemistry, but the analogous process in nuclear physics
   using neutrons had been foreseen as early as 1933 by Leo Szilard,
   although Szilard at that time had no idea with what materials the
   process might be initiated. Now, with the discovery of neutron-induced
   fission of heavy elements, a number of sensational articles were
   published in the press on the subject of nuclear chain reactions.
   Before the meeting in Washington was over, several other experiments to
   confirm fission had been initiated, and positive experimental
   confirmation was reported from four laboratories ( Columbia University,
   Carnegie Institution of Washington, Johns Hopkins University,
   University of California) in the February 15, 1939, issue of the
   Physical Review. By this time Bohr had heard that similar experiments
   had been made in his laboratory in Copenhagen about January 15. (Letter
   by Frisch to Nature dated January 16, 1939, and appearing in the
   February 18 issue.) Frédéric Joliot in Paris had also published his
   first results in the Comptes Rendus of January 30, 1939. From this time
   on there was a steady flow of papers on the subject of fission, so that
   by the time ( December 6, 1939) L. A. Turner of Princeton wrote a
   review article on the subject in the Reviews of Modern Physics nearly
   one hundred papers had appeared. Complete analysis and discussion of
   these papers have appeared in Turner's article and elsewhere.

   A major focus of early fission research was on producing a controllable
   nuclear chain reaction, which would mark the first harnessing of
   nuclear power. This led to the development of Chicago Pile-1, the
   world's first man-made critical nuclear reactor (which used uranium,
   the only natural nuclear fuel available in macroscopic quantities), and
   then to the Manhattan project to develop a nuclear weapon.

   Producing a fission chain reaction in uranium fuel is far from trivial.
   Early nuclear reactors did not use isotopically enriched uranium, and
   in consequence they were required to use large quantities of highly
   purified graphite as neutron moderation materials. Use of ordinary
   water (as opposed to heavy water) in nuclear reactors requires enriched
   fuel--- the partial separation and relative enrichment of the rare
   ^235U isotope from the far more common ^238U isotope. Typically,
   reactors also require inclusion of extremely chemically pure neutron
   moderator materials such as deuterium (in heavy water), helium,
   beryllium, or carbon, usually as the graphite (The high purity is
   required because many chemical impurities such as the boron-10
   component of natural boron, are very strong neutron absorbers and thus
   poison the chain reaction).

   Production of such materials at industrial scale had to be solved for
   nuclear power generation and weapons production to be accomplished. Up
   to 1940, the total amount of uranium metal produced in the USA was not
   more than a few grams and even this was of doubtful purity; of metallic
   beryllium not more than a few kilograms; concentrated deuterium oxide (
   heavy water) not more than a few kilograms; and finally carbon had
   never been produced in quantity with anything like the purity required
   of a moderator.

   The problem of producing large amounts of high purity uranium was
   solved by Frank Spedding using the thermite process. Ames Laboratory
   was established in 1942 to produce the large amounts of natural
   (unenriched) uranium that would be necessary for the research to come.
   The success of the Chicago Pile-1 which used unenriched (natural)
   uranium, like all of the atomic "piles" which produced the plutonium
   for the atomic bomb, was also due specifically to Szilard's realization
   that very pure graphite could be used for the moderator of even natural
   uranium "piles". In wartime Germany, failure to appreciate the
   qualities of very pure graphite led to reactor designs dependent on
   heavy water, which in turn was denied the Germans by allied attacks in
   Norway, where heavy water was produced. These difficulties prevented
   the Nazis from building a nuclear reactor capable of criticality during
   the war.

   Unknown until 1972, when French physicist Francis Perrin discovered the
   Oklo Fossil Reactors, nature had beaten humans to the punch by engaging
   in large-scale uranium fission chain reactions, some 2,000 million
   years in the past. This ancient process was able to use normal water as
   a moderator, only because 2,000 million years in the past, natural
   uranium was "enriched" with the shorter-lived fissile isotope ^235U, as
   compared with the natural uranium available today.

   For more detail on the early development of nuclear reactors and
   nuclear weapons, see Manhattan Project.

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