The Fission Process

More than 300 different nuclides have been observed as the primary products of fission. The term fission products usually refers to the primary fission products, i.e., the fission fragments and their daughters resulting from radioactive decay and neutron absorption. Only a few of the primary fission products are stable, the rest being beta-emitting radionuclides. As a fission-product radionuclide undergoes beta decay, its atomic number increases whereas its mass number remains constant. The direct yield of a fission-product nuclide is the fraction of the total fissions that yield this nuclide, essentially as a direct-fission fragment. The cumulative 

This situation with regard to yield and radioactive decay at each mass number is illustrated for mass number 90 in Fig. 2.14. For accurate estimation of the amount of any nuclide produced at a given time, the differential equations appropriate to such a system of yields and decays must be set up and solved. This is illustrated in Secs. 6.3 throu 6.5 for selected fission-product nuclides of mass 135 and masses 147, 149, 151, and 152, which are important neutron-absorbing poisons in thermal reactors. 

In the steady state, when atoms undergoing fission are in equilibrium with their radioactive fission products, the energy released per fission is distributed approximately as in Table 2.10. 

At a time t in days after fission, the products of a single fission undergo beta decay at a rate fi(t) given by 

Mass number Fission by sJow neutrons Fission by fast neutrons  

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When a uranium-235 atom undergoes fission, it splits 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 different ways. For example, while one atom of 292U is splitting to give isotopes of rubidium (Z = 37) and cesium (Z = 55), another may break up to give isotopes of bromine (Z = 35) and lanthanum (Z = 57), while still another atom yields isotopes of zinc (Z = 30) and samarium (Z = 62) ... 

The stable neutron-to-proton ratio near the middle of the periodic table, where the fission products are located, is considerably smaller (—1-2) than that of uranium-235 (1.6). Hence the immediate products of the fission process contain too many neutrons for stability ... 

The two nuclei on the right side are just two of the many possible products of the fission process. Since more than one neutron is released in each process, the fission reaction is a self-propagating, or chain reaction. Neutrons released by one fission event may induce other fissions. When fission reactions are run under controlled conditions in a nuclear reactor, the energy released by... 

Samarium is the 39th most abundant element in the Earths crust and the fifth in abundance (6.5 ppm) of all the rare-earths. In 1879 samarium was first identified in the mineral samarskite [(Y, Ce U, Fe) (Nb, Ta, Ti )Ojg]. Today, it is mostly produced by the ion-exchange process from monazite sand. Monazite sand contains almost all the rare-earths, 2.8% of which is samarium. It is also found in the minerals gadolmite, cerite, and samarskite in South Africa, South America, Australia, and the southeastern United States. It can be recovered as a byproduct of the fission process in nuclear reactors. 

It has been known for many years that the fission products observed in the field or in the laboratory some time after the event are in fact not usually the species produced in fission at all but the result of one or several consecutive beta disintegrations of shorter lived isobaric precursors which are formed directly in the fission process. From the chemist s point of view this is important because the f -decay process is an actual transmutation of elements, and the time scale involved is frequently comparable with that for the formation of fallout particles. 

Unfortunately, precise knowledge of the distribution of direct yield among several competing isobars is generally not available furthermore, the radioactive half-lives involved are frequently completely unknown since the fission process gives rise directly to between 300 and 400 radioactive species, and the separation of such a complex mixture usually involves a time which is quite long compared with the lifetimes of interest. We do know that each isobaric chain is formed directly as a number of different isobars and that the width of the isobaric yield distribution is such that to account for 90% or more of a chain one must consider at least three or perhaps four chemical elements. 

Tn a typical fast breeder, most of the fnel is 238I (90 to 9.3%). The remainder of the fuel is in the form of fissile isotopes, which sustain the fission process. The majority of these fissile isotopes are in the form of 23 Pu and 241Pu, although a small portion of 235U call also be present. Normally, the fissile isotopes are located in a central core region that is surrounded by the fertile isotopes in the blanket region. This is illustrated in Fig. 30. 

Spent fuel from a reactor contains unused uranium as well as plutonium-239 which has been created by bombardment of neutrons during the fission process. Mixed with these useful materials are other highly radioactive and hazardous fission products, such as cesium-137 and strontium-90. Since reprocessed fuels contain plutonium, well suited for making nuclear weapons, concern has been expressed over the possible capture of some of this material by agents or terrorists operating on behalf of unfriendly governments that do not have a nuclear weapons capability. 

In a typical fast breeder nuclear reactor, most of the fuel is 238U (90 to 93%). The remainder of the fuel is in the form of fissile isotopes, which sustain the fission process. The majority of these fissile isotopes are in the form of 239Pu and 241Pu, although a small portion of 235U can also be present. Because the fast breeder converts die fertile isotope 238 U into the fissile isotope 239Pu, no enrichment plant is necessary. The fast breeder serves as its own enrichment plant. The need for electricity for supplemental uses in the fuel cycle process is thus reduced. Several of the early hquid-metal-cooled fast reactors used plutonium fuels. The reactor Clementine, first operated in the Unired States in 1949. utilized plutonium metal, as did the BR-1 and BR.-2 reactors in the former Soviet Union in 1955 and 1956, respectively. The BR-5 in the former Soviet Union, put into operation in 1959. utilized plutonium oxide and carbide. The reactor Rapsodie first operated in France in 1967 utilized uranium and plutonium oxides. 

The reader should be cautioned that understanding the fission process represents a very difficult problem. Some of the best minds in chemistry and physics have worked on the problem since the discovery of fission. Yet, while we understand many aspects of the fission process, there is no overall theoretical framework that gives a satisfactory account of the basic observations. 

Figure 11.1 Schematic view of the fission process. (From J. E. Gindler and J. R. Huizenga, Nuclear Fission in Nuclear Chemistry, Vol. II, L. Yaffe, Ed. Copyright 1968 Academic Press, Reprinted by permission of Elsevier.)...

Of the fast neutrons produced in fission, some of them will be moderated to thermal energies and will induce other fission reactions while others will be lost. The ratio of the number of neutrons in the next generation to that in the previous generation is called the multiplication factor k. If the value of k is less than 1, then the reactor is subcritical and the fission process is not self-sustaining. If the value of k is greater than 1, then the number of fissions will accelerate with time and the reactor is supercritical. The goal of reactor operation is to maintain the system in a critical state with k exactly equal to 1. The extreme upper limit for the multiplication factor would correspond to the mean number of neutrons per fission ( 2.5 for 235U(n,f)) if each neutron produces a secondary fission. 

In a reactor the fission process cannot be allowed to get out of control as it does in an atomic bomb. 

Ida Noddack s proposal of the fission process has awakened interest in the history of science since Gerald Holton unearthed the case in 1973. See T. Hopper, She was ignored. Ida Noddack and the Discovery of Nuclear Fission, Master s thesis, Stanford University, 1990, and F. Habashi, Ida Noddack proposes of nuclear fission, in M.F, Rayner-Canham and G.W. Rayner-Canham (eds.), A Devotion to their Science Pioneer Women in Radioactivity (Philadelphia Chemical Heritage Foundation - Me Gill s University Press, 1997), 217-225. 

The discovery of fission was a complete surprise and also a great shock, because it shattered fundamental ideas of nuclear behavior that had guided the investigation. The surprise was evident in the events of December 1938. On December 10, Enrico Fermi was awarded the Nobel Prize in physics. He and his group in Rome had been the first to irradiate uranium with neutrons and to propose that transuranium elements had been formed in the process. In his Nobel lecture, Fermi was so confident of the first two, elements 93 and 94, that he referred to them by name ausonium and hesperium. But at that very moment, the Berlin team of Otto Hahn, Lise Meitner, and Fritz Strafimann was on the verge of identifying barium among the uranium products. By the end of the year, they understood that uranium had split, explained the fission process, and concluded that the transuranium elements were false. When Fermi published his Nobel lecture, he added a footnote to that effect, but by then ausonium and hesperium were themselves footnotes (if that) in the history of science. [1]... 

A typical nuclear reactor utilizes uranium oxide, whose uranium content is approximately 3 percent uranium-235, and 97 percent uranium-238, by mass. During the fission reaction, the uranium-235 is consumed and fission products form. As the amount of uranium-235 decreases and the amounts of fission products increase, the fission process becomes less efficient. At some point, the spent nuclear fuel is removed and stored. Some of the radioactive fission products, because of their radioactivity and long half-lives, must be stored securely for thousands of years. Thus, nuclear waste management poses a tremendous challenge. 

You will open the shipping container, transfer the solution to a volumetric flask, and dilute the original solution as needed for your purposes. This experiment must be planned and scheduled so that the time interval between the fission process and initial counting is sufficiently short to measure shorter-lived radionuclides before they can no longer be measured by you. 

The fission process produces radioactive as well as stable nuclides with masses ranging from 72 to 167 and with two broad peaks in the regions of 95 and 138. The masses are identified rather than the specific nuclides because in fission many short-lived nuclides are produced that quickly decay by beta... 

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