Actinides fission barrier

Figure 11.2 shows some of the basic features of fission barriers. In Figure 11.2, the fission barriers as estimated from the liquid drop model for a range of actinide nuclei are shown. The fission barrier height decreases, and the maximum (saddle point)... 

Figure 11.2 Qualitative features of the fission barriers for actinide nuclei. (From H. C. Britt, Fission Properties of the Actinides in Actinides in Perspective, N. Edelstein, Ed. Copyright 1982 Pergamon Press, Ltd. Reprinted by permission of H. C. Britt.)...

As an exercise, let us compare the spontaneous fission half-lives of two nuclei with barrier heights of 5 and 6 MeV, respectively, and barrier curvatures of 0.5 MeV. One quickly calculates that the spontaneous fission half-lives of these two nuclei differ by a factor of 3 x 105. The barrier heights and curvatures in this example are relevant for the actinides and illustrate the difficulty that a 1-MeV uncertainty in the fission barrier height corresponds to a factor of 105 in the spontaneous fission half-life. 

The initiated radioactive inventory for spent reactor fuel consists of actinides, fission products and activation products. As noted previously, (Gi. 21) the shorter lived fission products, such as Sr and Cs, and transuranic elements, such as Pu, Pu, are the main contributors to the radioactivity. However, performance assessments strongly indicate that the waste form matrix and the near field engineered barriers (e.g. clay backfill, etc.), can successfully retain and prevent any migration to the far field viromnent for one thousand years and probably much longer (> lO years). After the first thousand years the long lived nuclides such as Cs, Sn, Tc and Se among the fission products and the actinides Np, Pu, Pu, and Am become the major concern. 

For X < I, the deformation energy has a local minimum at the spherical shape. For the common actinide nuclei (0.68 < x< 0.76) the deformation energy also has a maximum with positive energy relative to the ground state at positive values of the deformation parameter (fl2o)- That defines the fission barrier. Numerical calculations including multipoles up to 1 16 led to fission barrier heights... 

All presently available information on fission-barrier parameters in the actinide mass r ion is shown in Fig. 5.27. 

Schematic illustration of the delayed-fission process. The potential energy of the daughter nucleus as a function of deformation is shown, displaying the double-humped fission barrier prevalent in the actinides. Region I is the inner, or ground-state, well (first minimum) region II is the outer, or shape isomer, well (second minimum). The horizontal arrows indicate the various kinds of deformation toward fission...

As researchers performed experiments that advanced them along the row of actinide elements on the Periodic Table, the general trends with increasing atomic number were smaller production probabilities expressed as cross sections (a consequence of the diminishing fission barrier and higher fission probabilities), an increased probability of decay by a-particle emission (a consequence of increasing a-decay Q values) and shorter half-lives. For the elements below fermium, spontaneous fission is not an important decay mode. Experimental work was dominated by radiochemical techniques in which atomic number was determined by chemical properties and atomic mass was determined by mass spectrometry and the connections of nuclei to one another by the processes of radioactive decay. The physical separation and detection methods that were used in later work were developed in the 1960s. 

Shape Isomers can be found in the second minimum of the fission barriers of actinides. If the nucleus is prepared in the lowest state in the second minimum it is much more deformed than in any of the states in the first minimum. Thus any transition out of the second minimum will require the rearranging of all nucleons, which leads to the observed very small matrix elements and thus the formation of an isomeric state. These isomers are also known as fission isomers. 

Because the sequence of neutron captures inevitably leads to looFrn which has a fission half-life of only a few seconds, the remaining three actinides, loiMd, 102N0 and losLr, can only be prepared by bombardment of heavy nuclei with the light atoms jHe to foNe. This raises the mass number in multiple units and allows the f Fm barrier to be avoided even so, yields are minute and are measured in terms of the number of individual atoms produced. 

Apatite is being considered as a barrier that will prevent the leakage of radioactive nuclei from the radioactive waste storage. Because of the similarity in the chemical and spectral features REE have been chosen as a model of the fission products of the actinides. For this reason it is of importance to recognize whether the elements are incorporated in the bulk of the barrier, or adsorbed on the surface where they can be subjected to leaching out (Martin et al. 1996 Martin et al. 1999a Martin et al. 1999b). 

Figure 14.11. Logarithm of the ratio of the cross sections (Tn,f and for various nuclides of the actinides as a function of the difference between the neutron binding energy B(n) and the energy barrier of fission E. (According to G. T. Seaborg The Transuranium Elements. Yale University Press 1958 Addison-Wesley Publ. Comp, Reading, Mass., S, 166/167 S. 240/241.)...

This document summarizes the efforts of the Source Term Working Group to complete the tasked objectives under the lASAP. It presents a detailed discussion of the fission product, actinide, and activation product inventories at each Kara Sea disposal site and a detailed description, with assumptions, of the models used to predict potential release of the radionuclides into the Kara Sea. Results of the release scenario models, reliability of the model input parameters, and an analysis of the sensitivity of the results to changes in the protective barrier lifetimes and SNF corrosion rates are then presented. The potential for recriticality of the SNF bearing cores and considerations for potential remedial measures are next addressed. Finally, conclusions are drawn with respect to the radionuclide releases at each Kara Sea disposal site from the SNF and activated components. 

The first barrier to sea water ingress is the bitumen which covers the whole SGI. This bitumen is assumed to degrade linearly in 100 years from full to zero effectiveness, modelled by the bitumen factor k(, = O.Olt (t 100 years). The fuel, and hence the fission products and actinides, is surrounded above and below by at least 300 mm of solidified Pb-Bi coolant, and multiple layers of SS. Using the most pessimistic estimates of corrosion rates and thicknesses, the minimum time for water ingress to the fuel via corrosion directly from above or below is over 40 000 years. This timescale is much longer than that of ingress through the EPR tubes and ECTs hence, this method will be assumed to be the primary means of release and is studied in more detail below. 

However, because of the difficulties and lack of information in estimating the rates of radionuclide release from grain boundaries, the model assumes an instantaneous release from the fuel surface and grain boundaries once sea water comes into contact with the fuel. As a conservative estimate, this is assumed to account for 20% of the total fission product and actinide inventory in the fuel. Similarly, since the extent of damage to the Zr-Nb alloy cladding around the fuel is unknown, the model assumes that cladding is ineffective as a barrier to sea water and radionuclide release. 

Unit 421 was encased in concrete and as discussed earlier, a slowly degrading lifetime of 100 years was assumed for this containment barrier. A similar lifetime was assumed for the Furfurol(F) encapsulating the SNF. As the concrete barrier becomes more and more porous, activation products are released from the outside of the RPV. Then the breather hole into the interior of the RPV is corroded open in the year 2005, allowing fuel and interior SS corrosion to begin. The other RPV penetrations and barriers begin to open up in the year 2035, shown by the peak in release rates for the fission products, 370 GBq-a and actinides, 0.2 GBq-a L Coupled with the continuing steel corrosion, the total peak release rate is 370 GBq a. ... 

Overall Scenario A release rates are shown in Figure 26. From the dumping in 1981, initially, no active material is expected to appear due to the hull and bitumen barriers. When it does start, the initial fission product and actinide release is less than 0.0001 GBq-a at the year 2105 when the fission product and actinide inventory in the corroded left board SG starts to appear. Corrosion of the outer surfaces of the activated RPVs by that time contributes about 8 GBq-a. ... 

Molten salt is a secondary barrier to prevent radionuclide releases to the environment (fission products and actinides dissolved in salt)... 

The actinides are the most important nuclides to be investigated in a P T strategy from the point of view of potential hazards (source term without any barriers) however, some long lived mobile fission products (such as I, Tc) and actinides such as neptunium represent the main residual hazard (taking into account engineered and geological barriers) over a long term period of time. 

When reprocessing SNF, it is assumed that the extracted fission products first are vitrified and then, after necessary cooling, are enclosed in special containers providing a multi-barrier shielding and transported to be finally disposed in deep geological formations. Minor actinides (except for curium) are not separated from plutonium and are used in the reactor as a fuel component. Curium is extracted and transported to the temporary repository for 100-150-year cooling. Upon being cooled, all curium isotopes (except for curium-245) are transformed into plutonium isotopes. Then this isotopic mixture is used to produce new fuel for the reactor. 

Fuel transportation within the reactor mono-block with solidified lead bismuth coolant creates an additional technical barrier to prevent fuel theft. Solidified lead bismuth coolant in the reactor mono-block also eliminates the risks of nuclear and radiation accidents in transportation. It may be expedient to concentrate SNF reprocessing at certain factories. In this, technological support of the non-proliferation regime may be provided through the application of the process of SNF reprocessing, in which 2% of fission products and all minor actinides remain in the re-fabricated fuel. The accounting and control of such fuel is simplified, because its handling requires special equipment. 

The release of a significant fraction of the stored radioactivity requires the breaching of multiple barriers. In the first place, most of the fission products and actinides are embedded in the fuel pellet matrix, and large-scale escape would only occur if the fuel were to melt. The fuel is enclosed in zircaloy cladding and the core is enclosed within the pressure vessel which forms part of a sealed primary circuit. Finally, the whole of the primary arcuit is enclosed within a containment structure which is specifically designed to minimize release of radioactivity to the environment. 

The microscopic stabilization of a Z = 114 nucleus results in a spherical nucleus that is more strongly bound than predicted by the macroscopic model. This effect produces a barrier to deformations leading to fission where there would otherwise be none [10, 12, 13, 23, 25-27]. At the time of these model calculations, the Periodic Table ended at the extreme limit of the actinides (Z = 103), with some experimental evidence for observation of the first transactinide elements. The overall trend with increasing atomic number was shorter half-lives and decreasing resistance to decay by spontaneous fission. The shell-model calculations indicated that well beyond the limits of the known elements the trends might reverse, allowing an extension of the Periodic Table [9]. This led to the concept of an Island of Stability . The term superheavy elements was coined to describe the nuclides occupying the Island. 

Evaporation residues arising in complete-fusion reactions between actinide targets and radioactive-beam particles are controlled by the same < r /Ff > and dynamical hindrance effects as are the reaction products from stable-ion beam irradiations. It has been observed that fusion cross sections for reactions with neutron-rich radioactive beam particles can be enhanced over those with stable-isotope beams at the same Z, possibly due to an effective lowering of the fusion barrier with the increasing neutron number of the projectile facilitated by neutron flow in the dinuclear reaction intermediate [226, 454, 458]. It is unclear how dynamical hindrance effects and a reduced resistance to deexcitation by fission at high excitation energies in heavier systems will influence the formation of evaporation residues. It has been suggested that the formation of products at the... 

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