Chain reaction A self-sustaining

Chain reaction a self-sustaining series of reactions in which the products of one reaction, such as neutrons, initiate more of the same reaction. One such particle can therefore start a whole series of reactions. 

In 1934 the Hungarian physicist Leo Szilard filed a patent with the British Patent Office. It was based on an idea, nothing more - an idea about how to harness nuclear energy. The Joliot-Curies had shown that bombarding nuclei with particles can induce radioactive decay artificially. And the work of Bothe and Chadwick had demonstrated that some radioactive nuclei emit neutrons. So what would happen if neutrons induced nuclear decay that led to more neutrons The result might be a chain reaction a self-sustaining release of nuclear energy. 

Note in this reaction that one neutron starts the fission of the uranium nucleus and that the fission produces 3 neutrons. (It is also possible for a given fission event to produce either fewer than 3 neutrons or more than 3.) These product neutrons can cause the fissioning of 3 other uranium atoms, releasing 9 more neutrons. If each of these 9 neutrons succeeds in splitting a uranium atom, the next step in the reaction produces 27 neutrons, and so on. Such a sequence, illustrated in Figure 4.22, is called a chain reaction—a self-sustaining reaction in which the products of one reaction event stimulate further reaction events. 

Chain reaction A self-sustaining reaction in which the products of one fission event stimulate further events. 

Nuclear chain reaction. A self-sustaining sequence of nuclear fission reactions. (23.5)... 

Celsius (°C) scale A temperature scale often used by scientists. On this scale, water freezes at 0 °C and boils at 100 °C at 1 atm pressure. Room temperature is approximately 22 °C. chain reaction A self-sustaining chemical or nuclear reaction yielding energy or products that cause further reactions of the same kind. 

R-type semiconductor. Semiconductors in which an electron-rich impurity is added to enhance conduction. (23.3,25.6) nuclear binding energy. The energy required to separate the nucleons in a nucleus. (20.2) nuclear chain reaction. A self-sustaining reaction sequence of fission reactions. 

Nuclear fission is the process in which a heavier nucleus (usually less stable) splits into smaller nuclei and neutrons. The process releases a large amount of energy and neutrons that can set up a chain reaction (or self-sustaining nuclear fission reactions) with more and more uncontrollable releases of energy (highly exothermic reactions) and neutrons. Examples of fission include the following ... 

CHAIN REACTION. In the context of nuclear materials, chain reaction refers to a series of reactions occurring in ilssionable materials. During these reactions, neutrons produced by fission reactions induce other fission reactions. A fission chain reaction becomes self-sustaining when one new fission is produced by each previous fission. See also NUCLEAR PILE. 

Am undergo fission with thermal neutrons of these isotopes and Pu are the most important as they are most readily obtainable. Other heavy nuclei require fast neutrons to induce fission such neutrons are much more difficult to control into a self-sustaining chain-reaction. 

Uranium-235 is of even greater importance because it is the key to utilizing uranium. 23su while occuring in natural uranium to the extent of only 0.71%, is so fissionable with slow neutrons that a self-sustaining fission chain reaction can be made in a reactor constructed from natural uranium and a suitable moderator, such as heavy water or graphite, alone. 

A key parameter in determining the possibiUty of a self-sustained chain reaction is the value of k for an infinite medium, k. In the four-factor formula,... 

In early 1941, 0.5 )-lg of Pu was produced (eqs. 3 and 4) and subjected to neutron bombardment (9) demonstrating that plutonium undergoes thermal neutron-induced fission with a cross section greater than that of U. In 1942, a self-sustaining chain reaction was induced by fissioning 235u... 

Commercially, there are only two nuclear species that will function in a self-sustaining nuclear fission chain reaction and Pu (plutonium). 

Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

FIGURE 17.24 A self-sustaining chain reaction, in which neutrons are the chain carriers, takes place when induced fission produces more than one neutron per fission event. These newly produced neutrons can stimulate fission in increasingly greater numbers of other nuclei. 

Because the isotope uranium-235 is fissionable, meaning that it produces free neutrons that cause other atoms to split, it generates enough free neutrons to make it unstable. When the unstable U-235 reaches a critical mass of a few pounds, it produces a self-sustaining fission chain reaction that results in a rapid explosion with tremendous energy and becomes a nuclear (atomic) bomb. The first nuclear bombs were made of uranium and plutonium. Today, both of these fuels are used in reactors to produce electrical power. Moderators (control rods) in nuclear power reactors absorb some of the neutrons, which prevents the mass... 

The three neutrons produced when uranium splits have the ability to split other U-235 nuclei and start a self-sustaining chain reaction. Whether a chain reaction takes place depends on the amount of fissionable material present. The more fissionable material that is present, the greater the probability that a neutron will interact with another U-235 nucleus. The reason for this involves the basic relationship between surface area and volume as mass increases. If a cube with a length of 1 unit is compared to a cube of 2 units, it is found that the surface area to volume ratio of the 1 unit cube is twice that of the 2 unit cube (Figure 17.6). This shows that volume increases at a greater rate than surface area as size increases. The probability that neutrons escape rather than react also depends on the surface area to volume ratio. The higher this ratio is the more likely neutrons escape. When a U-235 nucleus contained in a small mass of fissionable uranium is bombarded by a neutron, the... 

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