The nuclear fission is the phenomenon by which the core of a heavy atom (core which contains many Nucléon S, the such cores of Uranium and of Plutonium) is divided into several lighter nuclides. This nuclear Reaction also results in the emission of neutrons and a very important release of energy (≈ 200 MeV, to compare with energies chemical reactions which are about the eV).
Discovered nuclear fission
The results of the bombardment of uranium cores by neutrons had already appeared interesting and completely intriguing. Initially studied by Enrico Fermi and his/her colleagues in 1934, they were correctly interpreted only several years later.
January 16th, 1939, Niels Bohr arrived at the the United States to spend several months to the Université of Princeton, where it was in a hurry to discuss certain theoretical problems with Albert Einstein. Right before its departure of Denmark, two of his/her colleagues, Reads Meitner and Otto Frisch, had informed him of their assumption according to which the absorption of a neutron by a uranium core causes sometimes the scission of this one in two roughly equal parts, as well as the release of an enormous quantity of energy: they called this phenomenon " fission nucléaire". This assumption was based on the important discovered one of Hahn and Strassmann (published in Naturwissenschaften at the beginning of the month of January 1939) which showed that the bombardment of uranium by neutrons produced an isotope of barium.
Bohr had promised to keep secret the interpretation of Meitner and Frisch until they publish an article in order to ensure to them the paternity of discovered and interpretation, but on board boat on the way for the United States, he spoke about it with Leon Rosenfeld, while forgetting to ask him to respect the secrecy.
As of its arrival, Rosenfeld spoke about it with all the physicists about Princeton, and the news was spread with the other physicists, such Enrico Fermi of the Université of Columbia. The conversations between Fermi, John R. Dunning and G.B. Pegram led to research in Columbia of the ionizing rays produced by the fragments of the core of uranium obtained after this famous " fission".
January 26th, 1939, was held a conference of theoretical physics with Washington DC, organized jointly by the Université George Washington and Carnegie Institution of Washington. Fermi left New York to take part in this conference before the launching of the experiments of fission in Columbia. Bohr and Fermi discussed the problem of fission, Fermi mentioning the possibility in particular that neutrons can be emitted during the process. Although it is only one assumption, its consequences i.e. the possibility of a Chain reaction were obvious. Many articles with feeling were published in the press on this subject. Before the end of the conference in Washington, several other experiments were launched to confirm the thesis of the fission of the core.
February 15th, 1939, in the Physical Review four laboratories announced positive tests (Université of Columbia, Carnegie Institution of Washington, Université Johns-Hopkins, the University of California). At this time, Bohr knew that similar experiments had been undertaken in laboratory of Copenhagen (Denmark) about on January 15th (Letter of Frisch to Nature dated January 16th, 1939 and appeared in the number of February 18th). Frederic Joliot in Paris had also published its first results in the Comptes-rendus of January 30th, 1939. As from this moment there, there was a regular publication of articles on fission, in such a way that, in the Review off Modern Physics of December 6th, 1939, L.A. Turner of Princeton counted of it almost a hundred.
There exist two types of fissions: the spontaneous fission and the induced fission .
Note: atomic nuclei being able to fission are known as " fissiles" or " fissibles". Such cores have obligatorily a Atomic number equal to or higher than 89: they train the family of the Actinides.
The phenomenon of spontaneous fission was discovered in 1940 by G. NR. Flerov and K.A. Petrzak while working on uranium 238 cores.
One speaks about spontaneous nuclear fission when the core disintegrates in several fragments without preliminary absorption of a Corpuscule (Particule). This type of fission is not possible that for the extremely heavy cores, because the binding energy per nucleon is then smaller than for the lately formed fairly heavy cores.
Induced fission takes place when a heavy core captures another particle (generally a Neutron) and that the core composed then formed disintegrates in several fragments.
The induced fission of Uranium 235 by absorption of a neutron is the reaction of this type most known. It is of the type:
X and Y being two fairly heavy and generally radioactive cores: they are called fission product .
Thus the induced fission of a Uranium 235 core can give two fission products, the krypton and the barium, accompanied by three neutrons:
Induced fissions most usually used are the fission of Uranium 235, uranium 238 and plutonium 239.
See also: fissile Isotope
During fission, neutrons, said are immediately emitted prompt neutrons . Then, after the emission of these prompt neutrons, the fission products start to disintegrate by disintegration β and emission of neutrons after disintegrations β. As they are released after the fast neutrons, the neutrons released just after disintegrations β are called delayed neutrons .
The probability for a neutron of fissioning a fissile core depends on the energy of this last. One distinguishes classically the fast neutrons , directly resulting from a preceding fission, and the thermal neutrons or slow , to which one made practically lose all their energy by many collisions with light cores, such as hydrogen (in water, for example), the Deutérium (in the heavy Eau) or even carbon (in Graphite).
The following table indicates the number of released neutrons on average and by fission by thermal neutrons according to the core considered:
Distribution of the masses of the fission products
The distribution in mass of the fission products follows a curve " in bumps of chameau". One also speaks about bimodal curve: it has two maxima. It should be known that more than one hundred Nucléide S different can be released during the fission of uranium. However, all these nuclides have a Atomic number between Z=33 and Z=59. Fission creates cores of Mass number (many nucleons) around A=95 (brominates, krypton, zirconium) for one of the fragments and of A=139 (iodine, xenon, barium) for the other.
A symmetrical distribution (A=118 for Uranium 235) of the masses of the fission products (0,1% of fissions) or a fission in three fragments (tertiary fission, 0,005% of fissions) is very rare.
Each Uranium 235 core which undergoes fission releases from energy and thus from heat.
The origin of this energy finds its explanation in the energy balance between the initial core and the two produced cores: the protons of the same core are pushed back vigorously by their electrostatic loads, and this more especially as their number is high (Coulomb energy), energy corresponding growing more quickly than proportionally to the number of protons. Fission thus results in a release of energy, which is mainly transmitted in the fission products and the neutrons in the form of kinetic energy, which is transformed quickly into heat.
The heat produced during the fission of fissile cores of Uranium 235 or Plutonium 239 can then be used to transform water into vapor, thus making it possible to actuate a turbine being able to directly produce mechanical energy then via an alternator, electricity. It is this technique which is with work in the nuclear reactors intended to produce electricity.
During a reaction of induced nuclear fission, the absorption of a neutron by a fissile core allows the release of several neutrons, and each emitted neutron can in its turn break another fissile core. The reaction continues itself thus: it is the Chain reaction. This chain reaction takes place only if one neutron at least emitted during a fission is ready to cause a new fission.
The following table indicates the number of neutrons released on average by neutron (thermal) captured according to the core considered:
This table differs from the preceding one by the fact that it refers to all the neutrons entered the fissile core, and not only with those which give place to a fission.
One sees here very well why natural uranium is not used directly in the engines: uranium 238 qu' it contains in great proportion consumes too many neutrons which do not give place to a fission! To use it, it is necessary the to enrich Uranium 235 .
In a reactive medium, the speed to which is held this chain reaction is measured by multiplication the Factor.
A neutron which enters in collision with a fissile core can form with this one an excited made up core, or be simply absorbed (captures neutron). For Uranium 235, the proportion of captured neutrons is of approximately 16% for thermal neutrons (or slow neutrons); 9,1% for fast neutrons.
In the case of induced fission, the average lifespan of the made up core is about 10 -14 s. The core is fissioned, and the fragments separate at high speed: at the end of 10−17 S, these fragments, distant of 10 -10 m, emit, we saw it, of the neutrons.
Following de-energizings γ, photons γ are emitted after 10-14 S, whereas the fragments crossed 10-7 Mr. the fragments stop at the end of 10-12 S approximately, after having crossed a distance from 50 µm (these values are given for a material of density 1, such as ordinary water).
The kinetic energy of the fragments and the particles emitted following a fission ends up transforming into thermal energy, by the effect of the collisions and the interactions with the atoms of the crossed matter, except concerning the Neutrino S, inevitably emitted in disintegrations β, and which always escape from the medium (they can cross the Earth without interacting).
The following table indicates how is distributed released energy following the fission of a Uranium 235 atom, induced by a thermal neutron (these data are averages calculated on a great number of fissions).
Concept of critical mass
It is not enough that the multiplication factor of the neutrons is larger than 1 so that the chain reaction discusses: on the one hand, the neutrons are unstable and can disintegrate, but this plays little, because their average life time is of almost fifteen minutes, but especially, they can leave the medium where one tries to make a chain reaction. It is necessary that they have a collision before to leave, if not they do not take part any more in the chain reaction. The average thickness of the fissile medium must thus be enough large to ensure a probability sufficient for the neutrons to meet a fissile core. This brings to the concept of critical mass of the fissile element, which is a mass below which one cannot keep neutrons sufficient any more, whatever the form of the fissile load, to maintain the reaction. This explains why one cannot have nuclear mini-engines or atomic mini-bombs.
Bonin (Bernard), Klein (Etienne), Cavedon (Jean-Marc), Me, U235, radioactive atom , Flammarion, 2001
Bröcker (Bernhard), Atlas of atomic and nuclear physics , the pochotèque one, the Book of Pocket, 1997
Collective, Physics and the Elements , University of all the knowledge, Odile Jacob, 2002
- Nuclear plant
- atomic Energy binding
- nuclear Nuclear energy
- Fission product
- Nuclear reactor
Site of the Commissariat à l'Energie Atomique, nuclear fission part
SCK.CEN Centers study of Nuclear energy Mol, Belgium
TPE of pupils of First on nuclear fission
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