Actinide Dioxides Research Articles

Modelling thermal properties of actinide dioxide fuels

Modelling of the behaviour of fuels and targets containing minor actinides is still difficult because of very limited information on their thermal and mechanical properties. Integral analysis based on the sound physical models and on the similarity principle can be very useful in this situation. In the current article, a combination of macroscopic and microscopic approaches is used for development of an equation of state (EOS) for actinide dioxide fuels. Based on simple but physically complete models of phonon and electron spectra, the EOS was deduced in quasi-harmonic approximation, and useful relationships bounding thermal and mechanical properties were obtained. They were firstly tested with calculation of the specific heat and the coefficient of thermal expansion of UO 2 and ThO 2. A good agreement with the experimental data was demonstrated in the temperature range of 30–1600 K. Then the model was successfully applied to NpO 2 and PuO 2. Lack of experimental data on thermal properties of AmO 2 and CmO 2 in open literature did not allow the reliable comparison.

Perturbative approach toJmixing inf-electron systems: Application to actinide dioxides

We present a perturbative model for crystal-field calculations, which keeps into account the possible mixing of states labelled by different quantum number J. Analytical J-mixing results are obtained for a Hamiltonian of cubic symmetry and used to interpret published experimental data for actinide dioxides. A unified picture for all the considered compounds is proposed by taking into account the scaling properties of the crystal-field potential.

Molecular Dynamics Studies of Minor Actinide Dioxides

The molecular dynamics (MD) calculation was performed for minor actinide (MA: Np and Am) dioxides in the temperature range from 300 to 2,500K to evaluate the thermophysical properties viz., the lattice parameter, thermal expansion coefficient, compressibility, heat capacity, and thermal conductivity. The Morse-type potential function added to the Busing-Ida type potential was employed for the ionic interactions. The interatomic potential parameters were determined by fitting to the experimental data of the lattice parameter. The usefulness and applicability of the MD method to evaluate the thermophysical properties of minor actinide dioxides were discussed.

Behavior of actinide dioxides under pressure: UO2andThO2

The structural high-pressure properties of $\mathrm{Th}{\mathrm{O}}_{2}$ and $\mathrm{U}{\mathrm{O}}_{2}$ have been studied in diamond-anvil cells up to maximum pressures of 80 and 69 GPa, respectively. These results show that both thorium and uranium dioxides exhibit an identical sequence of structural transitions under pressure; both transform rather sluggishly to the cotunnite-type (orthorhombic $Pnma$) high-pressure structure. This study which was performed under better hydrostatic conditions than previous experiments has enabled us to determine reliable compressibility parameters for the two dioxides: $\mathrm{Th}{\mathrm{O}}_{2}$: ${B}_{0}=198(2)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and ${B}_{0}^{\ensuremath{'}}=4.6(3)$; $\mathrm{U}{\mathrm{O}}_{2}$: ${B}_{0}=207(2)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and ${B}_{0}^{\ensuremath{'}}=4.5(4)$. In the case of $\mathrm{U}{\mathrm{O}}_{2}$ the ambient pressure cubic phase was still found to be present at $69\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which is in contradiction with earlier measurements. With regards to these results and re-calculations performed on other actinide dioxides $\mathrm{Pu}{\mathrm{O}}_{2}$ and $\mathrm{Am}{\mathrm{O}}_{2}$, we obtain a different evolution of the bulk moduli through the actinide dioxide series.

Crystal field effect in light actinide dioxides and oxychalcogenides—a unified phenomenological description

Abstract The electronic properties of the actinide ions in the series of semi-conducting, antiferromagnetic compounds: dioxides, AnO 2 and oxychalcogenides, AnOY, where An=U, Np and Y=S, Se, are re-examined from the point of view of the consistency of the crystal field (CF) model. The discussion is based on the supposition that the effective metal–ligand interaction solely determines the net CF effect in non-metallic compounds. The main question we address here is, whether a reliable, consistent description of the CF effect in terms of the intrinsic parameters can be achieved for this particular family of compounds. Encouraging calculations reported previously for the AnO 2 and UOY series serve as a reference data in the present estimation of electronic structure parameters for neptunium oxychalcogenides.

Molecular Dynamics Studies of Minor Actinide Dioxides

The molecular dynamics (MD) calculation was performed for minor actinide (MA: Np and Am) dioxides in the temperature range from 300 to 2,500 K to evaluate the thermophysical properties viz., the lattice parameter, thermal expansion coefficient, compressibility, heat capacity, and thermal conductivity. The Morse-type potential function added to the Busing-Ida type potential was employed for the ionic interactions. The interatomic potential parameters were determined by fitting to the experimental data of the lattice parameter. The usefulness and applicability of the MD method to evaluate the thermophysical properties of minor actinide dioxides were discussed. (author)

High Pressure Theoretical Studies of Actinide Dioxides

Actinide dioxides (ThO 2 , UO 2 , Pu 2 etc.) compounds have the CaF 2 -type structure at ambient pressure and temperature. Under high pressure, they exist in the PbCl 2 -type structure, belonging to space group Pnma [1]. We have studied crystal structures under high pressure in actinide dioxides by means of first-principles self-consistent total-energy calculations with the non-local Perdew, Burke and Ernzerhof (PBE) exchange correlation using the full-potential linear-muffin-tin-orbital (FPLMTO) method. The atomic equilibrium volume, bulk modulus and transition pressure for actinide dioxides were calculated, covering the full pressure range for which the mentioned experiments have been done [2].

Calculated variation trends across the actinide dioxide series: electronic structure and charge transfer transitions

Variation trends across the AnO 2 series of An5f-electron covalent bonding and An5f→O3s, O2p→An5f charge transfer energies, are studied using DV-Xα relativistic spin-polarised computation of electronic structures applied to 11 ‘AnO 8’ clusters, from ThO 8 to FmO 8. It is found that the binding energies of 5f orbitals, therefore the An5f–O2p hybridisation, are increasing though the 5f orbitals are localising across the An series. In our calculations, the 3s and 3p ionic-like orbitals of O 2− ions are included for the first time as LCAO-MO bases. Then, the conduction band is a mixing of O3s and An6d and its lower edge corresponds to an O3s-dominated state. Moreover, the calculated charge transfer (CT) energies of An5f→O3s and O2p→An5f transitions show the so-called tetrad effect when CT energies, respectively increasing and decreasing across the AnO 2 series. It is pointed out that the tetrad effect here comes mainly from the special spin-polarised pattern of 5f levels and the increasing general trend of 5f binding energies.

Simultaneous determination of X-ray Debye temperature and Grüneisen constant for actinide dioxides: PuO 2 and ThO 2

The lattice vibrations of PuO 2 and ThO 2 were examined between room temperature and 1274 K using a high temperature X-ray diffractometer. The temperature factors for the metal atom, B Pu and B Th in the dioxides were evaluated by the Rietveld analysis. Debye temperature was calculated using the temperature factor. It was confirmed that the Debye temperature decreased with increasing temperature. From the temperature dependence of the Debye temperature, the Grüneisen constants were evaluated to be 1.62 for PuO 2 and 1.51 for ThO 2. The Debye temperatures modified for the thermal expansion of PuO 2 and ThO 2 were 429 and 463 K, respectively.

Thermal expansions of NpO 2 and some other actinide dioxides

Thermal expansions of the stoichiometric actinide dioxides (ThO 2, UO 2, NpO 2 and PuO 2) were investigated between room temperature and 1300 K using a high temperature X-ray diffraction method. The lattice parameter of NpO 2 at high temperatures was expressed as α T (pm) = 542.03 + 4.28 × 10 −3 T + 9.07 × 10 −7 T 2 − 1.36 × 10 −10 T 3. Based on excellent reproducible data, thermal expansion of NpO 2 was determined. Thermal expansions of the other actinide dioxides showed good agreement with the respective recommended literature values. The linear thermal expansion coefficients ( α) of actinide dioxides calculated at 1200 K were in inverse relation to their melting points. At room temperature, however, the α value of UO 2 was found to be higher than those of the other actinide dioxides and this result was discussed.

HEAT CAPACITY OF ThO2

The heat capacity Ｃpof ThO2 can be calculated as the phonon part of Ｃp for other actinide dioxides used as fuel in nuclear reactors. Precise determination of the phonon part of Ｃp of actinide dioxides is helpful to find out the contributions of other factores to Ｃp. In this paper we have, through studying the heat capacity of ThO2, developed a general method applicable to the study of Ｃp of other solids. In the developed method the three type-different experimental measurements made on a solid-heat capacity, thermal expansion and Debye-Waller factor-can be brought together for comparison. The application of this method to the study of Ｃp of ThO2 has enabled us to propose a better description of Ｃp of ThO2 than the generally accepted expression.

Heat capacity of actinide dioxides

We analyse available experimental information on the vibrational heat capacity of ThO 2, with particular emphasis on anharmonic effects, and on the similarities in the vibrational properties of actinide dioxides of the CaF 2 type structure. The result is then used in a discussion of the heat capacity of UO 2. The contribution from crystal-field split 5f-electron states is calculated from published spectral data, which allows remaining nonvibrational and controversial contributions to the heat capacity to be separated out. An important aim of the paper is also to demonstrate a method of analysing high-temperature thermodynamic data.

Theory of core-level spectroscopy in actinide systems

We analyze the actinide 4f core X-ray photoemission spectra for actinide dioxide series, ThO 2, UO 2−BkO 2, using the impurity Anderson model. Systematic variations of physical parameters, such as the charge transfer energy and the covalency hybridization strength, are investigated with the change of the actinide elements. We also calculate uranium 5d core X-ray absorption spectra (5d-XAS) of UO 2 and other U compounds taking into account the effect of multiplet coupling. We discuss an important role of the interplay between the intra-atomic multiplet coupling and the inter-atomic hybridization on the 5d-XAS.

Neutron inelastic experiments on actinide dioxides: Search for crystal field excitations

Neutron scattering has proved to be an effective probe of high-energy magnetic excitations. Recent measurements on AnO 2 (An=U, Np, Pu) performed at spallation source-based spectrometers have made important contributions in the understanding of the crystal field potential in such systems. The experimental results are consistent with a fairly constant potential across the series and suggest that the ground state wave functions may be expressed almost entirely within a single J-manifold.

The electronic structure of actinide dioxides

The f-electron elements in actinide dioxide crystals have cubic site symmetry. Nevertheless, our understanding of their electronic structure has advanced rather slowly. Now, in addition to the magnetic susceptibility, magnetic resonance, optical and X-ray absorption data, there is information from both elastic and inelastic neutron magnetic scattering experiments. This report compares experimental data with results from two types of calculations: discrete-variational X α molecular-orbital studies and traditional f n crystal-field studies. In this way, we not only gain an insight into the single-site crystal-field parameters but we learn something about the significance and the approximate magnitude of the interactions between the f electrons on neighboring actinide sites.

Systematic Analysis of Core Photoemission Spectra for Actinide Di-Oxides and Rare-Earth Sesqui-Oxides

We carry out systematic analysis of 4f core photoemission spectra for actinide di-oxides AnO 2 (An = Th ∼ Bk), using the impurity Anderson model including the exchange interaction J between 5f electrons. The effect of J is important in explaining the observed photoemission spectra especially for PuO 2 , AmO 2 and CmO 2 . It is shown that PuO 2 and BkO 2 are strongly mixed valence compounds, and that a crossover between Mott-Hubbard-type and charge-transfer-type insulators occurs around NpO 2 . A previous analysis of 3d core photoemission spectra for rare-earth sesqui-oxides R 2 O 3 (R = La ∼ Yb) is briefly reviewed, and the results for AnO 2 and R 2 O 3 are compared

Raman spectra of some actinide dioxides and of EuF2

Abstract Raman spectra have been obtained for the first time from polycrystalline samples of NpO2, PuO2 and EuF2. Raman spectra were also obtained from polycrystalline samples of ThO2, UO2 and CeO2 for comparison with published results. Each of these compounds exhibits the CaF2 (fluorite)-type cubic crystal structure. The observed Raman active phonon (T2g) frequencies were used in conjunction with reported optically active IR phonon (T1u) frequencies to calculate the force constants for both the cation-anion stretching (K) and anion-anion repulsion (F).

Powder neutron diffraction and magnetic susceptibility of 248CmO 2

We report the first neutron diffraction experiments on a sample of approximately 55 mg of CmO 2, using the rare isotope 248Cm. Preparative techniques have assured that the sample is close to stoichiometry and X-ray patterns show only the expected f.c.c. lines. The neutron diffraction experiments confirm that the sample is nearly stoichiometric and rule out the possibility that the oxygen atoms form a superlattice to accommodate a mixture of Cm 3+ and Cm 4+ ions. Magnetic susceptibility measurements give a μ eff value of (3.36 ± 0.06) μ B for the effective paramagnetic moment, consistent with many other determinations of this property in Cm(IV) compounds, which have been interpreted in the past as indicating substantial Cm 3+ impurity. In view of the contradiction between bulk susceptibility on the one hand and thermogravimetric and diffraction results on the other hand, we suggest that a re-evaluation of the electronic ground states in the actinide dioxides may be needed. Some other contradictory experimental results are given.

Neutron inelastic experiments on actinide dioxides: Search for crystal-field levels in NpO2

Early experiments at the pulsed neutron source at Argonne National Laboratory (ANL) showed that it was possible to observe crystal-field (CF) levels at energy transfers above 150 meV in UO2. More recent investigations at the UK pulsed source at Rutherford Laboratory have improved the resolution and seen a series of well-resolved peaks. Based on these measurements the CF parameters are V4=−130 meV and V6=+30 meV. Since these parameters should be independent of the precise An ion, there is interest in looking at the tetravalent oxides UO2, NpO2, and PuO2. We have now performed experiments at ANL on NpO2 at temperatures between 15 and 200 K. We find no sharp peak in the excitation spectrum up to energy transfers of about 250 meV. This result is contrary to the expected strong transition at about 53 meV using the above CF parameters in a complete intermediate coupling calculation. The observed phonon spectrum of NpO2, in comparison with that of ThO2, shows larger widths in all of the three vibrational bands above ∼30 meV, which may indicate strong interaction between CF levels with optic phonons in NpO2.

Many-Body Theory for Spectra of f-Electron Systems

Spectroscopic and thermodynamic properties of some 4f and 5f compounds are studied using the Anderson impurity model. The large degeneracy of the f-level allows us to treat 1/Nf as a small parameter. Exact results are obtained for Nf = ∞ and T = 0, and even for Nf = 1 or 2 accurate results are obtained if corrections of order 1/Nf are included. Comparison with experimental core level photoemission spectra determines the parameters of the model. With these parameters the model has been used to study other electron spectroscopies, such as M4,5XAS, 4f PES and BIS, for Ce compounds. The model gives a fairly good description of these spectroscopies. The values of the parameters place the known Ce intermetallics in the Kondo limit, where the 4f spectral function has a narrow resonance at the Fermi energy, which has been seen spectroscopically. This narrow resonance is related to a large density of low-lying excitations, which explains the large low temperature susceptibility and specific heat. The model has also been applied to the 4f photoemission spectra of some Pr and Nd compounds. The agreement with experiment is satisfactory. Finally the model has been used to study the 4f core spectrum of some actinide dioxides. This determines the importance of the hybridization between the 5f state and the O 2p states, and provides an interpretation of a satellite as well as the photon energy dependence of the spectrum close to the 3d → 5f resonance.