Nuclear chain reaction 22133 224700526 2008-07-10T00:56:50Z Alain r 389047 [[Image:Fission chain reaction.svg|300px|thumb|A possible [[nuclear fission]] chain reaction. 1. A [[uranium-235]] atom absorbs a [[neutron]], and fissions into two new atoms (fission fragments), releasing three new neutrons and a large amount of binding energy. 2. One of those neutrons is absorbed by an atom of [[uranium-238]], and does not continue the reaction. Another neutron leaves the system without being absorbed. However, one neutron does collide with an atom of uranium-235, which then fissions and releases two neutrons and more binding energy. 3. Both of those neutrons collide with uranium-235 atoms, each of which fissions and releases a few neutrons, which can then continue the reaction.]] A '''nuclear chain reaction''' occurs when one [[nuclear reaction]] causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g. <sup>235</sup>U) or the fusion of light isotopes (e.g. <sup>2</sup>H and <sup>3</sup>H). The nuclear chain reaction is unique since it releases several million times more energy per reaction than any [[chemical reaction]]. ==History== The concept of a nuclear chain reaction was first realized by [[Leó Szilárd]] in [[1933]]. He filed a patent for his idea of a simple nuclear reactor the following year.<ref>[http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=GB630726 esp@cenet document view<!-- Bot generated title -->]</ref> In 1936, Szilárd attempted to create a chain reaction using [[beryllium]] and [[indium]], but was unsuccessful. In 1939, Szilárd and [[Enrico Fermi]] discovered neutron multiplication in uranium, proving that a chain reaction was indeed possible.{{Fact|date=November 2007}} This discovery prompted [[Einstein-Szilárd letter|the letter]] from [[Albert Einstein]] to President [[Franklin D. Roosevelt]] warning of the possibility that [[Nazi Germany]] might be attempting to build an atomic bomb.<ref>http://www.aip.org/history/einstein/ae43a.htm, http://www.atomicarchive.com/Docs/Begin/Einstein.shtml</ref> Enrico Fermi created the first artificial self-sustaining nuclear chain reaction, called [[Chicago Pile-1]] (CP-1), in a [[racquets (sport)|racquets]] court below the bleachers of [[Stagg Field]] at the [[University of Chicago]] on December 2, 1942. Fermi's experiments at the University of Chicago were part of [[Arthur H. Compton]]'s [[Metallurgical Laboratory]] facility, which was part of the [[Manhattan Project]]. In 1956, Paul Kuroda of the [[University of Arkansas]] postulated that a natural fission reactor may have once existed. Since nuclear chain reactions only require natural materials (such as water and uranium), it is possible to have these chain reactions occur where there is the right combination of materials within the Earth's crust. Kuroda's prediction was verified with the discovery of [[Natural nuclear fission reactor|natural self-sustaining nuclear chain reactions]] at [[Oklo]] in Gabon, Africa in September 1972.<ref>[http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml Oklo: Natural Nuclear Reactors - Fact Sheet<!-- Bot generated title -->]</ref> ==Fission chain reaction== Fission chain reactions occur because of interactions between [[neutrons]] and [[fissile]] isotopes (such as <sup>235</sup>U). The chain reaction requires both the release of neutrons from fissile isotopes undergoing [[nuclear fission]] and the subsequent absorption of some of these neutrons in fissile isotopes. When an atom undergoes nuclear fission, a few neutrons (the exact number depends on several factors) are ejected from the reaction. These free neutrons will then interact with the surrounding medium, and if more fissile fuel is present, some may be absorbed and cause more fissions. Thus, the cycle repeats to give a reaction that is self-sustaining. [[Nuclear power plants]] operate by precisely controlling the rate at which nuclear reactions occur, and that control is maintained through the use of several redundant layers of safety measures. Moreover, the materials in a nuclear reactor core and the uranium enrichment level make a nuclear explosion impossible, even if all safety measures failed. On the other hand, [[nuclear weapons]] are specifically engineered to produce a reaction that is so fast and intense it cannot be controlled after it has started. When properly designed, this uncontrolled reaction can lead to an explosive energy release. ===Nuclear fission fuel=== Nuclear fission weapons must use an extremely high quality, highly-enriched fuel exceeding the critical size and geometry ([[Critical mass (nuclear)|critical mass]]) in order to obtain an explosive chain reaction. The fuel for a nuclear fission reactor is very different, usually consisting of a low-enriched oxide material (e.g. UO<sub>2</sub>). It is impossible for a nuclear power plant to undergo an explosive nuclear chain reaction. Chernobyl was a steam explosion, not a nuclear explosion.<ref name=Lamarsh>{{cite book |last=Lamarsh |first=John |coauthors=Baratta, Anthony |title=Introduction to Nuclear Engineering |year=2001 |publisher=Prentice Hall |isbn=0-201-82498-1 }}</ref> Furthermore, all power plants licensed in the United States require a negative [[void coefficient]] of reactivity, which completely eliminates the possibility of the accident that occurred at Chernobyl (which was due to a positive void coefficient). ===Fission Reaction Products=== When a heavy atom undergoes nuclear fission it breaks into two or more fission fragments. Also, several free neutrons, [[gamma rays]], and [[neutrinos]] are emitted, and a large amount of energy is released. The sum of the masses of the fission fragments and ejected neutrons is actually less than the mass of original atom and incident neutron. The mass difference is accounted for in the release of energy according to the equation E=mc²: :<math>\frac{E}{c^2} = m_\mbox{original}-m_\mbox{final}</math> Due to the extremely large value of the [[speed of light]], c, a small decrease in mass causes a tremendous release in energy. While typical chemical reactions release energies on the order of a few [[electron volt|eV]]s (e.g. the binding energy of the electron to hydrogen is 13.6&nbsp;eV), nuclear fission reactions typically release energies on the order of hundreds of millions of eVs. Two typical fission reactions are shown below with average values of energy released and number of neutrons ejected: :<math>{}^{235}U + \mbox{neutron} \rightarrow \mbox{fission fragments} + 2.4\mbox{ neutrons} + 192.9\mbox{ MeV}</math><ref name=Duderstadt>{{cite book |last=Duderstadt |first=James |coauthors=Hamilton, Louis |title=Nuclear Reactor Analysis |year=1976 |publisher=John Wiley & Sons, Inc |isbn=0-471-22363-8 }}</ref> :<math>{}^{239}Pu + \mbox{neutron} \rightarrow \mbox{fission fragments} + 2.9\mbox{ neutrons} + 198.5\mbox{ MeV}</math><ref name=Duderstadt /> Note that these equations are for fissions caused by slow-moving (thermal) neutrons. The average energy released and number of neutrons ejected is a function of the incident neutron speed.<ref name=Duderstadt /> Also, note that these equations exclude energy from neutrinos since these subatomic particles are extremely non-reactive and, therefore, rarely deposit their energy in the system. ==Timescales of nuclear chain reactions== ===Prompt neutron lifetime=== The '''prompt neutron lifetime''', ''l'', is the average time between the emission of neutrons and either their absorption in the system or their escape from the system.<ref name=Lamarsh /> The term lifetime is used because the emission of a neutron is often considered its "birth," and the subsequent absorption is considered its "death." For thermal (slow-neutron) fission reactors, the typical prompt neutron lifetime is on the order of 10<sup>-4</sup> seconds, and for fast fission reactors, the prompt neutron lifetime is on the order of 10<sup>-7</sup> seconds.<ref name=Duderstadt /> These extremely short lifetimes mean that in 1 second, 10,000 to 10,000,000 neutron lifetimes can pass. ===Mean generation time=== The '''mean generation time''', Λ, is the average time from a neutron emission to a capture that results in fission.<ref name=Duderstadt /> The mean generation time is different from the prompt neutron lifetime because the mean generation time only includes neutron absorptions that lead to fission reactions (not other absorption reactions). The two times are related by the following formula: :<math>\Lambda = \frac{l}{k}</math> In this formula, k is the effective neutron multiplication factor, described below. ===Effective neutron multiplication factor=== The [[four factor formula|'''effective neutron multiplication factor''']], k, is the average number of neutrons from one fission that cause another fission. The remaining neutrons either are absorbed in non-fission reactions or leave the system without being absorbed. The value of k determines how a nuclear chain reaction proceeds: *''k'' < 1 (sub-critical mass): The system cannot sustain a critical reaction, and any beginning of a chain reaction dies out over time. For every fission that is induced in the system, an average ''total'' of 1/(1&nbsp;&minus;&nbsp;''k'') fissions occur. *''k'' = 1 (critical mass): Every fission causes an average of one more fission, leading to a fission (and power) level that is constant. Nuclear power plants operate with k = 1. *''k'' > 1 (super-critical mass): For every fission in the material, it is likely that there will be "k" fissions after the next ''mean generation time.'' The result is that the number of fission reactions increases exponentially, according to the equation <math>e^{(k-1)t/\Lambda}</math>, where t is the elapsed time. Nuclear weapons are designed to operate under this state. There are two subdivisions of supercriticality: prompt and delayed. In an infinite medium, the multiplication factor is given by the [[four factor formula]]. ===Prompt and delayed supercriticality=== Not all neutrons are emitted as a direct product of fission, some are instead due to the [[radioactive decay]] of some of the fission fragments. The neutrons that occur directly from fission are called "[[prompt neutron]]s," and the ones that are a result of radioactive decay of fission fragments are called "delayed neutrons." The fraction of neutrons that are delayed is called β, and this fraction is typically less than 1% of all the neutrons in the chain reaction.<ref name=Duderstadt /> The delayed neutrons allow a nuclear reactor to respond several 100 times more slowly than just prompt neutrons would alone.<ref name=Lamarsh /> Without delayed neutrons, changes in reaction rates in nuclear reactors would occur at speeds that are too fast for humans to control. The region of supercriticality between k = 1 and k = 1/(1-β) is known as '''delayed supercriticality'''. It is in this region that all nuclear power reactors operate. The region of supercriticality for k > 1/(1-β) is known as '''prompt supercriticality''', which is the region in which nuclear weapons operate. The change in k needed to go from critical to prompt critical is defined as a [[dollar (disambiguation)|dollar]]. ==Neutron multiplication in nuclear weapons== Nuclear fission weapons require a mass of fissile fuel that is prompt supercritical. For a given mass of fissile material the value of k can be increased by increasing the density. Since the probability per distance traveled for a neutron to collide with a nucleus is proportional to the material density, increasing the density of a fissile material can increase k. This concept is utilized in the [[Nuclear_weapon_design#Implosion_method|implosion method]] for nuclear weapons. In these devices, the nuclear chain reaction begins after increasing the density of the fissile material with a conventional explosive. In the [[gun-type fission weapon]] two subcritical pieces of fuel are rapidly brought together. The value of k for a combination of two masses is always greater than that of its components. The magnitude of the difference depends on distance, as well as the physical orientation. The value of k can also be increased by using a [[neutron reflector]] surrounding the fissile material Once the mass of fuel is prompt supercritical, the power increases exponentially. However, the exponential power increase cannot continue for long since k decreases when the amount of fission material that is left decreases (i.e. it is consumed by fissions). Also, the geometry and density are expected to change during detonation since the remaining fission material is torn apart from the explosion. ===Predetonation=== Detonation of a nuclear weapon involves bringing fissile material into its optimal supercritical state very rapidly. During part of this process, the assembly is supercritical, but not yet in an optimal state for a chain reaction. Free neutrons, in particular from [[spontaneous fission]]s, can cause the device to undergo a preliminary chain reaction that destroys the fissile material before it is ready to produce a large explosion, which is known as '''predetonation'''. To keep the probability of predetonation low, the duration of the non-optimal assembly period is minimized and fissile and other materials are used which have low spontaneous fission rates. In fact, the combination of materials has to be such that it is unlikely that there is even a single spontaneous fission during the period of supercritical assembly. In particular, the gun method cannot be used with plutonium (see [[nuclear weapon design]]). ==Fusion chain reaction== In a more generalized sense, a [[nuclear fusion]] reaction can be considered a nuclear chain reaction: it occurs under extreme pressure and temperature conditions, which are maintained by the energy released in the fusion process. == See also == * [[Chain reaction]] * [[Critical mass (nuclear)|Critical mass]] * [[Four factor formula]] * [[Criticality accident]] * [[Nuclear physics]] * [[Nuclear reaction]] * [[Nuclear weapon design]] * [[Nuclear criticality safety]] * [[Nuclear reactor physics]] == References == <references /> == External links == * [http://www.atomicarchive.com/Movies/Movie1.shtml Nuclear Chain Reaction Animation] * [http://alsos.wlu.edu/adv_rst.aspx?query=nuclear+chain+reaction&method=exact&source=all&genre=all&disc=all&level=all&selection=keyword&sortby=creator&results=10&period=15 Annotated bibliography on nuclear chain reactions from the Alsos Digital Library] [[Category:Nuclear physics|Chain reaction, nuclear]] [[da:Kædereaktion]] [[fa:واکنش زنجیره‌ای هسته‌ای]] [[fr:Réaction en chaîne (nucléaire)]] [[ko:핵 연쇄 반응]] [[he:תגובת שרשרת גרעינית]] [[hi:नाभिकीय शृंखला अभिक्रिया]] [[lt:Grandininė branduolinė reakcija]] [[hu:Láncreakció]] [[ml:ആണവ ചെയിന്‍ റിയാക്ഷന്‍]] [[ru:Цепная ядерная реакция]] [[sr:Нуклеарна ланчана реакција]] [[ta:அணுக்கரு தொடர்வினை]] [[tr:Zincirleme çekirdek tepkimesi]] [[ur:مرکزی زنجیری تعامل]] [[zh:核連鎖反應]]