The electronic structure of Pu chalcogenides is investigated making use of static around-mean-field LDA+U and dynamical LDA+HIA (Hubbard-I) methods. The LDA+U calculations provide correct non-magnetic ground state for PuX (X = S, Se, Te) with 5f-manifold non-integer filling (∼5.6(PuS)-5.7(PuTe)). This is an indication of a mixed valence ground state which is a combination of f5 and f6 multiplets. The photoelectron spectra are calculated in good agreement with experimental data. The 5f-manifold three-peaks feature near EF is well reproduced by LDA+HIA, and follows from mixed valence character of the ground state.

Sweden plans to dispose of spent nuclear reactor fuel in a deep geologic repository in granitic rock. The conditions in the repository in the long term will be reducing and water is not expected to contact the fuel until after more than 1000 years. At that time, most of the beta- and gamma-active nuclides will have decayed away and the radiation will be dominated by alpha decay. In order to simulate the radiolysis field for dissolution of spent fuel with age more than 1000 years we have used uranium dioxide containing 5% U-235 and 0, 5, or 10% U-233. The 10% U-233 gives an alpha activity appropriate to about 3000 years after disposal. Samples were testied in a synthetic groundwater with low ionic strength and with the chemical composition dominated by sodium bicarbonate and calcium chloride. Tests were run in triplicate using an atmosphere of nitrogen (1atm), hydrogen (10 bar), hydrogen (10 bar) plus an iron strip in the solution, nitrogen (1 atm) plus an iron strip in the solution, hydrogen (10 bar) plus an iron strip in the solution, hydrogen (10 bar) without the iron strip. Each of these test conditions was run for 2 consecutive periods of at least 21 days. The results showed that the dissolution behavior of the samples was the same for both nitrogen atmosphere and hydrogen atmosphere. The amount of U dissolved under these conditions clearly showed the enhancement of dissolution due to oxidation of the sample surface by radiolysis products. When an iron strip was added to the solution, the amount of dissolution decreased dramatically indicating that the Fe(II) ions released from the corroding iron were able to react with most of the radiolysis products before they could oxidize the uranium dioxide surface.

In most deep disposal concepts, large amounts of hydrogen are expected to be produced by the anoxic corrosion of massive iron containers. At repository temperatures, hydrogen is quite inert and is not expected to contribute to the redox capacity of the deep groundwaters. In several recent works, a large impact of dissolved hydrogen on the dissolution of the LWR or MOX fuel and UO2(s) doped with 233U or 238Pu has been observed. For hydrogen concentrations above a certain limit, the dissolution rates of these highly radioactive materials drop to very low values. A discussion of the results obtained with spent fuel or α-doped UO2 in the presence of a range of hydrogen concentrations is presented. Typical for all measurements under such conditions are the very low long term concentrations of uranium and other redox-sensitive radionuclides, such as Tc and the minor actinides. The concentrations of U are systematically lower than the values measured during UO2(s) solubility measurements carried out in the presence of strong reducing agents. Measurements of the radiolytic oxygen after long leaching periods result in values below detection limit. The investigation of the surface of spent fuel or UO2(s) pellets doped with 233U by XPS after long periods of testing shows absence of oxidation. The kinetics of the release of non-redox sensitive elements such as Sr and Cs, used to estimate fuel matrix dissolution rates, is also discussed. An attempt is made to propose potential mechanisms responsible for the observed behaviour, based mainly on data from studies on the interaction of water adsorbed on the surfaces of metal oxides or actinide oxides with radiation. Another important effect observed in recent studies is the existence of a threshold for the specific alpha activity below which no measurable influence of the alpha radiolysis on the uranium release from UO2 is observed. The importance of such a threshold for the behaviour of spent fuel under repository conditions encompassing very long time scales will be discussed, as well as the necessity to better investigate the mechanisms of recombination reactions in a thin water layer on the surface of actinide oxides affected by α-radiolysis.

The radiochemical decomposition of molecules in storage environment which could lead to the corrosion of container or the formation of dangerous gas mixtures is a critical problem for radioactive materials. The complexity of the chemical system makes numerical models suitable for the reproduction mechanisms and the prediction of phenomena. In this study, a mathematical model for the dose rate distribution in external medium surrounding an α emitter actinide material has been proposed. The model has been implemented in a Monte Carlo scheme. An evaluation of the dose rate in the surrounding medium as a function of the sample size was shown and a discussion of the expected reactivity was made.

Expected to provide a clue about the origin of zero moment in the bulk phase of Plutonium, Pu1-xAmx alloys have attracted a great attention, in which upon doping the system transforms from the Kondo lattice to the diluted impurity limit. To understand the electronic structure and the magnetic properties of Pu in different crystal environments, we performed fully self-consistent first-principles calculations of the PuAm system based on the local density approximation (LDA) combined with static (LDA+U) and dynamic corrections (LDA+DMFT) for on-site electron correlations. The electronic structure strongly depends on the level of approximation for correlation effects. The exchange interactions between Pu 5f electrons and the Kondo screening strength were estimated and compared, which provide a new insight to Pu magnetism.

The behavior of plutonium (Pu) oxides in the presence of water/moisture in a confined space and the associated issues of hydrogen and oxygen generation due to radiolysis have important implications for the storage and transportation of Pu-bearing materials. This paper reviews the results of recent studies of gas generation in the Pu-O-H system, including the determination of release rates via engineering-scale measurement. The observations of the significant differences in gas generation behavior between “pure” Pu-bearing materials and those that contain salt impurities are addressed. In conjunction with the discussion of these empirical observations, the work also addresses recent scientific advances in the investigations of the Pu-O-H system using state-of-the-art ab initio electronic structure calculations, as well as advanced synchrotron techniques to determine the electronic structure of the various Pu-containing phases. The role of oxidizing species such as the hydroxyl radical from the radiolysis of water is examined. Discussed also is the challenge in the predictive ab-initio calculations of the electronic structure of the Pu-H-O system, due to the nature of the 5f valence electrons in Pu. Coupled with the continuing material surveillance program, it is anticipated that this work may help determine the electronic structure of the various Pu-containing phases and the role of impurity salts on gas generation and the long-term stability of oxygen/hydrogen-containing plutonium oxides beyond PuO2.

Photoelectron Spectroscopy (PES) [1] and X-ray Absorption Spectroscopy (XAS, Figure 1) [2-4] have contributed greatly to our improved understanding of Pu electronic structure. From these and related measurements, the following has been determined.

By combining the local density approximation (LDA) with dynamical mean field theory (DMFT), we report a systematic analysis of the spectral properties of δ-plutonium with varying 5f occupancy. The LDA Hamiltonian is extracted from a tight-binding (TB) fit to full-potential linearized augmented plane-wave (FP-LAPW) calculations. The DMFT equations are solved by the exact quantum Monte Carlo (QMC) method and the Hubbard-I approximation. We show the strong sensitivity of the spectral properties to the 5f occupancy, which suggests using this occupancy as a fitting parameter in addition to the Hubbard U. By comparing with PES data, we conclude that the “open shell” 5f5 configuration gives the best agreement, resolving the controversy over 5f “open shell” versus “close shell” atomic configurations in δ-Pu.

Measurements on URu2-xRexSi2 single crystals indicate that substitution of Re for Ru in URu2Si2 reduces the transition temperature of the hidden order state and quickly destroys superconductivity. At intermediate Re concentrations, weak ferromagnetism emerges and non Fermi liquid (NFL) behavior is observed in the low-temperature specific heat and electrical resistivity. A scaled Arrott analysis of the magnetization indicates the onset of ferromagnetism at x =0.15, where the hidden order disappears, and that the quantum phase transition is associated with novel critical exponents.

Recently we reported the observation of near-infrared photoluminescence from metal-centered 5f electronic excited states of PuO22+ doped into polycrystalline Cs2U(Pu)O2Cl4.[1] Photoluminescence dynamics following pulsed excitation show complicated decay patterns suggesting that multiple luminescent states are involved. Here we report the results of two recent sets of experiments showing that photoluminescence processes depend significantly on the energy of photoexcitation. In the first case, decay kinetics following excitation at a lower energy are missing an in-growth term that is present when exciting at higher energy. In the second case, we have observed that lower excitation energy produces significantly reduced number of emission transitions than higher excitation energy. Both observations suggest that higher energy excitation populates feeder states that decay to emitting states, causing signal from the latter to have an in-growth followed by a decay characteristic of their intrinsic lifetimes, whereas lower energy excitation leads to more direct population of luminescent states.

The gallium-stabilized Pu-2.0 at. % Ga alloy undergoes a partial or incomplete low-temperature martensitic transformation from the metastable δ phase to the gallium-containing, monoclinic α′ phase near -100 °C. This transformation has been shown to occur isothermally and it displays anomalous double-C kinetics in a time-temperature-transformation (TTT) diagram, where two nose temperatures anchoring an upper- and lower-C describe minima in the time for the initiation of transformation. The underlying mechanisms responsible for the double-C behavior are currently unresolved, although recent experiments suggest that a conditioning treatment—wherein, following an anneal at 375 °C, the sample is held at a sub-anneal temperature for a period of time—significantly influences the upper-C of the TTT diagram. As such, elucidating the effects of the conditioning treatment upon the δ⟶α′ transformation can provide valuable insights into the fundamental mechanisms governing the double-C kinetics of the transition. Following a high-temperature anneal, a differential scanning calorimeter (DSC) was used to establish an optimal conditioning curve that depicts the amount of α′ formed during the transformation as a function of conditioning temperature for a specified time. With the optimal conditioning curve as a baseline, the DSC was used to explore the circumstances under which the effects of the conditioning treatment were destroyed, resulting in little or no transformation.

Nowadays, nuclear energy is one of the options for developed countries in order to maintain the demand of electric energy. One of the problems of this kind of energy generation is the residual waste form after a fuel cycle (spent fuel). These kind of material is so difficult to characterize -due to their composition and the thermal treatment in the reactor- that exhaustive studies are necessaries for a complete knowledge, helping to build, with complete reliability, a very safety underground facility. In this way, the option known as Deep Geological Repository (DGR) is been developed by each country taking part in the nuclear energy industry. The unique via for the migration to the biosphere of the radionuclides -actinides and lanthanides content in the spent fuel pellet (UO2) after the closing of the deep geological repository is by the water transport phenomena. It is a fundamental question to know how much time they will spend in their trip; and the first step is the rate of liberation of these radionuclides from the spent fuel pellet. In this way, the matrix dissolution rate of the spent fuel pellet no dependent on the specific surface area after a normalization by the initial value- is a key parameter to begin the performance assessment for any deep geological repository and the specific surface value is, following the Matrix Alteration Model (MAM) sensitivity analysis, one of the most important parameters controlling the radionuclides liberation. In this way, several measurements were carried out to obtain values in different conditions for different sieves of UO2 powder, treated as fresh fuel. First of all, the specific surface area was measured with a multi-point isothermal procedure with N2 and Kr, the both. The values obtained were presented in order to obtain a general law for the evolution with the particle size. These data are part of a bigger project about the complete description of the spent fuel analogous; very useful to obtain new dissolution rates for the spent fuel under repository simulated conditions.

Defect reactions involving charged species are commonplace in nuclear fuels fabrication and burn-up. Even the simplest of these fuels, uranium dioxide (UO2), typically involves the nominal charge states of +3, +4, and +5 or +6 in U and -1 and -2 states in O. Simulations that attempt to model evolutionary processes in the fuels require tracking changes among these charge states. At the atomistic level, modeling defect reactions poses a particularly vexing problem. Typical potential energy surfaces do not have this type of physical phenomena built into them. Those models that do attempt to model charge-defect reactions do not have especially strong physical bases for the models. For instance, most do not obey established limits of charge behavior at dissociation or lack internal consistency. This work presents substantial generalizations to earlier work of Perdew et al. No matter the size of the system, total system hamiltonians can be decomposed into subsystem or site hamiltonians and coulombic interactions. Site hamiltonians can be evaluated in a spectral representation, once an integer number of electrons are assigned. For both pair and individual site hamiltonians a dilemma emerges in that many sites are better understood as possessing a fractional charge. The dilemma is how to weight the site integer-charge states in a physically consistent manner. One approach to solving the dilemma results in two distinct charge-dependent energy contributions emerge, arising from intra- and inter-subsystem charge transfer. Further analysis results in a model of the intra-subsystem charge-transfer that can accommodate the mixed valence states of either U or O in nuclear fuels. Mixed valence properties add complications to the model that originate in the phenomenological fact that it typically requires different amounts of energy to increase or decrease charge. As a result of the inherent complexity one has the option of using multiple charges, a concept with strong ties to shell models, or modeling parameters not directly related to charge as functions of charge. This latter approach is illustrated by invoking a minimization principle that does preserve the important dissociation limits of Perdew et al., in order to complete the model.

Dynamic nonlinear localization, an emerging new area of materials physics, has the potential to fundamentally change our understanding of materials properties. These intrinsically localized modes (ILM) have been found forming in uranium above about 450 K. Comparisons with data from the literature shows that these ILMs influence many properties, including heat capacity, thermal transport, thermal expansion, and mechanical deformation. The existence of ILMs helps to explain many anomalies in uranium, some of which have been known for more than fifty years. Here, the possibility that ILMs may also play a role in the properties of plutonium is considered. The mechanism helps to explain the success of an “Invar-like” two-level model for the thermal expansion of Pu without the need to invoke local magnetic states, which have been ruled out experimentally. It also helps to explain an excitation observed in δ-plutonium that has energy consistent with the two-level model, but momentum dependence consistent with lattice vibrations. This excitation, energy and momentum dependence, is consistent with the non-equilibrium generation of ILMs and the thermal excitation properties of these ILMs are consistent with a two-level model. High-temperature inelastic x-rays scattering measurements of the phonon dispersion curves of δ-plutonium are needed to test this hypothesis.

A theory is presented which describes the photoemission spectra of actinide compounds starting from the atomic limit of isolated actinide ions. The multiplets of the ion are calculated and an additional term is introduced to describe the interaction with the sea of conduction electrons. This leads to complex mixed-valent ground states, which describes well the rich spectrum observed for PuSe. In particular, the three-peak feature, which is often seen in Pu and Pu compounds in the vicinity of the Fermi level originates from f6 → f5 emission. The theory is further applied to PuSb, PuCoGa5 and Am.

With the current level of actinide materials used in civilian power generation and the need for safe and efficient methods for the chemical separation of these species from their daughter products and for long-term storage requirements, a detailed understanding of actinide chemistry is of great importance. Due to the unique bonding properties of the f-elements, the lanthanides are commonly used as structural and chemical models for the actinides, but differences in the bonding between these 4f and 5f elements has become a question of immediate applicability to separations technology. This brief overview of actinide coordination chemistry in the Raymond group at UC Berkeley/LBNL examines the validity of using lanthanide analogs as structural models for the actinides, with particular attention paid to single crystal X-ray diffraction structures. Although lanthanides are commonly accepted as reasonable analogs for the actinides, these comparisons suggest the careful study of actinide materials independent of their lanthanide analogs to be of utmost importance to present and future efforts in nuclear industries.

A new extractant for the separation of actinide(III) and lanthanide(III), bis(o-trifluoromethylphenyl)phosphinic acid (O-PA) was synthesized. The synthetic route employed mirrors one that was employed to produce the sulfur containing analog bis(o-trifluoromethylphenyl)dithiophosphinic acid (S-PA). Multinuclear NMR spectroscopy was used for elementary characterization of the new O-PA derivative. This new O-PA extractant was used to perform Am(III)/Eu(III) separations and the results were directly compared to those obtained in identical separation experiments using S-PA, an extractant that is known to exhibit separation factors of ∼100,000 at low pH. The separations data are presented and discussed in terms comparing the nature of the oxygen atom as a donor to that of the sulfur atom in extractants that are otherwise identical.

Very diverse Pu compounds exhibit strikingly universal features in their valence-band photoemission (PES) spectra. The conjecture that such features represent the 5f5 final state multiplet has been corroborated by LDA+Hubbard I calculations, meaning that the ground state has a mixed 5f5-5f6 character. Later on, more elaborated DMFT techniques (one crossing approximation, QMC) led to similar conclusions, providing quantitative explanation of such intermeate-valent situation in more details. Analogies in PES spectra of δ-Pu and other Pu systems suggest that the situation envisaged for δ-Pu is relevant for a large group of Pu compounds. Here we show that the around mean field LDA+U in conjunction with the Hubbard I approximation, which describes well the non-magnetic ground state for δ-Pu, captures in reality properties of a large group of Pu (as well as e.g. Am) compounds, reproducing correctly the onset of magnetism and size of magnetic moments.

A plutonia stabilised zirconia doped with yttria and erbia has been selected as inert matrix fuel (IMF) at PSI. The results of experimental irradiation tests on yttria-stabilised zirconia doped with plutonia and erbia pellets in the Halden research reactor as well as a study of zirconia solubility are presented. Zirconia must be stabilised by yttria to form a solid solution such as MAz(Y,Er)yPuxZr1-yO2-ζ where minor actinides (MA) oxides are also soluble. (Er,Y,Pu,Zr)O2-ζ (with Pu containing 5% Am) was successfully prepared at PSI and irradiated in the Halden reactor. Emphasis is given on the zirconia-IMF properties under in-pile irradiation, on the fuel material centre temperatures and on the fission gas release. The retention of fission products in zirconia may be stronger at similar temperature, compared to UO2. The outstanding behaviour of plutonia-zirconia inert matrix fuel is compared to the classical (U,Pu)O2 fuels. The properties of the spent fuel pellets are presented focusing on the once through strategy. For this strategy, low solubility of the inert matrix is required for geological disposal. This parameter was studied in detail for a range of solutions corresponding to groundwater under near field conditions. Under these conditions the IMF solubility is about 109 times smaller than glass, several orders of magnitude lower than UO2 in oxidising conditions (Yucca Mountain) and comparable in reducing conditions, which makes the zirconia material very attractive for deep geological disposal. The behaviour of plutonia-zirconia inert matrix fuel is discussed within a burn and bury strategy.

The electronic structure of plutonium is studied within the density-functional theory (DFT) model. Key features of the electronic structure are correctly modeled and bonding, total energy, and electron density of states are all consistent with measure data, although the prediction of magnetism is not consistent with many observations. Here we analyze the contributions to the electronic structure arising from spin polarization, orbital polarization, and spin-orbit interaction. These effects give rise to spin and orbital moments that are of nearly equal magnitude, but anti-parallel, suggesting a magnetic-moment cancellation with a zero total moment. Quantifying the spin versus orbital effects on the bonding, total energy, and electron spectra it becomes clear that the spin polarization is much less important than the orbital correlations. Consequently, a restricted DFT approach with a non-spin polarized electronic structure can produce reasonable equation-of-state and electron spectra for δ-Pu when the orbital effects are accounted for. Hence, we present two non-magnetic models. One in which the spin moment is canceled by the orbital moment and another in which the spin moment (and therefore the orbital moment) is restricted to zero.