See Also

Nuclear fission

For the generation of electrical power by fission, see Nuclear power plant Nuclear power plant

A nuclear power plant is a thermal [i] power station [i] in which the heat source is one or more nuclear reactor [i] ... 

Nuclear fission - also known as atomic fission - is a process in nuclear physics in which the nucleus Atomic nucleus

The nucleus of an atom [i] is the very dense region in its center consisting of proton [i]s and neutron [i] ... 

 of an atom splits into two or more smaller nuclei as fission product Fission product

Fission products are the residues of fission [i] processes. ... 

s, and usually some by-product particles. Hence, fission is a form of elemental transmutation. The by-products include free neutron Neutron

In physics [i], the neutron is a subatomic particle [i] with no net electric charge [i] and a mass [i] o ... 

s, photon Photon

In modern physics [i], the photon is the elementary particle [i] responsible for electromagnetic phenomena [i] ... 

s usually in the form gamma rays Gamma ray

Gamma rays are an energetic form of electromagnetic radiation [i] produced by radioactive decay [i] or ... 

, and other nuclear fragments such as beta particles Beta particle

[i]s emitted by certain types of [[radioactive]... 

 and alpha particles Alpha particle

Alpha particles are a highly ionizing [i] form of particle radiation [i] which have low pene ... 

.

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Timeline

1939   Nuclear fission discovered independently by Lise Meitner Lise Meitner

Lise Meitner was an Austria [i]n physicist [i] who studied radioactivity [i] and nuclear physics [i] ... 

 and Otto Hahn

1958   U.S. United States

The United States of America, also known as the United States, the U.S., the U.S.A., a... 

 B-47 B-47 Stratojet

The Boeing [i] B-47 Stratojet jet bomber [i] was a medium range and size bomber capable of flying at hig ... 

 bomber accidentally drops an atom bomb Nuclear weapon

A nuclear weapon derives its destructive force from nuclear reaction [i]s of fission [i] ... 

 on Mars Bluff, South Carolina South Carolina

South Carolina is a state [i] in the Southern [i] region of the United States [i] ... 

. Its conventional explosives destroy a house and injure several people, but no nuclear fission occurs.



Encyclopedia

For the generation of electrical power by fission, see Nuclear power plant Nuclear power plant

A nuclear power plant is a thermal [i] power station [i] in which the heat source is one or more nuclear reactor [i] ... 



Nuclear fission - also known as atomic fission - is a process in nuclear physics in which the nucleus Atomic nucleus

The nucleus of an atom [i] is the very dense region in its center consisting of proton [i]s and neutron [i] ... 

 of an atom splits into two or more smaller nuclei as fission product Fission product

Fission products are the residues of fission [i] processes. ... 

s, and usually some by-product particles. Hence, fission is a form of elemental transmutation. The by-products include free neutron Neutron

In physics [i], the neutron is a subatomic particle [i] with no net electric charge [i] and a mass [i] o ... 

s, photon Photon

In modern physics [i], the photon is the elementary particle [i] responsible for electromagnetic phenomena [i] ... 

s usually in the form gamma rays Gamma ray

Gamma rays are an energetic form of electromagnetic radiation [i] produced by radioactive decay [i] or ... 

, and other nuclear fragments such as beta particles Beta particle

[i]s emitted by certain types of [[radioactive]... 

 and alpha particles Alpha particle

Alpha particles are a highly ionizing [i] form of particle radiation [i] which have low pene... 

. Fission of heavy elements is an exothermic reaction Exothermic reaction

In chemistry [i], an exothermic reaction is one that releases heat [i]. ... 

 and can release substantial amounts of useful energy Energy

In general, the concept [i] of energy refers to "the potential for causing changes." The word is used in ... 

 both as gamma rays and as kinetic energy of the fragments .

Nuclear fission is used to produce energy for nuclear power Nuclear power

Nuclear power is the controlled use of nuclear reactions [i] to release energy for work including propulsion [i] ... 

 and to drive explosion of nuclear weapon Nuclear weapon

A nuclear weapon derives its destructive force from nuclear reaction [i]s of fission [i] ... 

s. Fission is useful as a power source because some materials, called nuclear fuel Nuclear fuel

Nuclear fuel is any material that can be consumed to derive nuclear energy [i], by analogy to chemical fuel [i] ... 

s, 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 Nuclear reactor

A nuclear reactor is a device in which nuclear chain reaction [i]s are initiated, controlled, and sustai ... 

 or at a very rapid uncontrolled rate in a nuclear weapon Nuclear weapon

A nuclear weapon derives its destructive force from nuclear reaction [i]s of fission [i] ... 

.

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 Gasoline

Gasoline, also called petrol, is a petroleum [i]-derived liquid [i] mixture consisting primarily o ... 

, making nuclear fission a very tempting source of energy; however, the waste products Radioactive waste

Radioactive waste is waste type [i] containing radioactive [i] chemical element [i]s ... 

 of nuclear fission are highly radioactive Radioactive decay

Radioactive decay is the set of various processes by which unstable atomic nuclei [i] ... 

 and remain so for millennia, giving rise to a nuclear waste problem Radioactive waste

Radioactive waste is waste type [i] containing radioactive [i] chemical element [i]s ... 

. 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 Radioactive decay

Radioactive decay is the set of various processes by which unstable atomic nuclei [i] ... 

 in that it can be harnessed and controlled via a chain reaction: free neutrons Neutron

In physics [i], the neutron is a subatomic particle [i] with no net electric charge [i] and a mass [i] o ... 

 released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions. Chemical Chemical element

A chemical element, often called simply an element, is a substance [i] that can... 

 isotopes that can sustain a fission chain reaction are called nuclear fuel Nuclear fuel

Nuclear fuel is any material that can be consumed to derive nuclear energy [i], by analogy to chemical fuel [i] ... 

s, and are said to be fissile. The most common nuclear fuels are 235U Uranium-235

Uranium-235 is an isotope [i] of uranium [i] that differs from the element's other common isotope, uranium-238 [i]... 

  and 239Pu . These fuels break apart into a range of chemical elements with atomic masses near 100 . Most nuclear fuels undergo spontaneous fission only very slowly, decaying mainly via an alpha Alpha particle

Alpha particles are a highly ionizing [i] form of particle radiation [i] which have low pene... 

/beta Beta particle

[i]s emitted by certain types of [[radioactive]... 

 decay chain over periods of millennia to eons. In a nuclear reactor Nuclear reactor

A nuclear reactor is a device in which nuclear chain reaction [i]s are initiated, controlled, and sustai ... 

 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 fuel atom that undergoes fission, which is why nuclear fission is used as an energy source Energy development

ergy development is the ongoing effort to provide sustainable, accessible energy [i] resources through know ... 

. By contrast, most chemical Chemical reaction

A chemical reaction is a process that results in the interconversion of chemical substance [i]s . ... 

 oxidation Redox

Redox reactions include all chemical processes [i] in which atoms have their oxidation number [i] ... 

 reactions 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 Electromagnetic radiation

Electromagnetic radiation is generally described as a self-propagating wave [i] in space with electric [i] ... 

 in the form of gamma ray Gamma ray

Gamma rays are an energetic form of electromagnetic radiation [i] produced by radioactive decay [i] or ... 

s; in a nuclear reactor, the energy is converted to heat Heat

In physics [i], heat, symbolized by Q, is defined as energy in transit. ... 

 as the particles and gamma rays collide with the atoms that make up the reactor and its working fluid, usually water Water

Water is a taste [i]less, odor [i]less substance that is essential to all known forms of life [i] and i ... 

 or occasionally heavy water.
-
- Nuclear fission of heavy elements produces energy because the specific binding energy Binding energy

Binding energy is the energy [i] required to disassemble a whole into separate parts. ... 

  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 from a single reaction, after their kinetic energy has been dissipated, is less than the mass of the original fuel nucleus. The excess mass ?m is associated with the released energy which carries it away, according to Einstein's relation E=mc² E=mc²

In physics [i] E = mc2 is an important and well-known equation [i], which states an equivalence betw ... 

, where the mass is ?m. In comparison, the specific binding energies of many lighter elements [elements 1 through approximately 12 ] are also significantly less than that of intermediate-mass nuclei, so if the lighter elements undergo nuclear fusion Nuclear fusion

In physics [i], nuclear fusion is the process by which multiple nuclei [i] join together ... 

 , this process also releases heat energy .
-
-
-
- The variation in specific binding energy with atomic number is due to the interplay of the two fundamental forces acting on the component nucleons that make up the nucleus. Nuclei are bound by an attractive strong nuclear force between nucleons, which overcomes the electrostatic repulsion VSEPR theory

Valence shell electron pair repulsion theory is a model [i] in chemistry [i] that aims to generall ... 

 between protons. However, the strong nuclear force acts only over extremely short ranges, since it follows a Yukawa potential Yukawa potential

A Yukawa potential is a potential [i] of the form
... 

. 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 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 , while the other nucleus with mass 135 u is still not far out of the range of the most tightly bound nuclei .
-

-
- 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; however this process works better for heavier elements which have room in outer nuclear orbitals for the necessary extra neutrons. 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 Radioactive decay

Radioactive decay is the set of various processes by which unstable atomic nuclei [i] ... 

 high level waste from nuclear reactors. Fission products tend to be beta emitters Beta particle

[i]s emitted by certain types of [[radioactive]... 

, emitting Beta decay

In nuclear physics [i], beta decay is a type of radioactive [i] decay in which a beta particle [i] is e ... 

 fast-moving electron Electron

The electron is a fundamental [i] subatomic particle [i] that carries an electric charge [i]... 

s 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 Alpha particle

Alpha particles are a highly ionizing [i] form of particle radiation [i] which have low pene... 

 emitter and hence not particularly dangerous unless ingested. Once a fuel element Nuclear reactor

A nuclear reactor is a device in which nuclear chain reaction [i]s are initiated, controlled, and sustai ... 

 has been used, the remaining fuel material is intimately mixed with highly radioactive fission products that emit energetic beta particles Beta particle

[i]s emitted by certain types of [[radioactive]... 

 and gamma rays Gamma ray

Gamma rays are an energetic form of electromagnetic radiation [i] produced by radioactive decay [i] or ... 

. 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 Yucca Mountain

Yucca Mountain is a ridge-line [i] in Nye County [i], Nevada [i], composed of volcanic [i]... 

 until the fission products decay into non-radioactive stable isotopes.
-
-

Spontaneous and induced fission; chain reactions

- Many heavy elements, such as uranium Uranium

Uranium is a chemical element [i] in the periodic table [i] that has the symbol U and atomic number [i] ... 

, thorium Thorium

Thorium is a chemical element [i] in the periodic table [i] that has the symbol Th and atomic number [i] ... 

, and plutonium Plutonium

Plutonium is a radioactive [i], metal [i]lic chemical element [i].... 

, undergo both spontaneous fission, a form of radioactive decay Radioactive decay

Radioactive decay is the set of various processes by which unstable atomic nuclei [i] ... 

 and induced fission,a form of nuclear reaction Nuclear reaction

style="float:right; margin-left:1em; width:300px; "> [i] ... 

. Elemental isotopes that undergo induced fission when struck by a free neutron Neutron

In physics [i], the neutron is a subatomic particle [i] with no net electric charge [i] and a mass [i] o ... 

 are called fissionable; isotopes that undergo fission when struck by a thermal Neutron temperature

The neutron temperature, also called the neutron energy, indicates a free neutron's [i] ... 

, slow moving neutron are also called fissile. A few particularly fissile and readily obtainable isotopes are called nuclear fuel Nuclear fuel

Nuclear fuel is any material that can be consumed to derive nuclear energy [i], by analogy to chemical fuel [i] ... 

s 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 Critical Mass

Critical Mass is a bike ride typically held on the last Friday of every month in cities [i] around ... 

 or, if the assembly is almost entirely made of a nuclear fuel, a critical mass Critical Mass

Critical Mass is a bike ride typically held on the last Friday of every month in cities [i] around ... 

. The word "critical" refers to a cusp in the behavior of the differential equation Differential equation

In mathematics [i], a differential equation is an equation [i] in which the derivative [i]s of a function [i]... 

 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 Radioactive decay

Radioactive decay is the set of various processes by which unstable atomic nuclei [i] ... 

, 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 Mass

Mass is a property of a physical [i] object that quantifies the amount of matter [i] and energy [i] ... 

 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 and decay by beta emission Beta decay

In nuclear physics [i], beta decay is a type of radioactive [i] decay in which a beta particle [i] is e ... 

 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 weapon Nuclear weapon

A nuclear weapon derives its destructive force from nuclear reaction [i]s of fission [i] ... 

s, by jacketing the weapon with 238U to react with neutrons released by nuclear fusion Nuclear fusion

In physics [i], nuclear fusion is the process by which multiple nuclei [i] join together ... 

 at the center of the device.
-
-

Fission reactors

- Critical fission reactors are the most common type of nuclear reactor Nuclear reactor

A nuclear reactor is a device in which nuclear chain reaction [i]s are initiated, controlled, and sustai ... 

. 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 Radioactive decay

Radioactive decay is the set of various processes by which unstable atomic nuclei [i] ... 

 or particle accelerator Particle accelerator

A particle accelerator is a device that uses electric [i] and/or magnetic field [i]s to p ... 

s 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 reactor Nuclear power plant

A nuclear power plant is a thermal [i] power station [i] in which the heat source is one or more nuclear reactor [i] ... 

s
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 Submarine

A submarine is a specialized watercraft [i] that can operate underwater [i].... 

.
- * research reactors are intended to produce neutrons and/or activate radioactive sources for scientific, medical, engineering, or other research purposes.
- * breeder reactor Fast breeder reactor

The fast breeder or fast breeder reactor is a fast neutron reactor [i] designed to breed fuel ... 

s
are intended to produce nuclear fuels in bulk from more abundant isotopes. The most common type makes 239Pu from the naturally very abundant 238U .
-
- 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. . 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 Heat engine

In engineering [i] and thermodynamics [i], a heat engine performs the conversion of heat [i] energy [i] ... 

 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 Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

. 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 Nuclear reactor

A nuclear reactor is a device in which nuclear chain reaction [i]s are initiated, controlled, and sustai ... 

.
-
-

Fission bombs

- One class of nuclear weapon Nuclear weapon

A nuclear weapon derives its destructive force from nuclear reaction [i]s of fission [i] ... 

, a fission 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 . Development of nuclear weapons was the motivation behind early research into nuclear fission: the Manhattan Project Manhattan Project

The Manhattan Project refers to the effort to develop the first nuclear weapon [i]s during World War II [i] ... 

 of the U.S. military Military of the United States

The military of the United States, officially known as the United States [i] Armed Forces [i] ... 

 during World War Two World War II

World War II, or the Second World War, was a worldwide [i] conflict [i] fought betwe ... 

 carried out most of the early scientific work on fission chain reactions, culminating in the Little Boy Little Boy

Little Boy was the codename [i] of the atomic bomb [i] which was dropped on Hiroshima [i] ... 

 and Fat Man Fat Man

"Fat Man" was the codename of the atomic bomb [i] that was detonated over Nagasaki [i], Japan [i] ... 

 bombs that were exploded over Hiroshima Hiroshima

The Japanese city of is the capital of Hiroshima Prefecture [i], and the largest city in the Chugoku region [i]... 

 and Nagasaki Nagasaki

listen, literally "long peninsula [i]", is the capital [i] and the largest city [i] o ... 

, Japan Japan

is an island country [i] in East Asia [i]. ... 

 in August of 1945.
-
- Even the first fission bombs were thousands of times more explosive Explosive material

Any explosive material has the following characteristics:
... 

 than a comparable mass of chemical explosive Explosive material

Any explosive material has the following characteristics:
... 

. For example, Little Boy weighed a total of about four tons , and yielded an explosion equivalent to about 15,000 tons of TNT Trinitrotoluene

Trinitrotoluene is an explosive [i].... 

, destroying a large part of the city of Hiroshima Hiroshima

The Japanese city of is the capital of Hiroshima Prefecture [i], and the largest city in the Chugoku region [i]... 

. Modern nuclear weapons 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 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 . It would be extremely difficult to convert a nuclear reactor Nuclear reactor

A nuclear reactor is a device in which nuclear chain reaction [i]s are initiated, controlled, and sustai ... 

 to cause a true nuclear explosion , and similarly difficult to extract useful power from a nuclear explosive .
-
- The strategic importance of nuclear weapons is a major reason why the technology Technology

Despite its cultural pervasiveness, technology is an elusive concept.... 

 of nuclear fission is politically sensitive. Viable fission bomb designs are within the capabilities of bright undergraduates , but nuclear fuel to realize the designs is thought to be difficult to obtain .

History

The results of the bombardment of uranium by neutrons had proved interesting and puzzling. First studied by Enrico Fermi Enrico Fermi

Enrico Fermi was an Italian [i] physicist [i] most noted for his work on beta decay [i], the deve ... 

 and his colleagues in 1934, they were not properly interpreted until several years later.

On January 16 1939, Niels Bohr Niels Bohr

Niels Bohr was a Danish [i] physicist [i] who made fundamental contributions to understanding ... 

 of Copenhagen Copenhagen

Copenhagen is the capital [i] of Denmark [i] and the country's largest city , at present made up of 16 ... 

, Denmark Denmark

The Kingdom of Denmark is the smallest and southernmost of the Nordic countries [i].... 

, arrived in the United States United States

The United States of America, also known as the United States, the U.S., the U.S.A., a... 

 to spend several months in Princeton, New Jersey Princeton, New Jersey

Princeton, New Jersey, is located in Mercer County [i], New Jersey [i], United States [i] ... 

, and was particularly anxious to discuss some abstract problems with Albert Einstein Albert Einstein

Albert Einstein was a German [i]-born theoretical physicist [i]. ... 

. Just before Bohr left Denmark, two of his colleagues, Otto Robert Frisch Otto Robert Frisch

Otto Robert Frisch, Austrian [i]-British [i] physicist [i]. ... 

 and Lise Meitner Lise Meitner

Lise Meitner was an Austria [i]n physicist [i] who studied radioactivity [i] and nuclear physics [i] ... 

 , 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 they dubbed "nuclear fission" .

The occasion for this hypothesis was the important discovery of Otto Hahn and Fritz Strassmann in Germany 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 Princeton University

Princeton University is a coeducation [i]al private university [i] located in Princeton, New Jersey [i]. ... 

, and from them the news spread by word of mouth to neighboring physicists including Enrico Fermi Enrico Fermi

Enrico Fermi was an Italian [i] physicist [i] most noted for his work on beta decay [i], the deve ... 

 at Columbia University Columbia University

Columbia University is a private [i] university [i] whose main campus lies in the Morningside Heights [i] ... 

. 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 Uranium

Uranium is a chemical element [i] in the periodic table [i] that has the symbol U and atomic number [i] ... 

 nucleus. On January 26, 1939, there was a conference on theoretical physics at Washington, D.C. Washington, D.C.

Washington, D.C. is the capital [i] city [i] of the United States of America [i]. ... 

, sponsored jointly by the George Washington University George Washington University

The George Washington University is a private, coeducational, non-sectarian university located in Washington, D.C. [i] ... 

 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 Leó Szilárd

Le Szilrd was a Hungarian [i]-American [i] physicist [i] who conceived the nuclear chain reaction [i] ... 

, 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 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. Frédéric Joliot in Paris Paris

native_name = Ville de Paris
|common_name = 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 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 , and then to the Manhattan project Manhattan Project

The Manhattan Project refers to the effort to develop the first nuclear weapon [i]s during World War II [i] ... 

 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 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 Deuterium

Deuterium, also called heavy hydrogen, is a stable isotope [i] of hydrogen [i] with a natural abundance [i] ... 

 , helium Helium

|-
| 3He || 0.000137%* || colspan="4" | He is stable [i] with 1 neutron [i]
... 

, beryllium Beryllium

Beryllium is the chemical element [i] in the periodic table [i] that has the symbol Be and atomic number [i] ... 

, or carbon, usually as the graphite Graphite

Graphite is one of the allotropes of [i] carbon [i]. ... 

 .

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 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 Thermite

A thermite reaction is a type of aluminothermic reaction [i] in which aluminium [i] metal [i] is oxidized [i] ... 

 process. Ames Laboratory was established in 1942 to produce the large amounts of natural uranium that would be necessary for the research to come. The success of the Chicago Pile-1 which used unenriched 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 Natural nuclear fission reactor

A natural nuclear fission reactor is a uranium [i] deposit where analysis of isotope ratios has shown th ... 

, 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 reactor Nuclear reactor

A nuclear reactor is a device in which nuclear chain reaction [i]s are initiated, controlled, and sustai ... 

s and nuclear weapon Nuclear weapon

A nuclear weapon derives its destructive force from nuclear reaction [i]s of fission [i] ... 

s, see Manhattan Project Manhattan Project

The Manhattan Project refers to the effort to develop the first nuclear weapon [i]s during World War II [i] ... 

.

External links

  • Historical account complete with audio and teacher's guides from the American Institute of Physics History Center
  • Nuclear Fission Explained
  • What is Nuclear Fission?
  • A simple explanation of the process of nuclear fission