Nuklearna fisija – razlika između verzija
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[[Datoteka:Nuclear fission.svg|mini|desno|300px|Jedna od mogućih reakcija nuklearne fisije: atom uranija-235 hvata spori neutron i raspada se na dva nova atoma (fisijski produkti – barij-141 i kripton-92), oslobađajući 3 nova neutrona i ogromnu količinu energije vezanja (200 MeV).]]
[[Datoteka:Stdef2.png|300px|desno|mini|Model tekuće kapljice atomske jezgre]]
Linija 55 ⟶ 56:
Za odvijanje lančane reakcije odlučne su dvije veličine: '''neutronski prinos''' ''k'' i '''trajanje fisijske generacije''' ''τ'' u lančanoj reakciji. Trajanjem jedne fisijske generacije naziva se prosječno vrijeme između dviju uzastopnih fisija (da bi fisijski neutroni bili emitirani iz neke jezgre i dospjeli do drugih fisibilnih jezgara potrebno je neko vrijeme). Neutronski prinos ''k'' je omjer broja neutrona nastalih u fisijskom procesa prema broju neutrona nastalih u prethodnom fisijskom procesu. Lančana je reakcija nadkritična ako je k > 1, podkritična ako je k < 1. Ako je k = 1, lančana reakcija održava se trajno s istim brojem fisija u jediničnom obujmu. Kontrolom neutronskog prinosa kontrolira se broj neutrona, koriste se štapovi od [[kadmij]]a koji se uvlače u reaktorsku jezgru i apsorbiraju neutrone.
== Fizički pregled ==
=== Mehanizam ===
[[Image:UFission.gif|250px|right|thumb|Vizuelna reprezentacija događaja indukovanje nuklearne fuzije u kome se sporo krećući neutron apsporbuje u jezgru atoma uranijuma-235, usled čega dolazi do fisije u dva lakša elementa (fisiona produkta) koji se brzo kreću i dodatne neutrone. Najveći deo oslobođene energije je u obliku kinetičke brzine fisionih produkata i neutrona.]]
[[Image:ThermalFissionYield.svg|thumb|300px|Fisioni product yields by mass for [[thermal neutron]] fission of [[Uranium-235|U-235]], [[Pu-239]], a combination of the two typical of current nuclear power reactors, and [[Uranium-233|U-233]] used in the [[thorium cycle]].]]
Nuklearna fisija 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 fuel]]s, and are said to be ''[[fissile]]''. The most common nuclear fuels are [[uranium-235|<sup>235</sup>U]] (the isotope of [[uranium]] with an [[atomic mass]] of 235 and of use in nuclear reactors) and [[Plutonium-239|<sup>239</sup>Pu]] (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 particle|alpha]]/[[beta particle|beta]] [[decay chain]] over periods of [[millennium|millennia]] to [[Eon (geology)|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 neutron]]s (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.<ref>S. Vermote, et al. (2008) [http://books.google.com/books?id=6IkykKNob6gC&pg=PA259 "Comparative study of the ternary particle emission in 243-Cm (nth,f) and 244-Cm(SF)"] in ''Dynamical aspects of nuclear fission: proceedings of the 6th International Conference.'' J. Kliman, M. G. Itkis, S. Gmuca (eds.). World Scientific Publishing Co. Pte. Ltd. Singapore.</ref>
=== Energetika ===
==== Ulaz ====
[[Image:Stdef2.png|150px|right|thumb|The stages of binary fission in a liquid drop model. Energy input deforms the nucleus into a fat "cigar" shape, then a "peanut" shape, followed by binary fission as the two lobes exceed the short-range [[nuclear force]] attraction distance, then are pushed apart and away by their electrical charge. In the liquid drop model, the two fission fragments are predicted to be the same size. The nuclear shell model allows for them to differ in size, as usually experimentally observed.]]
The fission of a heavy nucleus requires a total input energy of about 7 to 8 million [[electron volt]]s (MeV) to initially overcome the [[nuclear force]] which holds the nucleus into a spherical or nearly spherical shape, and from there, deform it into a two-lobed ("peanut") shape in which the lobes are able to continue to separate from each other, pushed by their mutual positive charge, in the most common process of binary fission (two positively charged fission products + neutrons). Once the nuclear lobes have been pushed to a critical distance, beyond which the short range [[strong force]] can no longer hold them together, the process of their separation proceeds from the energy of the (longer range) [[Electromagnetic force|electromagnetic]] repulsion between the fragments. The result is two fission fragments moving away from each other, at high energy.
About 6 MeV of the fission-input energy is supplied by the simple binding of an extra neutron to the heavy nucleus via the strong force; however, in many fissionable isotopes, this amount of energy is not enough for fission. Uranium-238, for example, has a near-zero fission cross section for neutrons of less than one MeV energy. If no additional energy is supplied by any other mechanism, the nucleus will not fission, but will merely absorb the neutron, as happens when U-238 absorbs slow and even some fraction of fast neutrons, to become U-239. The remaining energy to initiate fission can be supplied by two other mechanisms: one of these is more kinetic energy of the incoming neutron, which is increasingly able to fission a [[fissionable]] heavy nucleus as it exceeds a kinetic energy of one MeV or more (so-called [[fast neutron]]s). Such high energy neutrons are able to fission U-238 directly (see [[thermonuclear weapon]] for application, where the fast neutrons are supplied by [[nuclear fusion]]). However, this process cannot happen to a great extent in a nuclear reactor, as too small a fraction of the fission neutrons produced by any type of fission have enough energy to efficiently fission U-238 (fission neutrons have a [[mode (statistics)|mode]] energy of 2 MeV, but a [[median]] of only 0.75 MeV, meaning half of them have less than this insufficient energy).<ref>J. Byrne (2011) ''Neutrons, Nuclei, and Matter'', Dover Publications, Mineola, NY, p. 259, ISBN 978-0-486-48238-5.</ref>
Among the heavy actinide elements, however, those isotopes that have an odd number of neutrons (such as U-235 with 143 neutrons) bind an extra neutron with an additional 1 to 2 MeV of energy over an isotope of the same element with an even number of neutrons (such as U-238 with 146 neutrons). This extra binding energy is made available as a result of the mechanism of [[semi-empirical mass formula|neutron pairing]] effects. This extra energy results from the [[Pauli exclusion principle]] allowing an extra neutron to occupy the same nuclear orbital as the last neutron in the nucleus, so that the two form a pair. In such isotopes, therefore, no neutron kinetic energy is needed, for all the necessary energy is supplied by absorption of any neutron, either of the slow or fast variety (the former are used in moderated nuclear reactors, and the latter are used in [[fast neutron reactor]]s, and in weapons). As noted above, the subgroup of fissionable elements that may be fissioned efficiently with their own fission neutrons (thus potentially causing a nuclear [[chain reaction]] in relatively small amounts of the pure material) are termed "[[fissile]]." Examples of fissile isotopes are U-235 and plutonium-239.
==== Izlaz ====
Typical fission events release about two hundred million [[Electronvolt|eV]] (200 MeV) of energy for each fission event. The exact isotope which is fissioned, and whether or not it is fissionable or fissile, has only a small impact on the amount of energy released. This can be easily seen by examining the curve of [[binding energy]] (image below), and noting that the average binding energy of the [[actinide]] nuclides beginning with uranium is around 7.6 MeV per nucleon. Looking further left on the curve of binding energy, where the [[fission products]] cluster, it is easily observed that the binding energy of the fission products tends to center around 8.5 MeV per nucleon. Thus, in any fission event of an isotope in the actinide's range of mass, roughly 0.9 MeV is released per nucleon of the starting element. The fission of U235 by a slow neutron yields nearly identical energy to the fission of U238 by a fast neutron. This energy release profile holds true for thorium and the various minor actinides as well.<ref name=ENS>{{cite web|author=Marion Brünglinghaus|url=http://www.euronuclear.org/info/encyclopedia/n/nuclear-fission.htm |title=Nuclear fission |publisher=European Nuclear Society |accessdate=2013-01-04}}</ref>
By contrast, most [[chemical reaction|chemical]] [[oxidation]] reactions (such as burning [[coal]] or [[trinitrotoluene|TNT]]) release at most a few [[Electronvolt|eV]] per event. So, nuclear fuel contains at least ten million times more [[Energy density|usable energy per unit mass]] 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 ray]]s; 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]] or [[molten salt]]s.
When a [[uranium]] nucleus fissions into two daughter nuclei fragments, about 0.1 percent of the mass of the uranium nucleus<ref name="bulletin1950">Hans A. Bethe (April 1950), [http://books.google.com/books?id=Mg4AAAAAMBAJ&pg=PA99 "The Hydrogen Bomb"], ''Bulletin of the Atomic Scientists'', p. 99.</ref> appears as the fission energy of ~200 MeV. For uranium-235 (total mean fission energy 202.5 MeV), typically ~169 MeV appears as the [[kinetic energy]] of the daughter nuclei, which fly apart at about 3% of the speed of light, due to [[Coulomb's law|Coulomb repulsion]]. Also, an average of 2.5 neutrons are emitted, with a [[mean]] kinetic energy per neutron of ~2 MeV (total of 4.8 MeV).<ref>These fission neutrons have a wide energy spectrum, with range from 0 to 14 MeV, with mean of 2 MeV and [[mode (statistics)]] of 0.75 Mev. See Byrne, op. cite.</ref> The fission reaction also releases ~7 MeV in prompt [[gamma ray]] [[electromagnetic waves|photons]]. The latter figure means that a nuclear fission explosion or criticality accident emits about 3.5% of its energy as gamma rays, less than 2.5% of its energy as fast neutrons (total of both types of radiation ~ 6%), and the rest as kinetic energy of fission fragments (this appears almost immediately when the fragments impact surrounding matter, as simple [[heat]]). In an atomic bomb, this heat may serve to raise the temperature of the bomb core to 100 million [[kelvin]] and cause secondary emission of soft X-rays, which convert some of this energy to ionizing radiation. However, in nuclear reactors, the fission fragment kinetic energy remains as low-temperature heat, which itself causes little or no ionization.
So-called [[neutron bomb]]s (enhanced radiation weapons) have been constructed which release a larger fraction of their energy as ionizing radiation (specifically, neutrons), but these are all thermonuclear devices which rely on the nuclear fusion stage to produce the extra radiation. The energy dynamics of pure fission bombs always remain at about 6% yield of the total in radiation, as a prompt result of fission.
The total ''prompt fission'' energy amounts to about 181 MeV, or ~ 89% of the total energy which is eventually released by fission over time. The remaining ~ 11% is released in beta decays which have various half-lives, but begin as a process in the fission products immediately; and in delayed gamma emissions associated with these beta decays. For example, in uranium-235 this delayed energy is divided into about 6.5 MeV in betas, 8.8 MeV in [[antineutrino]]s (released at the same time as the betas), and finally, an additional 6.3 MeV in delayed gamma emission from the excited beta-decay products (for a mean total of ~10 gamma ray emissions per fission, in all). Thus, about 6.5% of the total energy of fission is released some time after the event, as non-prompt or delayed ionizing radiation, and the delayed ionizing energy is about evenly divided between gamma and beta ray energy.
In a reactor that has been operating for some time, the radioactive fission products will have built up to steady state concentrations such that their rate of decay is equal to their rate of formation, so that their fractional total contribution to reactor heat (via beta decay) is the same as these radioisotopic fractional contributions to the energy of fission. Under these conditions, the 6.5% of fission which appears as delayed ionizing radiation (delayed gammas and betas from radioactive fission products) contributes to the steady-state reactor heat production under power. It is this output fraction which remains when the reactor is suddenly shut down (undergoes [[scram]]). For this reason, the reactor [[decay heat]] output begins at 6.5% of the full reactor steady state fission power, once the reactor is shut down. However, within hours, due to decay of these isotopes, the decay power output is far less. See [[decay heat]] for detail.
The remainder of the delayed energy (8.8 MeV/202.5 MeV = 4.3% of total fission energy) is emitted as antineutrinos, which as a practical matter, are not considered "ionizing radiation." The reason is that energy released as antineutrinos is not captured by the reactor material as heat, and escapes directly through all materials (including the Earth) at nearly the speed of light, and into interplanetary space (the amount absorbed is minuscule). Neutrino radiation is ordinarily not classed as ionizing radiation, because it is almost entirely not absorbed and therefore does not produce effects (although the very rare neutrino event is ionizing). Almost all of the rest of the radiation (6.5% delayed beta and gamma radiation) is eventually converted to heat in a reactor core or its shielding.
Some processes involving neutrons are notable for absorbing or finally yielding energy — for example neutron kinetic energy does not yield heat immediately if the neutron is captured by a uranium-238 atom to breed plutonium-239, but this energy is emitted if the plutonium-239 is later fissioned. On the other hand, so-called [[delayed neutrons]] emitted as radioactive decay products with half-lives up to several minutes, from fission-daughters, are very important to [[nuclear reactor physics|reactor control]], because they give a characteristic "reaction" time for the total nuclear reaction to double in size, if the reaction is run in a "[[delayed criticality|delayed-critical]]" zone which deliberately relies on these neutrons for a supercritical chain-reaction (one in which each fission cycle yields more neutrons than it absorbs). Without their existence, the nuclear chain-reaction would be [[prompt critical]] and increase in size faster than it could be controlled by human intervention. In this case, the first experimental atomic reactors would have run away to a dangerous and messy "prompt critical reaction" before their operators could have manually shut them down (for this reason, designer [[Enrico Fermi]] included radiation-counter-triggered control rods, suspended by electromagnets, which could automatically drop into the center of [[Chicago Pile-1]]). If these delayed neutrons are captured without producing fissions, they produce heat as well.<ref>{{cite web|url=http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_1.html |title=Nuclear Fission and Fusion, and Nuclear Interactions|publisher=National Physical Laboratory|accessdate=2013-01-04}}</ref>
==Reference==
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