Actinide elements, transmutation

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

The redox reaction has been utilized in the separation of light actinide elements (U, Np, and Pu) with both ion-exchange process and solvent extraction process. For trivalent heavy actinides with Z> 94 (except No), separation of these actinide ions from lanthanide ions is required for safe storage of long-lived nuclear waste and transmutation of these nuclides. Fundamental researches have widely been carried out by several groups for the purpose of quantitative separation of transuranium elements. Recent topics on the development and application of solvent extraction for the separation of transuranium elements are briefly summarized below. 

Ernest O. Lawrence, inventor of the cyclotron) This member of the 5f transition elements (actinide series) was discovered in March 1961 by A. Ghiorso, T. Sikkeland, A.E. Larsh, and R.M. Latimer. A 3-Mg californium target, consisting of a mixture of isotopes of mass number 249, 250, 251, and 252, was bombarded with either lOB or IIB. The electrically charged transmutation nuclei recoiled with an atmosphere of helium and were collected on a thin copper conveyor tape which was then moved to place collected atoms in front of a series of solid-state detectors. The isotope of element 103 produced in this way decayed by emitting an 8.6 MeV alpha particle with a half-life of 8 s. 

As no technology can selectively transmute minor actinides to a degree meaningful for waste management while they are contained in the spent nuclear fuel, these elements must be separated from the neutron-absorbing elements before being properly transmuted. In the case of trivalent minor actinides, this preliminary step is further necessary because of the following reasons ... 

The straightforward way to obtain light actinides is by neutron irradiation of elements of lower atomic number. For example the production of Pa has been produced by the transmutation of Th with neutron produced in a high-flux nuclear reactor. 

So, it is expedient to separate them from the HLW that may be vitrified and incorporate these actinides into crystalline matrices (nuclear waste forms) or fabricate them into solid targets for transmutation in nuclear reactors or accelerators. There are a variety of processes for processing and partitioning the actinides - TRUEX (USA, Japan, Russia), DIAMEX (USA, Japan, EEC), TRPO (PRC), SANEX (USA, EEC, PRC), are under development [6]. The basic process in these technologies is extraction (or precipitation) of actinides from HLW solutions using special reagents. These methods provide for the separation of a high-actinide fraction or joint extraction of actinides, rare earths, and zirconium. The proportion of elemental concentrations in typical fractions is (in wt.%) actinides - 10-15, rare earths - 60-65, zirconium - 20-25 [7]. 

In the long term ( 600 y) the actinides dominate the risk picture. Continuous neutron irradiation of the actinides finally destroys all of them by fission (cf. Fig. 16.3). The annual production of americium and curium is 5 kg in a 1000 MW LWR, but considerably less in a FBR. Thus if pins of these elements are inserted in a FBR, more americium and curium is destroyed than formed it is estimated that 90% will have been transformed into fission products after 5-10 y. In the future, CTRs could be used for the same purpose. As an alternative it has been suggested to leave the americium and curium in the uranium returned in the LWR cycle. Wastes from transmutation processes will contain some amoimt of longlived nuclides, thus a safe final repository is still needed. 

The iiradiation experiment TRABANT (Transmutation and Suming of Actinides) was planned and executed in a trilateral collaboration CEA, FZK and Institute for Transuranium Element (ITU) within the framework of the CAPRA project This experiment contained three pins in capsules. 

For the purpose of transmutation, the minor actinides of concern in spent fuel are neptunium, americium, and curium although small amounts of the higher actinides, berkeUum, californium, einsteinium, and fermium can be made under the right conditions. The reaction pathways leading to the production of the transuranium elements are given in Fig. 61.4. 

A number of different strategies for transmuting the minor actinides have been suggested. In general, the different approaches depend on trade-offs between the effectiveness of the partitioning steps (chemical yields and acceptable losses of the desired elements) and the degree of transmutation that can be achieved by various types of devices. 

Over the past 10 years, modifications to the PUREX process have made it possible to more effectively separate neptunium. To effect the efficient separation of Np within the conventional PUREX process, Np is oxidized to VI state by nitrous add and is extracted in the first cycle along with U and Pu into the organic phase. The extracted Np( VI) follows the uranium stream and is later separated during the second purification cycle of uranium. In the RFC, the neptunium is sent to vitrification and disposed of as HLW but in an AFC option, the neptunium can be blended with MOX fuel or fabricated into special targets for later transmutation. The other minor actinides, ameridum and curium cannot be separated by reasonable modifications to the PUREX process. These elements will require the addition of special processing steps to separate them from the PUREX high-level waste stream. 

P T involves three steps (i) partitioning of long-lived radionuclides (minor actinides (Np, Am, Cm) and fission products (I, Tc, Cs)) from high level waste (ii) development of fuel and targets containing these long-lived elements in view of their (iii) transmutation in different burners (fission reactors and accelerator driven transmutation devices). The progress achieved in these three areas can be summarized as follows. 

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