Fission nuclear

Nuclear fission is a process in which a heavy nucieus (usuaiiy uranium or plutonium) splits into two parts, releasing neutrons and energy. 

In nuclear fission, big atoms break apart into smaller atoms. In nuclear fusion, the opposite occurs—small atoms combine together to make larger atoms. In both processes, however, a very large amount of energy is released. Thus, both processes can be used to make atomic bombs. Controlled fission has been used for over fifty years in the commercial production of electricity. There are serious technological difficulties that must be overcome. 

Uranium consists of about 99.3 percent U-238 and seven-tenths of 1 percent of U-235. It 

Notice that each fission event also results in the release of additional neutrons, an average of about 2.5 neutrons per fission. 

The release of additional neutrons is the reason a chain reaction can take place. The neutrons are quickly absorbed by surrounding uranium atoms so that the rate of fission increases exponentially. 

Nuclear fission is the breaking apart of atomic nuclei into two or more pieces. This can take place spontaneously in the case of the heaviest atoms. Neutron bombardment of atoms can also cause the nuclei to break apart. This process is called induced nuclear fission. Atoms that undergo this process are called fissionable. Some nuclides can undergo fission with slow (not very energetic, or thermal) neutrons. These atoms are called fissile. 

The neutron, being uncharged, is not repelled by the positive charge on the nucleus, and makes an ideal nuclear probe. Soon after the discovery of this property many experiments were carried out to try to make new elements that were more massive than uranium by bombarding heavy atoms, notably U itself, with neutrons. Two such elements that can be made this way are neptunium, Np, and plutonium, Pu  

Neptunium has a half-life of about 2 days and decays to plutonium-239 by way of (3-decay  

However, the experiments most often resulted in fission of the heavy nuclei into two more-or-less equal parts during the bombardment. For example, fission of uranium-235 can produce krypton, Rr, and barium, Ba  

In practice, a range of fission products with masses similar to krypton and barium are formed when uranium is irradiated with neutrons. This reaction is important, as it is used to produce nuclear power. There are two vital features that make this application possible first, the amount of energy liberated and, second, the number of neutrons produced. 

Nuclear fission was discovered in the late 1930s when nuclides bombarded with neutrons were observed to split into two lighter elements. 

To achieve the critical state, a certain mass of fissionable material, called the critical mass, is needed. If the sample is too small, too many neutrons escape before they have a chance to cause a fission event, and the process stops. 

Upon capturing a neutron, the 3 U nucleus undergoes fission to produce two lighter nuclides, more neutrons (typically three), and a large amount of energy. 

The nuclear fission process utilized in today s power-producing reactors is initiated by interaction between a neutron and a fissile nucleus, such as The nucleus then divides into 

The numbers assigned to each reactant or end product represent its mass in atomic mass units (amu). This unit is defined as the ratio of the mass of a neutral atom to one-twelfth the mass of an atom of C. In the present instance the mass of the products is less than that of the reactants  

A fraction 0.2I0675/235.0439I5 = 0.0008963 of the mass of the atom disappears in this fission reaction. This reduction in mass is a measure of the amount of energy released in this fission reaction. The Einstein equation (1.1) expressing the equivalence of energy and mass, 

Energy changes associated with a single nuclear event are commonly expressed in terms of millions of electron volts (MeV), defined as the amount of energy acquired by an electronic charge (1.602 X 10 C) when accelerated through a potential difference of 1,000,000 V. One MeV therefore equals 1.602 X 10 X 10 = 1,602 X 10 J 

Following the primary fission reaction, the radioactive fission products undergo radioactive disintegration, yielding beta particles and delayed gamma rays and ending up as stable fission products. Since the radioactive fission products have half-lives ranging from fractions of a 

1 Identify the balanced nuclear equation for the reaction represented by Mo(d,n) lTc. 

TABLE 20.4 Nuclear Binding Energies of U and Its Fission Products 

FiQUre 20.9 If a critical mass is present, many of the neutrons emitted during the fission process wUl be captured by other nuclei and a chain reaction will occur. 

The explosion forces the sections of fissionable material together to form an amount considerably larger than the critical mass. 

The first application of nuclear fission was in the development of the atomic bomb. How is such a bomb made and detonated The cracial factor in the bomb s design is the determination of the critical mass for the bomb. A small atomic bomb is equivalent to 20,000 tons of TNT (trinitrotoluene). Because 1 ton of TNT releases about 4 X 10 J of energy, 20,(XX) tons would produce 8 X lO J. Recall that 1 mole, or 235 g, of uranium-235 liberates 2.0 X 10 J of energy when it undergoes fission. Thus, the mass of the isotope present in a small bomb must be at least 

The process of nuclear fission was discovered in Germany more than half a century ago in 1938 by Lise Meitner (1878-1968) and Otto Hahn (1879-1968). With the outbreak of World War II a year later, interest focused on the enormous amount of energy released in the process. At Los Alamos, in the mountains of New Mexico, a group of scientists led by J. Robert Oppenheimer (1904-1967) worked feverishly to produce the fission, or atomic, bomb. Many of the members of this group were exiles from Nazi Germany. They were spurred on by the fear that Hitler would obtain the bomb first. 

Their work led to the explosion of the first atomic bomb in the New Mexico desert at 5 30 a.m. on July 16,1945. Less than a month later (August 6,1945), the world learned of this new weapon when another bomb was exploded over Hiroshima. This bomb killed 70,000 people and completely devastated an area of 10 square kilometers. Three days later, Nagasaki and its inhabitants met a similar fate. On August 14, Japan surrendered, and World War II was over. 

Several isotopes of the heavy elements undergo fission if bombarded by neutrons of high enough energy. In practice, attention has centered on two particular isotopes, and HPu. Both of these can be split into fragments by relatively low-energy neutrons. 

Our discussion concentrates on the uranium-235 isotope. It makes up only about 0.7% of naturally occurring uranium. The more abundant isotope, uranium-238, does not undergo fission. The first process used to separate these isotopes, and until recently the only one available, was that of gaseous effusion (Chapter 5). The volatile compound uranium hexafluoride, 

When a uranium-235 atom undergoes fission, it spHts into two unequal fragments and a number of neutrons and beta particles. The fission process is complicated by the fact that different uranium-235 atoms split up in many 

Identify the second nncUde formed in the fission reaction  

The reaction must proceed with conservation of mass number and of charge. The mass numbers are denoted by the superscripts, and the charges by the subscripts (i.e. the number of protons). 

The value of Z identifies the element as Sn (see the periodic table inside the front cover of the book). 

Nuclear power is now used in a number of countries as a source of electrical power. The fuel in all commercial nuclear reactors is uranium, but of naturally occurring uranium only 

7% is 92U, the radionuclide required for the fission process. Enrichment of the uranium is usually carried out but, even then, constitutes only a few per cent of the 

Isotopes of some elements with atomic numbers above 80 are capable of undergoing fission in which they split into nuclei of intermediate masses and emit one or more neutrons. Some fissions are spontaneous others require that the activation energy be supplied by bombardment. A given nucleus can split in many different ways, liberating enormous amounts of energy. Some of the possible fissions that can result from bombardment of fissionable uranium-235 with fast neutrons follow. The uranium-236 is a short-lived intermediate. 

Unless otherwise noted, all content on this page is Cengage Learning. 

Which isotopes of which elements undergo fission Experiments with particle accelerators have shown that every element with an atomic number of 80 or more has one or more isotopes capable of undergoing fission, provided they are bombarded at the right energy. Nuclei with atomic numbers between 89 and 98 fission spontaneously with long half-lives of 10 to 10 years. Nuclei with atomic numbers of 98 or more fission spontaneously with shorter half-lives of a few milliseconds to 60.5 days. One of the natural decay modes of the transuranium elements is via spontaneous fission. In fact, all known nuclides with mass numbers greater than 250 do this because they are too big to be stable. 

Most nuclides with mass numbers between 225 and 250 do not undergo fission spontaneously (except for a few with extremely long half-lives). They can be induced to undergo fission when bombarded with particles of relatively low kinetic energies. Particles that can supply the required activation energy include neutrons, protons, alpha particles, and fast electrons. For nuclei lighter than mass 225, the activation energy required to induce fission rises very rapidly. 

In Section 22-2, we discussed the stability of nuclei with even numbers of protons and even numbers of neutrons. We should not be surprised to learn that both and can be excited to fissionable states by slow neutrons much more easily than because they are less stable. It is so difficult to cause fission in that this isotope is said to be nonfissionable.  

Bombardment of nuclei by slow neutrons 3.5 Nuclear fission 

The transuranium elements (Z 93) are almost exclusively all man-made. Other man-made elements include technetium (Tc), promethium (Pm), astatine (At) and francium (Fr). 

Different nuclei show wide variations in their ability to absorb neutrons, and also in their probabilities of undergoing other nuclear reactions such probabilities are often expressed as the cross-section of a nucleus for a particular nuclear reaction. For example, the nuclides gC, fH and H have very low cross-sections with respect to the capture of thermal neutrons, but 5 cross-sections. 

Lise Meitner (1878-1968), one of a team of scientists who discovered nuclear fission. Meitner s work was recognized by naming element 109 meitnerium, Mt, in her honor. 

In the fission of uranium-235 there are many products it is not possible to write a single equation to show what happens. A representative equation is 

Notice that it takes a neutron to initiate the reaction. Notice also that the reaction produces three neutrons. If one or two of these collide with other fissionable uranium nuclei, there is the possibility of another fission or two. And the neutrons from those reactions can trigger others, repeatedly, as long as the supply of nuclei lasts. This is what is meant by a chain reaction (Fig. 20.15), in which a nuclear product of the reaction becomes a nuclear reactant in the next step, thereby continuing the process. 

The number of neutrons produced in the fission of 92U varies with each reaction. Some reactions yield two neutrons per uranium atom others, like that above, yield 

The protons and the neutrons in atomic nuclei are bound together with forces that are much greater than the forces that bind atoms together to form molecules. In fact, the energies associated with nuclear processes are more than a million times those associated with chemical reactions. This potentially makes the nucleus a very attractive source of energy. 

Because medium-sized nuclei contain the strongest binding forces ( jFe has the strongest binding forces of all), there are two types of nuclear processes that produce energy  

Combining two light nuclei to form a heavier nucleus. This process is called fusion. 

Splitting a heavy nucleus into two nuclei with smaller mass numbers. This process is called fission. 

As we will see, these two processes can supply amazing quantities of energy with relatively small masses of materials consumed. 

When heavier nuclei are bombarded by slow neutrons, the nuclei of lighter elements are formed. Besides the energy released, several neutrons are emitted. The disintegration of a heavier nucleus into lighter nuclei by neutron bombardment is called nuclear fission (nuclear division). 

In 1938, Otto Hahn and Fritz Strassman of Germany proved that the bombardment of uranium nuclei with neutrons produces several lighter and stable nuclei, each having about equal sizes. The produced nuclei are the radioisotopes of the lighter elements such as barium (Ba) and cerium (Ce). Thus, the foundations of a new useful method for the production of huge amount of energy were established. 

The element uranium is the element used for almost all fission processes. It has two natural isotopes. One of them is 238CI which, constitutes 99.3% of uranium ore, and the other is 235CJ, which constitutes 0.7% of uranium ore. Fissionable nuclei such as 235CJ and 239Pu are called fissile. Nuclear fission reactions occur 

A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate. Nuclear reactors are used for many purposes, but the most significant current uses are for the generation of electrical power and for the production of plutonium for use in nuclear weapons. Currently, all commercial nuclear reactors are based on nuclear fission. The amount of energy released by one kg 235U is equal to the energy from the combustion of 3000 tons of coal or the energy from an explosion of 20,000 tons of TNT (Trinitrotoluene, called commonly dynamite). 

In addition to the huge amount of energy released by nuclear fission reactions, another important result of such reactions is that more neutrons are produced than the number of neutrons used to bombard. The produced neutrons may also strike other 235(J isotopes and causes new fissions. The new nuclear fission reactions also produce neutrons with huge amounts of energy, and so on. This continuous process is said to be the atomic bomb, and is the basic principle of nuclear reactors. 

Both very heavy and very light nuclei are relatively unstable they decompose with the evolution of energy. 

Both of these can be split into fragments by relatively low-energy neutrons. 

Shortly after Chadwick s discovery, a group of physicists in Rome, led by Enrico Eermi, began to stndy the interaction of nentrons with the nnclei of various elements. The experiments prodnced a nnmber of radioactive species, and it was evident that the absorption of a nentron increased the N Z ratio in target nuclei above the stability line (see Fig. 19.1). One of the targets nsed was nranium, the heaviest naturally occurring element. Several radioactive prodncts resnlted, none of which had chemical properties characteristic of the elements between Z = 86 (radon) and Z = 92 (uranium). It appeared to the Italian scientists in 1934 that several new transuranic elements (Z 92) had been synthesized, and an active period of investigation followed. 

The operation of the first atomic bomb hinged on the fission of uranium in a chain reaction induced by absorption of neutrons. The two most abundant isotopes of uranium are and whose natural relative abundances are 

The fission of follows many different patterns, and some 34 elements have been identified among the fission prodncts. In any single fission event two particular nuclides are produced together with two or three secondary neutrons collectively, they carry away abont 200 MeV of kinetic energy. Usually, the daughter nuclei have different Z and A nnmbers, so the fission process is asymmetric. Three of the many pathways are 

FIGURE 19.9 The distribution of nuclides produced in the fission of has two peaks. Nuclei having mass numbers in the vicinity of A = 95 and A = 139 are formed with the highest yield those with A = 117 are produced with lower probability. 

FIGURE 19.10 In a self-propagating nuclear chain reaction, the number of neutrons grows exponentially during fission. 

Hoffinan, D. C. Ghiorso, A. and Seaborg, Glenn T., eds. (2000). The Transuranium People An Intimate Glimpse. London Imperial College Press. 

Morrissey, D. Loveland, W. T. and Seaborg, Glenn T. (2001). Introductory Nuclear Chemistry. New York John Wiley Sons. 

Rydberg, J. Liljenzin, J.-O. and Choppin, Gregory R. (2001). Radiochemistry and Nuclear Chemistry, 3rd edition. Woburn, AIA Butterworth-Heinemann. 

American Institute of Physics History Center. Available from http //www.aip. org/history/ . 

The curie is the unit used to express the amount of radioactivity produced by an element. One curie (Ci) is defined as the quantity of radioactive material giving 3.7 X 10 disintegrations per second. The basis for this figure is pure radium, which has an activity of 1 Ci/g. Because the curie is such a large quantity, the millicurie and microcurie, representing one-thousandth and one-millionth of a curie, respectively, are more practical and more commonly used. 

In nuclear fission a heavy nuclide splits into two or more intermediate-sized fragments when struck in a particular way by a neutron. The fragments are called fission products. As the atom splits, it releases energy and two or three neutrons, each of which can cause another nuclear fission. The first instance of nuclear fission was reported in January 1939 by the German scientists Otto Hahn (1879-1968) and Fritz Strassmann (1902-1980). Detecting isotopes of barium, krypton, cerium, and lanthanum after bombarding uranium with neutrons led scientists to believe that the uranium nucleus had been split. 

Upon absorption of a neutron, a heavy nuclide splits into two or more smaller nuclides (fission products). 

The mass of the nuclides formed ranges from about 70 to 160 amu. 

Two or more neutrons are produced from the fission of each atom. 

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Many of the most relevant projects under Brite-EuRam, plus some from "Standards, Measurements and Testing" (SMT), Esprit IlM, and the "Nuclear Fission Programme", have now been grouped in the network, with the participating projects clustered under the following main topics, under which a number of specific cluster activities have been defined ... 

Uranium is converted by CIF, BiF, and BrP to UF. The recovery of uranium from irradiated fuels has been the subject of numerous and extensive investigations sponsored by atomic energy agencies in a number of countries (55—63). The fluorides of the nuclear fission products are nonvolatile hence the volatile UF can be removed by distiUation (see Nuclearreactors Uraniumand uranium compounds). 

G. Kessler, Nuclear Fission Reactors, Spriager-Vedag, New York, 1983. 

Approximately 25—30% of a reactor s fuel is removed and replaced during plaimed refueling outages, which normally occur every 12 to 18 months. Spent fuel is highly radioactive because it contains by-products from nuclear fission created during reactor operation. A characteristic of these radioactive materials is that they gradually decay, losing their radioactive properties at a set rate. Each radioactive component has a different rate of decay known as its half-life, which is the time it takes for a material to lose half of its radioactivity. The radioactive components in spent nuclear fuel include cobalt-60 (5-yr half-Hfe), cesium-137 (30-yr half-Hfe), and plutonium-239 (24,400-yr half-Hfe). 

Properties. Strontium is a hard white metal having physical properties shown in Table 1. It has four stable isotopes, atomic weights 84, 86, 87, and 88 and one radioactive isotope, strontium-90 [10098-97-2] which is a product of nuclear fission. The most abundant isotope is strontium-88. 

Economic Aspects. The principal market for deuterium has been as a moderator for nuclear fission reactors fueled by unenriched uranium. The decline in nuclear reactor constmetion has sharply reduced the demand for heavy water. The United States has stopped large-scale production of D2O, and Canada is the only suppHer of heavy water at this time. Heavy water is priced as a fine chemical, and its price is not subject to market forces. 

The only large-scale use of deuterium in industry is as a moderator, in the form of D2O, for nuclear reactors. Because of its favorable slowing-down properties and its small capture cross section for neutrons, deuterium moderation permits the use of uranium containing the natural abundance of uranium-235, thus avoiding an isotope enrichment step in the preparation of reactor fuel. Heavy water-moderated thermal neutron reactors fueled with uranium-233 and surrounded with a natural thorium blanket offer the prospect of successful fuel breeding, ie, production of greater amounts of (by neutron capture in thorium) than are consumed by nuclear fission in the operation of the reactor. The advantages of heavy water-moderated reactors are difficult to assess. 

Production in Fission of Heavy Elements. Tritium is produced as a minor product of nuclear fission (47). The yield of tritium is one to two atoms in 10,000 fissions of natural uranium, enriched uranium, or a mixture of transuranium nucHdes (see Actinides and transactinides Uranium). 

As a result of nuclear fission the oxygen/metal ratio increases in the fuel across tire wide range of non-stoichiometry of UO2+J . The oxygen potential of... 

Within nuclear reactors, neutrons are a primary product of nuclear fission. By controlling the rate of the nuclear reactions, one controls the flux of neutrons and provides a steady supply of neutrons. For a diffraction analysis, a narrow band if neutron wavelengths is selected (fixing X) and the angle 20 is varied to scan the range of values. 

Nuclear (fission) reactors produce useful thermal energy from the fission (or disintegration) of isotopes such as and 94Pu . Fission of a heavy... 

Yttrium and lanthanum are both obtained from lanthanide minerals and the method of extraction depends on the particular mineral involved. Digestions with hydrochloric acid, sulfuric acid, or caustic soda are all used to extract the mixture of metal salts. Prior to the Second World War the separation of these mixtures was effected by fractional crystallizations, sometimes numbered in their thousands. However, during the period 1940-45 the main interest in separating these elements was in order to purify and characterize them more fully. The realization that they are also major constituents of the products of nuclear fission effected a dramatic sharpening of interest in the USA. As a result, ion-exchange techniques were developed and, together with selective complexation and solvent extraction, these have now completely supplanted the older methods of separation (p. 1228). In cases where the free metals are required, reduction of the trifluorides with metallic calcium can be used. 

In the early years of this century the periodic table ended with element 92 but, with J. Chadwick s discovery of the neutron in 1932 and the realization that neutron-capture by a heavy atom is frequently followed by j6 emission yielding the next higher element, the synthesis of new elements became an exciting possibility. E. Fermi and others were quick to attempt the synthesis of element 93 by neutron bombardment of but it gradually became evident that the main result of the process was not the production of element 93 but nuclear fission, which produces lighter elements. However, in 1940, E. M. McMillan and P. H. Abelson in Berkeley, California, were able to identify, along with the fission products, a short-lived isotope of... 

See also Nuclear Energy Nuclear Energy, Historical Evolution of the Use of Nuclear Fission Nuclear Fusion. 

The nucleus of an atom consists of protons and neutrons that are bound together by a nuclear force. Neutrons and protons are rearranged in a nuclear reaction in a manner somewhat akin to rearrang ing atoms in a chemical reaction. The nuclear reaction liberating energy in a nuclear power plant is called nuclear fission. The word fission is derived from fissure, which means a crack or a separation. A nucleus is separated (fissioned) into two major parts by bombardment with a neutron. 

See also Carnot, Nicolas Leonard Sadi Climatic Effects Engines Matter and Energy Nuclear Energy Nuclear Fission Refrigerators and Freezers Thermal Energy. 

See also Acid Rain Air Pollution Atmosphere Carson, Rachel Climatic Effects Disasters Environmental Economics Fossil Fuels Gasoline and Additives Gasoline Engines Government and the Energy Marketplace Nuclear Fission Nuclear Fusion Nuclear Waste. 

See also Nuclear Energy Nuclear Fission Nuclear Fusion. 

In 1938 Niels Bohr had brought the astounding news from Europe that the radiochemists Otto Hahn and Fritz Strassmann in Berlin had conclusively demonstrated that one of the products of the bom-bardmeiit of uranium by neutrons was barium, with atomic number 56, in the middle of the periodic table of elements. He also announced that in Stockholm Lise Meitner and her nephew Otto Frisch had proposed a theory to explain what they called nuclear fission, the splitting of a uranium nucleus under neutron bombardment into two pieces, each with a mass roughly equal to half the mass of the uranium nucleus. The products of Fermi s neutron bombardment of uranium back in Rome had therefore not been transuranic elements, but radioactive isotopes of known elements from the middle of the periodic table. 

Fermi and another European refugee, Leo Szilard, discussed the impact nuclear fission would have on physics and on the veiy unstable state of the world... 

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