Fissile actinides

Note S storage. T transmute by thermal (therm) or fast (fast) neutrons. F. Ans fissile actinides. 

There are three fast-flux reactors proposed for development the sodium cooled, the gas cooled, and the lead cooled. The fission cross sections for fast neutrons (high-energy spectrum neutrons) for all of the fissile actinides are nearly the same so the fast-flux reactors use all of the fissile actinides as fuel. The fast-flux isotopic fission cross sections are smaller than for thermal neutrons so the fraction of fissile isotopes (e.g., 235u 239pu, range of... 

VII.63. The use of INF isotopics in the criticality safety analysis means that any computational methods used to predict the isotopics should be validated, preferably against measured data. The reduced reactivity in INF is due to the decrease in fissile inventory and the increase in parasitic, neutron absorbing nuclides (non-fissile actinides and fission products) that build up during bumup. Broadhead [V11.23] and DeHart [V11.24] provide information to help identify the important nuclides that affect the reactivity of PWR irradiated fuel. The INF nuclides that can be omitted from a safety analysis are the parasitic absorbers that can only decrease k jj further if included in the analysis. Neutron absorbers that are not intrinsic to the fuel material matrix (gases, etc.) must also be eliminated. 

For the minor actinides, the transmutation process consists of the capture of one or more neutrons until a more fissionable isotope is formed. For the actinides, the most important transmutation reaction is fission because it results in the removal of the isotope from the minor actinide inventory and replaces it with two typically shorter-lived, less toxic fission fragments. With more energetic neutrons, the (n,2n) reaction is also useful because this reaction transforms fertile actinides with low fission probabilities to more fissile actinides with higher fission probabilities. Neutron capture reactions that produce less fissionable isotopes merely add to the inventory of minor actinides. 

Fission-produced neutrons can be absorbed by any of the materials present in the reactor, with relative probabilities proportional to the neutron-absorption cross sections of the nuclei in the materials. Even for the fissile nuchdes, neutron-capture reactions compete favorably with neutron-induced-fission reactions. For example, low-energy neutron irradiation of a sample of results in the production of about half as often as it results in fission. Fuel containing the fissile actinides always contains some of the even-mass actinide nuclides as well, which are... 

As for the future, it seems that experimental nuclear criticality safety studies with Pu, u, and U should be limited to specific experiments related to peculiar uses or processes. Experiments with fissile actinides should... 

Subcritical Mass Limits for Fissile Actinide Nuclides... 

Over the last six decades, a significant amount of work has been done in this area of nuclear science and technology. In the present review, an attempt has been made to highlight the recent developments in this branch of science concerning the actinide elements with special reference to thorium, uranium, and plutonium, specific isotopes of which are being used as fissile/fertile materials. 

Actinides, particularly the lighter ones, display multiple oxidation states and complex chemical behavior, which makes their chemistry quite fascinating. Some isotopes of these elements, such as 232Th, 233,235,238 and 239Pu, are important for the nuclear industry due to their utility as fissile/fertile materials. Therefore, the separation chemistry of different oxidation states of Th, U, and Pu need to be reviewed with respect to both basic as well as applied aspects. Some fundamental chemical properties of the lighter actinides, including oxidation states, hydrolysis, and complexation characteristics form the basis of their separation. 

Most radioactive particles and vapours, once deposited, are held rather firmly on surfaces, but resuspension does occur. A radioactive particle may be blown off the surface, or, more probably, the fragment of soil or vegetation to which it is attached may become airborne. This occurs most readily where soils and vegetation are dry and friable. Most nuclear bomb tests and experimental dispersions of fissile material have taken place in arid regions, but there is also the possibility of resuspension from agricultural and urban land, as an aftermath of accidental dispersion. This is particularly relevant to plutonium and other actinide elements, which are very toxic, and are absorbed slowly from the lung, but are poorly absorbed from the digestive tract. Inhalation of resuspended activity may be the most important route of human uptake for actinide elements, whereas entry into food chains is critical for fission products such as strontium and caesium. 

Outside of nuclear energy, the most well-known utilization of the actinides is in nuclear weapons. The fissile nature of and Pu has been exploited in the creation of nuclear arsenals throughout the world. Primarily found in the... 

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