In recent years there has been a renewed global interest in electricity production from nuclear power and several countries are announcing plans for new reactors construction,25
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in order to reduce the consumption of fossil fuels and the associated greenhouse gas emissions.Some studies have shown that nuclear energy can indeed be considered as a clean energy supply and it has been predicted that if nuclear energy became the prime source of energy supply, then the carbon dioxide emission could be significantly reduced.26 However, concerns regarding the safety of nuclear power plants and the
radioactive waste produced after nuclear fission processes are still far from being resolved.10, 27Nuclear waste is usually divided into categories. In UK, for example, there
are three types of nuclear fission waste: Low-level Waste (LLW) in the form of contaminated material,27 Intermediate-Level Waste (ILW) and the more serious High- Level Waste (HLW). This latter includes Spent Nuclear Fuels (SNF), Transuranic Waste (TRUW) and by-products of nuclear reprocessing, which comprise different isotopes of highly radioactive actinides, such as neptunium-237 and plutonium-239 with very long half-lives.27 Table 1.1 lists the known isotopes of the actinide elements, from Ac to No,
and the half-life of the most long-lived isotope.
Table 1.1. Masses of existing isotopes of actinides, from Ac to No, and half-life of the most long- lived isotope (shown in year for Ac – Fm, m: million and b: billion).5
Early Actinides Metal Ac Th Pa U Np Pu Am Isotopes 227-228 225, 232, 234-226- 235 231, 233-236 240 232- 240 237- 242, 244 238-240, 245 241- Longest- lived Isotope 227 232 232 238 237 244 243 Half- life/year 21.8 14 b 32500 4.47 b 2.14 m 80.8 m 7370 Later Actinides Metal Cm Bk Cf Es Fm Md No Isotopes 249 242- 250 249- 253 249- 252 257 258 259 Longest- lived Isotope 247 247 251 252 257 258 259 Half-
life/year 15.6 m 1400 900 1.29 100.5 52 days min 58
After neutron irradiation, the mass of the Spent Nuclear Fuel is mostly consisted by 95%
238U, 1% 235U, 1-2% Pu isotopes, 2-3% radioactive fission products and less than 0.1%
other trans-uranic elements.9 Handling the SNF is a complex issue and the waste can either
5
Currently, in United States, nuclear waste is disposed in cooling pools of nitric acid and
in dry storage casks at nuclear power plants,28 with the most famous nuclear waste repository being at Yucca mountain in Nevada. In 1987, the site was designated as a deep geological repository storage for SNF and other high level radioactive waste in the United States.29 However, the funding decreased significantly in 2011 and the project started to
be highly contested by the non-local public, the Western Shoshone peoples, and many politicians.30 In 2010, the U.S. government had also found an alternative deep geological
repository in New Mexico, known as Waste Isolation Pilot Plant (WIPP);31 however, on
February 5th 2014, an incident caused the leakage of airborne radiation consisting of
americium and plutonium particles and 21 people were exposed.32 This event raised the
question of whether or not WIPP could be a safe replacement for the Yucca Mountain
nuclear waste repository, as a destination for all the waste generated at U.S. commercial nuclear power plants.
An alternative strategy to nuclear waste repositories is to reprocess and re-use the fissile material to ultimately reduce both the volume and the activity of high level radioactive nuclear waste. In UK, for example, for several years this process had taken place at the Sellafield site with the new waste, generated after the reprocessing, being finally vitrified and sealed in stainless steel containers for dry storage.33 However, the current policy of
the UK government is to manage the nuclear waste through geological disposal.34
Reprocessing can theoretically recover up to 95% of the fissile material in Spent Nuclear Fuel.35 However, firstly, the fissionable material needs to be isolated from the SNF and
therefore several separation processes have to be performed. One example is the PUREX solvent extraction36 (Plutonium and Uranium Reduction EXtraction) which uses tri-n- butyl phosphate (TBP) as extracting ligand to extract uranium and plutonium (and with
slight modifications neptunium also) from SNF, so that they can be recycled as MOX
(mixed oxide) fuels. After the PUREX separation, the remaining waste of a highly radioactive raffinate contains over 99.9% of the fission product including lanthanide isotopes, 137Cs, 90Sr, 99Tc and the minor actinides 237Np, 243Am and 247Cm.37 This must
then be further divided to finally recycle the spent nuclear fuel as proposed in the
“Partitioning and Transmutation”(PT) strategy:38 the actinides have to be separated from
the lanthanide fission products and then neutron irradiation can be executed. The
separation is crucial, because the lanthanides have a high neutron absorption cross-section so that they can effectively compete with the actinides during neutron irradiation.38
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similarities between lanthanides and actinides.38 This is particularly evident with some
minor actinides such as Am and Cm; they tend to resemble the lanthanides, with their 5f
orbitals dropping in energy and becoming more core-like to a greater extent compared to the early actinides. This different chemical behaviour within the actinide series will be illustrated in more detail in section 1.3.
Despite several difficulties, extraction processes have already been established for a
correct separation of the material in the PUREX raffinate and some are listed in Scheme
1.1. The UREX39 (URanium EXtraction) method for the extraction of the remaining
uranium and technetium from the nitric acid solution of the dissolved PUREX raffinate,
the UNEX40 procedure (UNiversal EXtraction) for the simultaneous separation of
caesium, strontium and some actinides from radioactive acidic raffinate solutions, and
finally the SANEX39e,f (Selective ActiNide EXtraction), the TRUEX41 (TRansUranic
EXtraction) and the DIAMEX42(DIAMide Extraction) processes that help to separate the
trivalent actinides (americium and curium) and lanthanides from high acidity PUREX raffinate solutions.
Scheme 1.1. Schematic diagram showing the separation processes of SNF.
The most successful techniques adopted by the above methods are centred on liquid- liquid separations, taking advantage of selective binding of an actinide and/or lanthanide to specific ligands. For example, different N-heterocyclic chelating ligands such as the 6,6′-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2′-bipyridines (BTBPs) (with an example reported in Figure 1.1(a)), the 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen) (Figure 1.1(b)) and the 2,6-bis(5,6-di-n-propyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP, R1) or 2,6-bis(5,6-di-iso-propyl-1,2,4-triazin-3-yl)pyridine (iPr-BTP, R2) (Figure 1.1, (c)),
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have demonstrated extraction abilities towards Am(III) and Cm(III) with high selectivity over Ln(III) from nitric acid solutions.43
Figure 1.1. Examples of N-heterocyclic ligands for extraction of radioactive minor actinides from
lanthanides in SNF and corresponding separation factors for separation between Am(III) and
Eu(III) from nitric acid solution, (a) example of a 6,6′-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2′-
bipyridine (CyMe4-BTBP), (b) 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen);43a (c) 2,6-bis(5,6-di-n-propyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP, R1) and 2,6-bis(5,6-di-iso-propyl- 1,2,4-triazin-3-yl)pyridine (iPr-BTP, R2).43
The above N-donor ligands show high positive values for the separation factors
SFAm(III)/Eu(III) for Am(III) and Eu(III). These are given by the ratios between the
distribution ratios DAm(III)/Eu(III)for Am(III) and Eu(III), where a distribution ratio (D) is
defined as the ratio of the solute’s (i.e., the metal ion extracted) concentration in the organic phase over its concentration in the acidic aqueous phase after extraction (DM =
[M]org/[M]aq).
The higher affinity of these ligands towards An(III) ions over Ln(III) ions can be explained considering that the slightly higher charge density of the lanthanides makes them a little harder compared to actinides; thus, soft nitrogen donor ligands will preferentially coordinate to trivalent actinides over trivalent lanthanides.43 This selectivity
8
may also be traced to the fact that An-ligand bonds have a covalent character generally greater compared to the principally ionic Ln-ligand bonds.44
In turn, the higher covalency in An-ligand bonds, especially for the early actinides, is partially explainable considering the larger radial extension of the 5f orbitals of the actinides compared to the more contracted 4f orbitals of the lanthanides. Thus, there is possibility of an orbital overlap between the 5f orbitals of the actinides and the π* molecular orbitals of these N-heterocyclic ligands.43 However, recent computational
studies raised some doubts about the origin of this perceived covalency. For example, it has been suggested that the greater affinity of the BTBPs ligands for trivalent actinides over trivalent lanthanides, particularly for the isoelectronic pair Am(III)/Eu(III), can be explained considering a coincidental energy match between ligand and actinide orbitals. In this case, the covalency in An-ligand bonds is driven by “orbital energy degeneracy”
rather than “spatial orbital overlap”.45 The covalency in actinide complexes will be
discussed in more detail in section 1.3.
In conclusion, the nature of the interactions between the extractant ligands and the 4f lanthanide and 5f actinide ions represents an attractive and challenging subject, and its understanding is crucial to improve the efficiency of the PT strategy.38