Actinide elements, irradiated fuel

The research programme of the European Institute for Transuranium Elements was, from its very beginning, devoted to both basic research on advanced plutonium containing fuel and to fundamental research on actinide elements. Non-fuel actinide research in Europe started more than 20 years ago with the reprocessing of irradiated actinide samples. Since the first isolation and purification of transplutonium elements, actinide research developed steadily in close contact and cooperation with specialised laboratories in Western Europe and in the United States. 

In the chemistry of the fuel cycle and reactor operations, one must deal with the chemical properties of the actinide elements, particularly uranium and plutonium and those of the fission products. In this section, we focus on the fission products and then chemistry. In Figures 16.2 and 16.3, we show the chemical composition and associated fission product activities in irradiated fuel. The fission products include the elements from zinc to dysprosium, with all periodic table groups being represented. 

Origins. Most of the radioactive waste at SRP originates in the two separations plants, although some waste is produced in the reactor areas, laboratories, and peripheral installations. The principal processes used in the separations plants have been the Purex and the HM processes, but others have been used to process a variety of fuel and target elements. The Purex process recovers and purifies uranium and plutonium from neutron-irradiated natural uranium. The HM process recovers enriched uranium from uranium—aluminum alloys used as fuel in SRP reactors. Other processes that have been used include recovery of and thorium (from neutron-irradiated thorium), recovery of Np and Pu, separation of higher actinide elements from irradiated plutonium, and recovery of enriched uranium from stainless-steel-clad fuel elements from power reactors. Each of these processes produces a characteristic waste. 

The chemistry involved in the isolation and purification of the actinide elements from irradiated reactor fuel elemmts is further discussed in Chapter 21. Actinide chemistry in the ecosphere is discussed in 22.6. 

Chemistry used in the recovery of plutonium from irradiated fuel must provide a separation from all these elements, other fission and activation products, and the actinides (including a large amount of unburned uranium), and still provide a complete recovery of plutonium. The same issues apply to the recovery of uranium from spent thorium fuel. Most of the processes must be performed remotely due to the intense radiation field associated with the spent fuel. As in the enrichment of uranium, the batch size in the later steps of the reprocessing procedure, where the fissile product has become more concentrated, is limited by the constraints of criticality safety. There is a balance between maximizing the yield of the precious fissile product and minimizing the concentrations of contaminant species left in the final product These residual contaminants, which can be detected at very small concentrations using standard radiochemical techniques, provide a fingerprint of the industrial process used to recover the material. 

Table 3.6. Actinide element concentrations (g/kg HM) in irradiated LWR uranium fuel (initial enrichment 4.0%...

In terms of amount, by far the most significant of the synthetic actinide elements is plutonium. Nuclear power production by fission in uranium produces as a byproduct approximately 50 tons per year world-wide of a mixture of plutonium isotopes. About 250 tons of plutonium is estimated to be in the world plutonium inventory, some still in unprocessed spent fuel assemblies from nuclear reactors. World inventory of plutonium by the year 2000 has been estimated at 2400 tons [57], Plutonium produced for nuclear weapons is mainly Pu, but plutonium produced as a by-product of energy production contains substantial amounts of °Pu, Pu, and Pu and small amounts of Pu [64]. The plutonium in the environment is due, in decreasing order of importance, to the testing of nuclear weapons in the atmosphere, the re-entry into the atmosphere and disintegration of satellites equipped with Pu power sources, and the processing of irradiated uranium fuel from nuclear reactors. 

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. 

Actinides occurrence and preparation. With the exception of U and Th, the availability of the actinides of the first half of the series ranges from the g to kg scale that of the elements of the second half of the series from the mg scale for Cf to the sub-mg scale for Es. Isotopes of Np, Pu, Am, Cm can be available as byproducts of nuclear fuel processing other elements such as Ac, Cf, Bk, Es can be obtained by irradiation of selected isotopes in high flux reactors, or by reprocessing large quantities of ore (Pa). 

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. 

Irradiated uranium fuel elements contain fission product isotopes, activation products produced by atoms exposed to the intense neutron, ot, (3, and y radiation in the reactor core, and actinides, produced by neutron capture of the nuclei of atoms with atomic... 

As discussed in 19.10, Pu has been formed in natural uranium reactors at a later stage of the earth s evolution. Many thousands of tons of plutonium has been synthesized in commercial and military reactors the annual global production rate in nuclear power reactors in the year 2000 was 1000 tons/y, contained in the spent fuel elements. The nuclear reactions and chemical separation processes are presented in Chapters 19 and 21. The build-up of heavier elements and isotopes by n-irradiation of Pu in nuclear reactors is illustrated in Figures 16.2 and 16.3. The accumulated amount of higher actinides within the European commimity is many tons for Np, Pu and Am, and himdreds of kg of Cm the amounts in the United States and Russia are of the same magnitude. 

For metal fuel fabrication, the actinide metals are alloyed in an injection casting furnace that melts, mixes the alloy and injects the molten metal into quartz molds. After quick cooling, the quartz mold is removed from the metal pin, which is cut to length and undergoes quality assurance measurements. These pins are placed into new fuel cladding that contains a small amount of metallic sodium, which provides a thermal bond in early irradiation in the nuclear reactor. These fuel elements are welded closed and are ready for the reactor. Recent research in this area has focused on modifying the process to minimize the volatization of americium, which is a key component in U/TRU recovered for fast reactors and has a high vapor pressure. 

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