Transuranium nuclide production

The third principal component of environmental radioactivity is that due to the activities of humans, the anthropogenic radionuclides. This group of nuclides includes the previously discussed cases of 3H and 14C along with the fission products and the transuranium elements. The primary sources of these nuclides are nuclear weapons tests and nuclear power plant accidents. These events and the gross nuclide releases associated with them are shown in Table 3.1. Except for 14C and... 

Fig. 12. Production cross sections of transuranium nuclides in the interaction of 238U with 238U (solid lines) plotted versus mass number. Also shown are data for the l36Xe+238U interaction (dashed lines). FromM. Schadel et al. [104],...

The most important method of production of the first transuranium elements is neutron irradiation of uranium. After the discovery of the neutron by Chadwick in 1932, this method was applied since 1934 by Fermi in Italy and by Hahn in Berlin. The method is based on the concept that absorption of neutrons by nuclides with atomic number Z leads to formation of neutron-rich nuclides that change by fi decay into nuclides with atomic numbers Z - -1. Unexpectedly, the experiments carried out by Hahn and Strassmann led to the discovery of nuclear fission in 1938. 

Separation of Actinides from the Samples of Irradiated Nuclear Fuels. For the purpose of chemical measurements of burnup and other parameters such as accumulation of transuranium nuclides in irradiated nuclear fuels, an ion-exchange method has been developed to separate systematically the transuranium elements and some fission products selected for burnup monitors (16) Anion exchange was used in hydrochloric acid media to separate the groups of uranium, of neptunium and plutonium, and of the transplutonium elements. Then, cation and anion exchange are combined and applied to each of those groups for further separation and purification. Uranium, neptunium, plutonium, americium and curium can be separated quantitatively and systematically from a spent fuel specimen, as well as cesium and neodymium fission products. 

Pentreath, R,J., Harvey, B.R. Lovett, M.B. (1985) Chemical speciation of transuranium nuclides discharged into the marine environment. In Speciation of Fission and Activation Products in the Environment, eds. R.A. Bulman and J.R, Cooper, 312-25, Elsevier, London. 

Tc is available through the /l -decay of Mo (Fig. 2.1.B), which can be obtained by irradiation of natural molybdenum or enriched Mo with thermal neutrons in a nuclear reactor. The cross section of the reaction Mo(nih,v) Mo is 0.13 barn [1.5], Molybdenum trioxide, ammonium molybdate or molybdenum metal are used as targets. This so-called (n,7)-molybdenum-99 is obtained in high nuclidic purity. However, its specific activity amounts to only a few Ci per gram. In contrast, Mo with a specific activity of more than in Ci (3.7 10 MBq) per gram is obtainable by fission of with thermal neutrons in a fission yield of 6.1 atom % [16]. Natural or -enriched uranium, in the form of metal, uranium-aluminum alloys or uranium dioxide, is used for the fission. The isolation of Mo requires many separation steps, particularly for the separation of other fission products and transuranium elements that arc also produced. 

We simply define radiochemistry and nuclear chemistry by the content of this book, which is primarily written for chemists. The content contains fimdamental chapters followed by those devoted to applications. Each chapter ends with a section of exercises (with answers) and literature references. An historic introduction (Ch. 1) leads to chapters on stable isotopes and isotope separation, on unstable isotopes and radioactivity, and on radionuclides in nature (Ch. 2-5). Nuclear radiation - emission, absorbance, chemical effects radiation chemistry), detection and uses - is covered in four chapters (Ch. 6-9). This is followed by several chapters on elementary particles, nuclear structure, nuclear reactions and the production of new atoms (radio-nuclides of known elements as well as the transuranium ones) in the laboratory and in cosmos (Ch. 10-17). Before the four final chapters on nuclear energy and its environmental effects (Ch. 19-22), we have inserted a chapter on radiation biology and radiation protection (Ch. 18). Chapter 18 thus ends the fimdam tal part of radiochemistry it is essential to all students who want to use radionuclides in scientific research. By this arrangement, the book is subdivided into 3 parts fundamental ladiochemistry, nuclear reactions, and applied nuclear energy. We hope that this shall satisfy teachers with differrat educational goals. 

The main task in the monitoring of radionuclides in fission products (nuclides produced by primary and secondary processes associated with fission, generally of the uranium isotopes and U), transuranium nuclides, and transplutonium elements is the identification of nuclides and the determination of yield. A... 

Now the question whether transuranium elements were within range was again open. Edwin McMillan obtained a first hint when he irradiated thin uranium layers with neutrons (McMillan 1939). Whereas the fission products recoiled out of the target, the already known 23 min formed by neutron capture in remained in the target along with a P emitter with 2.3 d half-Kfe. A year later, he and Philip Abelson were able to show by a radiochemical milking experiment that the 2.3 d nuclide is the decay product of hence it is 93, the first... 

Successful production of fissionable nuclei, such as transuranium nuclides in nuclear reactions, depends mainly on two factors fusion cross section, fTftision> and survival probability,... 

Some experimental production cross sections of transuranium nuclides including transactinides in charged-particle-induced reactions of the U isotopes and Cm targets are summarized in Table 18.3. 

List of the studied transfer reactions for the production of transuranium nuclides in the bombardments of Cm with various kinds of incident particles... 

Most standards for nuclear criticality safety deal with U and, to a lessw degree. and Pu. However, increasing production df other transuranium nuclides that can be made critical has necessitated consideration of the fis e properties. and critical masses of these special materials. A work group (Table I) was formed to draft a new standard that would be an extension of the standard for criticality safety in Operations outside of reactors (NI6.1-197S/ANS- -8.1). Subcritical mass limits for 14 new nuclides (Tables II and. Ill), were developed however, fire major application of thb hew standard is expected to be te setting operational liihits in facilities that handle six important nuclides, namely, Np, " u. Pu, "Aih, and n m. 

In addition to the fission products, actinide nuclides are generated in the nuclear fuel during irradiation. The main starting reaction for the buildup of the transuranium nuclides is neutron capture in the nucleus leading to short-lived... 

Figure 3.7. Enhanced production of transuranium nuclides at the fuel pellet rim (autoradiographic image)...

The radioactivity of HLW s lasts for several thousands years before its radioactivity reduces to the level of natural uranium ore, because it includes long-lived fission products (FP s) and transuranium nuclides (TRU s). The radionuclides released from the stored waste can be transported via ground water flow from the repository. Scientifically, it is difficult to prove the safety of a HLW disposal facility because the long time scale involved is beyond human experience. 

The fuel cycle feedstock is natural or depleted uranium, and multi recycle through sequential cassette reload cycles achieves total fission consumption of the feedstock only fission product waste forms (and trace losses of transuranium nuclides) go to a geologic repository operated by the regional centre. These waste forms - lacking any transuranic component -decay to the equivalent radio toxicity levels of the original ore within 200-300 years... 

Fermi irradiated uranium with slow neutrons, and observed a variety of radioactivities that he tentatively identified as being transuranium elements [1]. We now know that these radioactive species were the products of the fission of the in the sample. Study of the chemical properties of these new nuclides led to the subsequent discovery of fission in 1939 [2, 3], Explanation of the fission process was closely connected to the creation of the liquid-drop model [4—6], in which the nucleus is treated like an incompressible charged fluid with surface tension. See Nuclear Structure of Superheavy Elements for more information on nuclear structure and the stability of the heaviest nuclides. 

The fate of actinide elements introduced into the environment is of course not merely a scientific issue. The disposal of the by-products of the nuclear power industry has become a matter of public concern. For each 1000 kg of uranium fuel irradiated in a typical nuclear reactor for a three-year period, about 50 kg of uranium are consumed. In addition to a large amount of energy evolved as heat, 35 kg of radioactive fission products and 15 kg of plutonium and transplutonium elements are produced. Many of the fission-product nuclides are stable, but others are highly radioactive. All of the fission products are isotopes of elements whose chemical properties are well-understood. The transuranium elements produced in the reactor by neutron capture, however, have unique chemical properties, which are reasonably well-understood but are not always easily inferred by extrapolation from the chemistry of the classical elements. Plutonium is fissile and can be recycled as a nuclear fuel in conventional or breeder reactors, but the transplutonium elements are not fissile to the extent of supporting a nuclear chain reaction, and in any event they are produced in amounts too small to be of interest for large-scale uses. The transplutonium elements must therefore be secured and stored. 

The main source of transuranium elements is the high-flux reactor, in which or heavier nuclei get transformed into higher-Z elements by multiple neutron capture. In the USA, there is a national program for the production of transuranium elements utilizing the high-flux reactor (HFIR) at Oak Ridge. The heaviest nuclide produced in the reactor is Fm. Neutron-deficient nuclides are synthesized in charged-particle accelerators and very neutron-rich nuclides with short half-lives are produced in reactors. 

Fuel leakages are qualitatively indicated by a sudden increase in the concentrations (spiking) of fission products, especially noble gases and iodine nuclides. The appearance of less volatile nuclides in the coolant (e.g., transuranium nuclides) suggests the occurrence of open defects. 

Gen-IV reactor systems of fast neutron spectrum type in fact include waste destruction as an integral part of the fuel cycle rather than as a separate process. In a stiU more ambitious project, based on an accelerator-driven fission system, all the waste products are continuously recycled and selected transuranium nuclides are destroyed. This is the purpose, for example, of the previously mentioned MYRRHA project. 

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