Transmutation (Latin transmutatio : metamorphosis) is the transformation of one chemical element into another. The historical alchemists used it to describe the desired transformation of base metals into gold or silver, for example with a philosopher's stone .
Element conversion is not possible with chemical means. However, it takes place in various types of radioactive decays and nuclear reactions ; this is also sometimes called transmutation in general. Conversion of elements through nuclear reactions takes place on an industrial scale, especially when generating energy in nuclear reactors .
Transmutation in nuclear waste disposal
Since the 1990s, special techniques have been called transmutation, with which radioactive waste is to be reduced in its dangerousness by converting the particularly long-lived radioactive components into shorter-lived ones through nuclear reactions with free neutrons . The main focus is on the minor actinides neptunium , americium and curium - more precisely: the nuclides Np-237, Am-241, Am-243 and Cm-245 - with their particularly long half-lives . In some of the concepts, plutonium is also to be converted, although plutonium can also be used in the form of uranium-plutonium- MOX fuel in today's (2015) nuclear power plants. Certain concepts also provide for the transmutation of long-lived fission products .
Before the transmutation, in most of the processes, the components to be processed - i.e. the minor actinides, depending on the strategy together with the plutonium and possibly the fission products - must be separated from the used ( spent ) reactor fuel . For this partitioning ( partitioning ) chemical methods have to be developed which are based on the existing reprocessing methods , e.g. B. the PUREX process . In addition to hydrometallurgical processes, research is also carried out on pyrometallurgical processes, namely electrochemical processes in molten salt.
Suitable transmutation reaction: nuclear fission
The predominant reactions of actinides with neutrons are nuclear fission and neutron capture . Splitting is the process desired to shorten the half-life; it also delivers usable energy at the same time. Neutron capture, on the other hand, only produces the next heavier, sometimes also long-lived nuclide.
The fission cross-section of nuclides with an even number of neutrons for incident thermal neutrons is small and only increases sharply for neutron energies above approx. 1 MeV . In contrast, the capture cross-section is the largest for all nuclides for thermal neutrons. For this reason, systems with a “fast”, ie not moderated, neutron spectrum are predominantly considered for transmutation .
Transmutation with critical reactors
Reactors with fast neutrons can be used to transmutate minor actinides. There have already been experiments on this, for example in the Phénix nuclear power plant in France and in the EBR-II plant in the USA.
The possibility of using it for transmutation is also mentioned for the concept of the dual-fluid reactor .
A project designed from the outset for transmutation and simultaneous energy generation was ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration), a 600-megawatt nuclear power plant planned in France with a fourth generation sodium-cooled reactor . The Astrid project was temporarily discontinued in 2019; a continuation of the project in the second half of the current century is planned. Other existing or under construction fast reactors, e.g. B. the Russian breeder reactor types BN-800 and BN-1200 can be operated in this way.
The transmutation of minor actinides in fast reactors can, however, lead to safety problems, as explained here using the BN-800 as an example. In the BN-800 reactor, it is currently possible to transmutate the minor actinides of a 1 GW nuclear power plant that occur in one year. We are working on increasing the amount. To do this, it is necessary to completely remove the uranium-238 from the reactor core and replace it with an inert placeholder. In this case it would be possible to transmute 90 kg of minor actinides per year, the output of around five light water reactors of the 1 GW class.
The transmutation of long-lived fission products (e.g. selenium -79, zirconium -93, technetium -99, palladium -107, iodine -129 and cesium -135) is demanding because of the very small capture cross-sections in the fast neutron spectrum. However, depending on the nuclide, other reactions, especially (n, alpha) reactions, can also be considered. In the 1990s, the transmutation of technetium-99 into short-lived fission products was demonstrated at the ALMR experimental reactor in Hanford - with a fast neutron spectrum . It was possible to transmute significantly more technetium-99 than was generated at the same time.
Thermal neutrons would also not be efficient for transmutation, since in this case irradiation times of well over 100 years would be required. There are considerations to optimize the spectrum of fast reactors by suitable moderators for the fission product transmutation.
Overall, it should be noted that long-lived fission products (mainly technetium-99 and cesium-135) compared to plutonium-239, uranium-235/238 or minor actinides have several orders of magnitude lower radiotoxicity , as can be seen in the figure above.
Transmutation with accelerator-powered reactors
The minor actinides may only form a small admixture in the fuel of critical reactors, because otherwise the criticality will not be achieved because of their insufficient generation factor. However, this restriction does not apply if the reactor is operated subcritically with an external neutron source “driven” by a particle accelerator . With the development of spallation neutron sources, power reactors of this type have moved into the realm of possibility. Such accelerator driven systems (ADS) could utilize all fissile nuclides to produce energy.
Two concepts have become particularly well known: the concept of Bowman et al. M. and the energy amplifier (energy amplifier ) according to Carlo Rubbia u. M. (sometimes referred to as "Rubbiatron"). Bowman's proposal is the technologically more sophisticated and "more radical" one (with transmutation of the fission products as well). But - at least until 2013 - it did not lead to detailed development work. Rubbia's proposal is closer to technologies that have already been tried and tested.
The European ADS demonstration plant MYRRHA (Multi-purpose hybrid Research Reactor for High-tech Applications) is to be built at the Mol research center in Belgium and will go into operation around 2030. An ADS test facility at the J-PARC accelerator center in Japan is under construction and is expected to start operating with transmutation fuel around 2020.
Recovery of weapons plutonium
A task related to waste processing is the peaceful use (and thus disposal) of existing weapons plutonium stocks. It can already take place in today's (2015) light water reactors with MOX fuel elements. In the process, however, the uranium content of the fuel creates new, albeit not weapons-grade, plutonium. It would be more effective and perhaps more economical to use modified reactors, such as B. from Galperin u. M. proposed under the name Plutonium incinerator (plutonium burner). In a conventional Westinghouse pressurized water reactor , instead of the normal fuel element type, a type with two concentric, differently charged zones ( seed and blanket ) would be used. The inner zone of each fuel bundle contains the plutonium but no uranium and has a high moderator-to-fuel ratio; the outer zone contains the breeding material thorium , from which uranium-233 is produced, which is consumed again directly in the reactor operation. As a power plant, the reactor would deliver the electrical output of 1,100 megawatts. Of the annual load of 1210 kg weapons plutonium, 702 kg would be removed by splitting. The remaining 508 kg would have a high proportion of even-numbered transuranic elements (Pu-240, Pu-242, Am-242, Cm-242, Cm-244) and a high level of spontaneous fission activity and would therefore be unsuitable for military weapons.
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