Sorption of U(VI) on clay and related minerals has been inspected experimentally and computationally because of its central role in safety considerations of geological repositories for highly radioactive waste. Np(V), which also has long half-life isotopes, has received considerably less attention. The purpose of the present study was to investigate computationally the adsorption of Np(V) on a clay-mineral surface and to compare it to adsorption of U(VI). As a sample case study, adsorption of Np(V) at the (110) edge surface of the common clay mineral montmorillonite was modeled. The density functional approach and periodic supercell models were applied. Mono- and bidentate adsorption complexes with coordination numbers 4 and 5 were inspected and compared to corresponding U(VI) species. While U(VI) prefers bidentate adsorption complexes with varying coordination numbers, Np(V) is more stable when monodentate-coordinated with a coordination number of four. In line with its smaller hydrolysis constant in aqueous solution, Np(V) shows a lower tendency to form monohydroxides on the mineral surface compared to U(VI). As no experimental geometry parameters are available for Np(V) adsorbed on montmorillonite, the results were compared tentatively to EXAFS data for adsorption at kaolinite and good agreement for the geometry changes due to adsorption was found for the more preferred adsorbed species.

New data on microstructure of 16 years old (La, Pu)PO4 monazite ceramics doped with 8.1 wt% of 238Pu are presented. It is shown that the sample consists from at least two phases differing in La/Pu ratio and small precipitates of Pu-phosphate. Possible mechanisms of the compositional heterogeneity are discussed. Formation of Pu-containing rhabdophane after sample storage in air is observed. This phenomenon together with gradual mechanical destruction of the ceramic pellet formation of submicron particles will likely increase rate of radionuclides loss from the monazite-based waste form. X-ray emission lines produced by recoil uranium ions from Pu decay are analysed. It is suggested that careful examination of their relative intensities may provide important information about behaviour of "hot" recoils in nuclear waste forms.

Computational modeling is an important aspect of the research on nuclear waste materials. In particular, atomistic simulations, when used complementary to experimental efforts, contribute to the scientific basis of safety case for nuclear waste repositories. Here we discuss the state-of-the-art and perspectives of atomistic modeling for nuclear waste management on a few cases of successful synergy of atomistic simulations and experiments. In particular, we discuss here: (1) the potential of atomistic simulations to investigate the uranium oxidation state in mixed-valence uranium oxides and (2) the ability of cementitious barrier materials to retain radionuclides such as 226Ra and 90Sr, and of studtite/metastudtite secondary peroxide phases to incorporate actinides such as Np and Am. The new contribution we make here is the computation of the incorporation of Sr by C-S-H (calcium silicate hydrate) phases.

The transport and deposition of uranium under hydrothermal conditions in the Earth’s crust has been a subject of ongoing study but is yet to be completely understood. In addition, there is little known about the fate of nuclear waste, consisting of uranium from spent fuel and other radioactive materials, upon storage in repositories or in nuclear reactor facilities. Because the nuclear waste often comes in contact with aqueous fluids in storage environments, studies of uranyl complexation with chloride and other ligands in aqueous media, to high temperature and pressure conditions, are needed. The primary purpose of this study was to investigate the speciation of aqueous uranyl (VI) chloride complexes, in solutions having a 0.05 M uranyl concentration and [Cl] concentrations ranging from 0.2 M to 6 M, under hydrothermal conditions. The aqueous uranyl chloride complexes in the samples were studied using Raman spectroscopy and the hydrothermal diamond anvil cell (HDAC), at temperatures up to 500 °C and pressures up to ~ 0.5 GPa. The uranyl bond stretching band feature occurring in the ~810 to 870 cm-1 region was fitted using the Voigt peak shape to determine the speciation of the equilibrium uranyl chloride complexes present in the samples. As expected, the n integer value of the UO2Cln+2-n complex species increases with the increase in temperature and chloride concentration, generally trending toward charge neutrality at high temperatures.

X-ray Absorption Spectroscopy is a technique of fundamental importance in nuclear waste management, as an element specific probe of speciation, which governs radionuclide solubility, immobilisation and migration. Here, we exploit recent developments in laboratory instrumentation for X-ray Absorption Spectroscopy, based on a Rowland circle geometry with a spherically bent crystal analyser, to demonstrate speciation in prototype ceramic and glass-ceramic waste forms. Laboratory and synchrotron XANES data acquired from the same materials, at the Ce and U L3 edges, were found to be in excellent quantitative agreement. We establish that analysable laboratory XANES data may be acquired, and interpreted for speciation, even from quite dilute absorber concentrations of a few mol%, albeit with data acquisition times of several hours. For materials with suitable absorber concentrations, this approach will enable routine element specific speciation studies to support rapid optimisation of radioactive waste forms and analysis of radiological materials in a purpose designed laboratory, without the risk associated with transport and manipulation at a synchrotron radiation facility.

The addition of Cr2O3 to modern UO2 fuel modifies the microstructure so that, through the generation of larger grains during fission, a higher proportion of fission gases can be accommodated. This reduces the pellet-cladding mechanical interaction of the fuel rods, allowing the fuels to be “burned” for longer than traditional UO2 fuel, thus maximising the energy obtained. We here describe the preparation of UO2 and Cr-doped UO2 using Hot Isostatic Pressing (HIP), as a potential method for fuel fabrication, and for development of analogue materials for spent nuclear fuel research. Characterization of the synthesised materials confirmed that high density UO2 was successfully formed, and that Cr was present as particles at grain boundaries and also within the UO2 matrix, possibly in a reduced form due to the processing conditions. In contrast to studies of Cr-doped UO2 synthesised by other methods, no significant changes to the grain size were observed in the presence of Cr.

A range of stoichiometric and mixed A-site cation brannerite glass-ceramics have been synthesised and characterised. The formation of UTi2O6 in glass is reliant on ensuring all uranium remains tetravalent by processing in an inert atmosphere. ThTi2O6 forms in glass under both inert and oxidising atmospheres due to the lack of other easily available oxidation states. CeTi2O6 could not be made to form within this glass system. The formation of A0.5B0.5Ti2O6 phases depends strongly on the oxidation states of the A and B cations available in the process atmosphere, with the most successful compositions having an average final oxidation state of (A,B)4+. Mixed cation brannerite compositions that formed in argon include U0.75Th0.25Ti2O6 and U0.71Ce0.29Ti2O6. Those forming in air include U0.23Th0.77Ti2O6, Th0.37Ce0.63Ti2O6, and U0.41Ce0.59Ti2O6.

A zirconolite glass-ceramic material is a candidate wasteform for immobilisation of chlorine contaminated plutonium residues, in which plutonium and chlorine are partitioned to the zirconolite and aluminosilicate glass phase, respectively. A preliminary investigation of chlorine speciation was undertaken by analysis of Cl K-edge X-ray Absorption Near Edge Spectroscopy (XANES), to understand the incorporation mechanism. Cl was found to be speciated as the Cl- anion within the glass phase, according to the characteristic chemical shift of the X-ray absorption edge. By comparison with Cl K-edge XANES data acquired from reference compounds, the local environment of the Cl- anion is most closely approximated by the mineral marialite, in which Cl is co-ordinate to 4 x Na and/or Ca atoms.

Microbial metabolism has the potential to control the biogeochemistry of redox-active radionuclides in a range of geodisposal scenarios. In this study, sediments from a high pH lime workings site were incubated under carefully controlled anaerobic conditions, at a range of alkali pH values with added electron donors and electron acceptors, to explore the limits and rates of bioreduction in a sediment system analogous to intermediate-level nuclear waste. There was a clear succession in the utilization of electron acceptors (in the order nitrate > Fe(III)-citrate > Fe(III) oxyhydroxide > sulfate), in accordance with calculated free energy yields and Eh values over the pH range 10–12. The rate and extent of bioreduction decreased at higher pH, with an upper limit for the processes studied at pH 12. The biochemical limits for such processes are discussed, alongside the potential impact of such forms of microbial metabolism on the solubility of a range of redox active radionuclides that will feature heavily in the safety case for the geological disposal of intermediate-level nuclear waste.

Previously, it has been suggested that metal ions complexed to humic acid in the environment might show slower dissociation than those added to humic substances in the laboratory, which has serious implications for the transport of radionuclides in the environment. The dissociation of lanthanide and anthropogenic actinide ions from humic substance complexes has been studied as a function of humic concentration and metal ion:humic concentration ratio. The results suggest that the apparently slower kinetics observed for metal ions complexed in the environment are probably due to the large humic concentrations that are used in those studies. Further, there is no evidence that the dissociation rate constant varies at very low metal ion concentrations. Although humic samples size-fractionated by ultrafiltration showed that more metal may be bound non-exchangeably, there was no evidence for different rate constants. Ultrafiltration of Eu(III)/humic acid mixtures did show a shift in Eu from smaller to larger fractions over a period of two days. Therefore, the results suggest that dissociation rate constants determined in the laboratory at metal ion concentrations higher than those expected in the environment may be used in predicting radionuclide mobility, provided that the humic acid concentration is in the range expected at the site.

A brief review of the status of our understanding of the occupation of the Pu 5f states is provided herein, with an emphasis upon experimental measurements.

Cladding tubes of water-cooled nuclear reactors are usually made of Zircaloy and are an important retaining element for radionuclides present in the fuel both during predisposal activities such as reloading of fuel assemblies from interim storage casks to final disposal casks and during final disposal in the case of canister breaching. However, cladding integrity is affected by various processes during reactor operation and beyond, e.g. fuel cladding chemical interaction and fission product precipitation onto the inner cladding surface. Using experimental and modelling methods, the radionuclide inventory of an irradiated Zircaloy-4 plenum section is analyzed. Quantities of 235/238U, 237Np, 238/239/240/241/242Pu, 241/243Am, 243/244Cm besides 14C, 55Fe, 125Sb, 154Eu, and 134/137Cs were (radio-)chemically determined in digested Zircaloy-4 subsamples. Measured inventories of activation products in the Zr-alloy are in good agreement with calculated values. However, amounts of actinides and fission products exceed the calculated inventory by factor ∼57 (minor actinides and non-volatile fission products) and ∼114 (137Cs). Excess of minor actinides and part of enhanced Cs inventory originate from fuel residues deposited on the inner cladding surface during fuel rod fabrication, whereas vast amount of cesium is volatilized from subjacent fuel pellets and transported to the plenum.

We investigate the origin of satellite features that appear in the high-resolution x-ray absorption spectra measured at the uranium M4 edge in compounds where the uranium atoms are in the U6+ oxidation state. We employ a material-specific Anderson impurity model derived from the electronic structure obtained by the density-functional theory.

Nagelschmidtite, Ca7P2Si2O16, is an end-member of continuous solid solution Ca2SiO4 – Ca3(PO4)2⋅2Ca2SiO4 within the pseudo-binary system Ca3(PO4)2 – Ca2SiO4 (whitlockite – larnite). This phase is capable to wide isomorphic exchanges in Ca, P and Si sites: Ca2+ = Sr2+; Ca2+ = Eu2+; Ca2+ + P5+ = (RE,An)3+ + Si4+, 2Ca2+ = Na+ + (RE,An)3+; 2Ca2+ = An4+ + ☐; Ca2+ + Si4+ = (RE,An)3+ + (Al,Fe)3+; Ca2+ + Si4+ = Na+ + P5+; 2Ca2+ = Na+ + (Al,Fe)3+; Ca2+ + P5+ = Na+ + S6+. It was found in metallurgical slags and geological formations. We revealed nagelschmidtite-type phase in vitrified phosphorus-bearing radioactive incinerator slags. The materials were glass-crystalline and contained nano-sized nagelschmidtite crystals distributed in vitreous matrix phase. Average chemical composition of the largest (few microns) crystals was recalculated to formula Na1.21K1.05Ca2.22Al2.02Fe0.46Si2.69P1.26U0.08O15.76. Significant oxygen misbalance suggests higher than U(IV) oxidation state for uranium – U(V) or U(VI). Capability of nagelschmidtite to be crystallized from melt makes it promising phase for actinides, rare earths and some other fission and corrosion products at using a melting route to nuclear waste forms including cold crucible induction melting and self-propagating high-temperature synthesis.

U60 ([UO2(O2)(OH)]60 60− in water) is a uranyl peroxide nanocluster with a fullerene topology and O h symmetry. U60 clusters can exist in crystalline solids or in liquids; however, little is known of their behavior at high pressures. We compressed the U60-bearing material: Li68K12(OH)20[UO2(O2)(OH)]60(H2O)310 ( $Fm\bar 3$ ; a = 37.884 Å) in a diamond anvil cell to determine its response to increasing pressure. Three length scales and corresponding structural features contribute to the compression response: uranyl peroxide bonds (<0.5 nm), isolated single nanoclusters (2.5 nm), and the long-range periodicity of nanoclusters within the solid (>3.7 nm). Li68K12(OH)20[UO2(O2)(OH)]60(H2O)310 transformed to a tetragonal structure below 2 GPa and irreversibly amorphized between 9.6 and 13 GPa. The bulk modulus of the tetragonal U60-bearing material was 25 ± 2 GPa. The pressure-induced amorphous phase contained intact U60 clusters, which were preserved beyond the loss of long-range periodicity. The persistence of U60 clusters at high pressure may have been enhanced by the interaction between U60 nanoclusters and the alcohol pressure medium. Once formed, U60 nanoclusters persist regardless of their associated long-range ordering—in crystals, amorphous solids, or solutions.

Borate is present in natural groundwaters and borate is also released into groundwaters when borosilicate glass, waste form for high level nuclear waste, is corroded. Borate can form an aqueous complex, AmHB4O72+, with actinides in +III oxidation state. In this work, we present our evaluation of the equilibrium constant for formation of AmHB4O72+ and the associated Pitzer interaction parameters at 25°C.

Using Nd(III) as an analog to Am(III), solubility data of Nd(OH)3(s) in NaCl solutions in the presence of borate ion from the literature, is used to determine Am(III) interactions with borate. The log10K for the formation reaction is 37.34. This evaluation is in accordance with the Waste Isolation Pilot Plant (WIPP) thermodynamic model in which the borate species include B(OH)3(aq), B(OH)4–, B3O3(OH)4–, B4O5(OH)42–, and NaB(OH)4(aq). The WIPP thermodynamic database uses the Pitzer model to calculate activity coefficients of aqueous species.

In addition, the equilibrium constant for dissolution of AmB9O13(OH)4(cr) at 25oC is evaluated from the solubility data on NdB9O13(OH)4(cr) in NaCl solutions, again using Nd(III) as an analog to Am(III). The log10K for the dissolution reaction is –79.30. In the evaluation for log10K for the dissolution reaction, AmHB4O72+ is also considered.

The equilibrium constant and Pitzer parameters evaluated by this study will be important to describe the chemical behavior of Am(III) in the presence of borate in geological repositories.

Because of steady increase in the availability of computing power, ab initio methods of computational materials science become everyday investigation tools in various research fields. This popularity of the first-principle-based atomistic modeling is in large part due to the performance of density functional theory (DFT), which could be used for simulations of chemically and structurally complex materials, including minerals, fluids and melts. However, because of intrinsic approximations, DFT is not always able to deliver reliable predictions. This is especially pronounced for f-elements bearing materials such as nuclear materials considered in nuclear waste management. Properties such as reaction enthalpies or electronic state are often badly predicted. In this contribution we discuss our experience with different computational methods, including the parameter free DFT+U method, in which the Hubbard U parameter is derived ab initio, for prediction of various properties of f electrons bearing materials. We show significant improvement obtained for structural and thermochemical parameters of various lanthanide-bearing ceramic materials and actinide-bearing molecular and solid compounds when the f electrons correlations are explicitly accounted for. Last, but not least, we demonstrate that complementary experimental and atomistic modeling studies result in superior and more complete characterization of challenging materials considered in nuclear waste management.

Recent nuclear forensics cases have focused primarily on plutonium (Pu) and uranium (U) materials. By definition however, nuclear forensics can apply to any diverted nuclear material. This includes neptunium (Np), an internationally safeguarded material like Pu and U, that could offer a nuclear security concern if significant quantities were found outside of regulatory control. This case study couples scanning electron microscopy (SEM) with quantitative analysis using newly developed specialized software, to evaluate a non-traditional nuclear forensic sample of Np. The results of the morphological analyses were compared with another Np sample of known pedigree, as well as other traditional actinide materials in order to determine potential processing and point-of-origin.

Glass-ceramics were developed initially for the immobilization of miscellaneous Pu-residues at the UK’s Sellafield site from which it was uneconomic to recover Pu for reuse. Renewed interest in the immobilization of a portion of the UK PuO2 stockpile has led to glass-ceramics being evaluated for bulk Pu immobilization. The Nuclear Decommissioning Authority (NDA) in the UK have proposed hot isostatic pressing (HIP) as a potential consolidation technique for the processing of these wasteforms. In this study, zirconolite based glass-ceramics were investigated to determine an optimum formulation. The yield of zirconolite is shown to vary with glass composition and glass fraction, such that a higher Al content favours zirconolite formation. The sample preparation process is discussed to highlight the importance of a high temperature heat-treatment during sample preparation to achieve high quality HIPed wasteforms.

We have synthesized and investigated electronic properties of several non-centrosymmetric actinide compounds, which do not have an inversion center in the crystal structure “globally” or “locally”, under high pressure. The Néel temperature of an antiferromagnet UIrSi3 with “globally” non-centrosymmetric structure increases with increasing pressure at a rate of 2.5 K/GPa up to 5 GPa. On the other hand, T Ns of U2Rh3Si5 and U2Ir3Si5, which are “locally” non-centrosymmetric compounds, decrease with -1 K/GPa and -0.5 K/GPa with increasing pressure, respectively. Here, U2Ir3Si5 is a new antiferromagnet crystallizing in the U2Co3Si5-type of orthorhombic structure. Below T N = 36.5 K, U2Ir3Si5 shows magnetic order-order transition at T 0 = 26.1 K with a first-order nature. Electrical resistivity in U2Ir3Si5 shows semiconducting-like behavior due to the formation of the super-zone gap in the antiferromagnetic state. T N and T 0 as well as semi-conducting-like behavior in resistivity are suppressed by external pressure.