Phase stability, electronic structures and elastic properties of (U,Np)O2 and (Th,Np)O2 mixed oxides.

Mixing enthalpies (ΔHmix) of U1-xNpxO2 and Th1-xNpxO2 solid solutions are derived from atomic scale simulations based on density functional theory (DFT) employing the generalised gradient approximation corrected with an effective Hubbard parameter (Ueff). The variation of structural and electronic properties of UO2 and NpO2 with collinear ferromagnetic (FM), collinear anti-ferromagnetic (AFM) and non-collinear anti-ferromagnetic arrangements of the uranium and neptunium magnetic moments are investigated while ramping up Ueff from 0 eV to 4 eV (the Ueff-ramping method). A combination of the Ueff-ramping method to treat the presence of metastable magnetic states and special-quasirandom structures (SQS) for the random distribution of Np atoms in UO2 and ThO2 is employed to calculate ΔHmix of U1-xNpxO2 and Th1-xNpxO2 mixed oxides (MOX). The effect of collinear FM and AFM ordering is also considered in determining the ΔHmix. The calculated ΔHmix of Th1-xNpxO2 MOX were positive compared to the end members and nearly symmetric around x = 0.5 and ΔHmix of the AFM configuration were higher compared to the FM configuration maximum by 0.19 kJ mol-1. The ΔHmix of U1-xNpxO2 MOX were negative up to U0.50Np0.50O2 with a maximum value of -1.21 kJ mol-1 for U0.4375Np0.5625O2 whereas Np-rich (U,Np)O2 MOX compositions exhibited ΔHmix close to zero. Values of ΔHmix for (Th,Np)O2 are consistent with a simple miscibility-gap phase diagram while those for (U,Np)O2 suggest more complex behaviour. Nevertheless, lattice parameter variation with composition still follows a Vegard's law relationship. Finally, single crystal elastic constants of pure oxides and MOX are reported. The linear-elasticity models describe the mixing energies to within an accuracy of approximately 1 kJ mol-1 for the U1-xNpxO2 and Th1-xNpxO2 MOX systems.

High temperature measurements and condensed matter analysis of the thermo-physical properties of ThO2.

Values are presented for thermal conductivity, specific heat, spectral and total hemispherical emissivity of ThO2 (a potential nuclear fuel material) in a temperature range representative of a nuclear accident - 2000 K to 3050 K. For the first time direct measurements of thermal conductivity have been carried out on ThO2 at such high temperatures, clearly showing the property does not decrease above 2000 K. This could be understood in terms of an electronic contribution (arising from defect induced donor/acceptor states) compensating the degradation of lattice thermal conductivity. The increase in total hemispherical emissivity and visible/near-infrared spectral emissivity is consistent with the formation of donor/acceptor states in the band gap of ThO2. The electronic population of these defect states increases with temperature and hence more incoming photons (in the visible and near-infrared wavelength range) can be absorbed. A solid state physics model is used to interpret the experimental results. Specific heat and thermal expansion coefficient increase at high temperatures due to the formation of defects, in particular oxygen Frenkel pairs. Prior to melting a gradual increase to a maximum value is predicted in both properties. These maxima mark the onset of saturation of oxygen interstitial sites.

Advances in the development of a dissolution method for the attribution of iridium source materials.

To assist in nuclear forensic investigations, new techniques are required to evaluate radioactive materials that may be discovered outside of regulatory control. Using a recently developed pressure digestion method for iridium powder, assessments have been made of this techniques suitability for undertaking iridium target material evaluations. In addition to determining the reaction conditions necessary for total dissolution, these investigations have provided an insight into the elemental impurities that are present within unirradiated iridium targets that are used in QSA Global radiography sources, and established the speciation of the iridium solutions that are formed during this process.

Thermophysical properties and oxygen transport in (Thx,Pu1-x)O2.

Using Molecular Dynamics, this paper investigates the thermophysical properties and oxygen transport of (Thx,Pu1-x)O2 (0 ≤ x ≤ 1) between 300-3500 K. In particular, the superionic transition is investigated and viewed via the thermal dependence of lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure. Oxygen diffusivity and activation enthalpy are also investigated. Below the superionic temperature an increase of oxygen diffusivity for certain compositions of (Thx,Pu1-x)O2 compared to the pure end members is predicted. Oxygen defect formation enthalpies are also examined, as they underpin the superionic transition temperature and the increase in oxygen diffusivity. The increase in oxygen diffusivity for (Thx,Pu1-x)O2 is explained in terms of lower oxygen defect formation enthalpies for (Thx,Pu1-x)O2 than PuO2 and ThO2, while links are drawn between the superionic transition temperature and oxygen Frenkel disorder.

A computational study on the superionic behaviour of ThO2.

This study reports the density functional theory (DFT) and classical molecular dynamics (MD) study of the lattice dynamical, mechanical and anionic transport behaviours of ThO2 in the superionic state. DFT calculations of phonon frequencies were performed at different levels of approximation as a function of isotropic dilation (ε) in the lattice parameter. With the expansion of the lattice parameter, there is a softening of B1u and Eu phonon modes at the X symmetry point of the Brillouin zone. As a result of the nonlinear decrease at the X point, the B1u and Eu phonon modes cross each other at ε = 0.03, which is associated with a sharp increase in the narrow peak of the phonon density of states, signifying a higher occupation and hence a higher coupling of these modes at high temperatures. The mode crossing also indicates anionic conductivity in the 〈001〉 direction leading to occupation of interstitial sites. Moreover, MD and nudged elastic band calculated diffusion barriers indicate that 〈001〉 is the easy direction for anion migration in the normal and superionic states. With a further increase in the lattice parameter, the B1u mode continues to soften and becomes imaginary at a strain (ε) of 0.036 corresponding to a temperature of 3430 K. The calculated temperature variation of single crystal elastic constants shows that the fluorite phase of ThO2 remains elastically stable up to the superionic regime, though the B1u phonon mode is imaginary in that state. This leads to anionic disorder at elevated temperatures. Tracking of anion positions in the superionic state as a function of time in MD simulations suggests a hopping model in which the oxygen ions migrate from one tetrahedral site to another via octahedral interstitial sites.

Development of Xe and Kr empirical potentials for CeO2, ThO2, UO2 and PuO2, combining DFT with high temperature MD.

The development of embedded atom method (EAM) many-body potentials for actinide oxides and associated mixed oxide (MOX) systems has motivated the development of a complementary parameter set for gas-actinide and gas-oxygen interactions. A comprehensive set of density functional theory (DFT) calculations were used to study Xe and Kr incorporation at a number of sites in CeO2, ThO2, UO2 and PuO2. These structures were used to fit a potential, which was used to generate molecular dynamics (MD) configurations incorporating Xe and Kr at 300 K, 1500 K, 3000 K and 5000 K. Subsequent matching to the forces predicted by DFT for these MD configurations was used to refine the potential set. This fitting approach ensured weighted fitting to configurations that are thermodynamically significant over a broad temperature range, while avoiding computationally expensive DFT-MD calculations. The resultant gas potentials were validated against DFT trapping energies and are suitable for simulating combinations of Xe and Kr in solid solutions of CeO2, ThO2, UO2 and PuO2, providing a powerful tool for the atomistic simulation of conventional nuclear reactor fuel UO2 as well as advanced MOX fuels.

Pipe and grain boundary diffusion of He in UO2.

Molecular dynamics simulations have been conducted to study the effects of dislocations and grain boundaries on He diffusion in [Formula: see text]. Calculations were carried out for the {1 0 0}, {1 1 0} and {1 1 1} [Formula: see text] edge dislocations, the screw [Formula: see text] dislocation and Σ5, Σ13, Σ19 and Σ25 tilt grain boundaries. He diffusivity as a function of distance from the dislocation core and grain boundaries was investigated for the temperature range 2300-3000 K. An enhancement in diffusivity was predicted within 20 Å of the dislocations or grain boundaries. Further investigation showed that He diffusion in the edge dislocations follows anisotropic behaviour along the dislocation core, suggesting that pipe diffusion occurs. An Arrhenius plot of He diffusivity against the inverse of temperature was also presented and the activation energy calculated for each structure, as a function of distance from the dislocation or grain boundary.

Genetics of superionic conductivity in lithium lanthanum titanates.

The self-diffusion of ions is a fundamental mass transport process in solids and has a profound impact on the performance of electrochemical devices such as the solid oxide fuel cell, batteries and electrolysers. The perovskite system lithium lanthanum titanate, La2/3-xLi3xTiO3 (LLTO) has been the subject of much academic interest as it displays very high lattice conductivity for a solid state Li conductor; making it a material of great technological interest for deployment in safe durable mobile power applications. However, so far, a clear picture of the structural features that lead to efficient ion diffusion pathways in LLTO, has not been fully developed. In this work we show that a genetic algorithm in conjunction with molecular dynamics can be employed to elucidate diffusion mechanisms in systems such as LLTO. Based on our simulations we provide evidence that there is a three-dimensional percolated network of Li diffusion pathways. The present approach not only reproduces experimental ionic conductivity results but the method also promises straightforward investigation and optimisation of the properties relating to superionic conductivity in materials such as LLTO. Furthermore, this method could be used to provide insights into related materials with structural disorder.

Thermal conductivity and energetic recoils in UO2 using a many-body potential model.

Classical molecular dynamics simulations have been performed on uranium dioxide (UO2) employing a recently developed many-body potential model. Thermal conductivities are computed for a defect free UO2 lattice and a radiation-damaged, defect containing lattice at 300 K, 1000 K and 1500 K. Defects significantly degrade the thermal conductivity of UO2 as does the presence of amorphous UO2, which has a largely temperature independent thermal conductivity of ∼1.4 Wm(-1) K(-1). The model yields a pre-melting superionic transition temperature at 2600 K, very close to the experimental value and the mechanical melting temperature of 3600 K, slightly lower than those generated with other empirical potentials. The average threshold displacement energy was calculated to be 37 eV. Although the spatial extent of a 1 keV U cascade is very similar to those generated with other empirical potentials and the number of Frenkel pairs generated is close to that from the Basak potential, the vacancy and interstitial cluster distribution is different.

A many-body potential approach to modelling the thermomechanical properties of actinide oxides.

A many-body potential model for the description of actinide oxide systems, which is robust at high temperatures, is reported for the first time. The embedded atom method is used to describe many-body interactions ensuring good reproduction of a range of thermophysical properties (lattice parameter, bulk modulus, enthalpy and specific heat) between 300 and 3000 K for AmO2, CeO2, CmO2, NpO2, ThO2, PuO2 and UO2. Additionally, the model predicts a melting point for UO2 between 3000 and 3100 K, in close agreement with experiment. Oxygen-oxygen interactions are fixed across the actinide oxide series because it facilitates the modelling of oxide solid solutions. The new potential is also used to predict the energies of Schottky and Frenkel pair disorder processes.

Point defect engineering strategies to retard phosphorous diffusion in germanium.

The diffusion of phosphorous in germanium is very fast, requiring point defect engineering strategies to retard it in support of technological application. Density functional theory corroborated with hybrid density functional calculations are used to investigate the influence of the isovalent codopants tin and hafnium in the migration of phosphorous via the vacancy-mediated diffusion process. The migration energy barriers for phosphorous are increased significantly in the presence of oversized isovalent codopants. Therefore, it is proposed that tin and in particular hafnium codoping are efficient point defect engineering strategies to retard phosphorous migration.

Strain-induced changes to the electronic structure of germanium.

Density functional theory calculations (DFT) are used to investigate the strain-induced changes to the electronic structure of biaxially strained (parallel to the (001), (110) and (111) planes) and uniaxially strained (along the [001], [110] and [111] directions) germanium (Ge). It is calculated that a moderate uniaxial strain parallel to the [111] direction can efficiently transform Ge to a direct bandgap material with a bandgap energy useful for technological applications.

Defect processes in orthorhombic LnBaCo2O5.5 double perovskites.

Static atomistic simulations based on the Born model were used to investigate intrinsic defect processes in orthorhombic LnBaCo(2)O(5.5) (Ln = Y, La, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb) double perovskites. It was found that Ln/Ba antisite disorder is the lowest energy defect reaction, with the large Ln cations giving rise to smaller antisite energies. On the oxygen sublattice the oxygen Frenkel disorder dominates and also decreases in energy with increasing Ln cation size. The lowest energy oxygen vacancy and interstitial positions are in the LnO(0.5) and CoO(2) layers respectively. Interestingly, the calculations indicate that oxygen vacancies cluster with Ba antisite defects (occupying Ln sites). This suggests that the transport of oxygen vacancies will be influenced not only by the oxygen Frenkel energy but also the antisite energy. We propose that PrBaCo(2)O(5.5) most efficiently balances these two competing effects as it has an oxygen Frenkel energy of just 0.24 eV per defect combined with a high antisite energy (0.94 eV), which ensures that the A cation sublattice will remain more ordered.

A theoretical study of intrinsic point defects and defect clusters in magnesium aluminate spinel.

Point and small cluster defects in magnesium aluminate spinel have been studied from a first principles viewpoint. Typical point defects that occur during collision cascade simulations are cation anti-site defects, which have a small formation energy and are very stable, O and Mg split interstitials and vacancies. Isolated Al interstitials were found to be energetically unfavourable but could occur as part of a split Mg-Al pair or as a three atom-three vacancy Al 'ring' defect, previously observed in collision cascades using empirical potentials. The structure and energetics of the defects were investigated using density functional theory (DFT) and the results compared to simulations using empirical fixed charge potentials. Each point defect was studied in a variety of supercell sizes in order to ensure convergence. It was found that empirical potential simulations significantly overestimate formation energies, but that the type and relative stability of the defects are well predicted by the empirical potentials both for point defects and small defect clusters.

Structure and mobility of defects formed from collision cascades in MgO.

We study radiation-damage events in MgO on experimental time scales by augmenting molecular dynamics cascade simulations with temperature accelerated dynamics, molecular statics, and density functional theory. At 400 eV, vacancies and mono- and di-interstitials form, but often annihilate within milliseconds. At 2 and 5 keV, larger clusters can form and persist. While vacancies are immobile, interstitials aggregate into clusters (In) with surprising properties; e.g., an I4 is immobile, but an impinging I2 can create a metastable I6 that diffuses on the nanosecond time scale but is stable for years.

Estrogen stimulation of P450 cholesterol side-chain cleavage activity in cultures of human placental syncytiotrophoblasts.

The present study determined whether estrogen has a role in regulating the P450 cholesterol side-chain cleavage enzyme (P450scc) and/or de novo/deesterification cholesterol pathways involved in progesterone biosynthesis within human syncytiotrophoblasts. Human placental syncytiotrophoblasts were cultured for 48 h with estradiol, and P450scc activity was determined by the formation of progesterone from 25-hydroxycholesterol. Estradiol at 10(-7) or 10(-6) M and 25-hydroxycholesterol increased mean (+/- SE) progesterone production by syncytiotrophoblasts (ng/0.5 x 10(6) cells) to a value (19.2 +/- 1.1) that was 104% (p < 0.001) higher than that of the untreated controls (9.4 +/- 0.8) and 52% higher (p < 0.001) than with 25-hydroxycholesterol alone (12.6 +/- 0.9). The stimulation of progesterone secretion apparently was not the result of a change in progesterone metabolism to its principal metabolite, because 20alpha-hydroxypregn-4-en-3-one represented a minor secretory component (0.7-1.7 ng/0.5 x 10(6) cells) under these conditions, and levels were not substantially altered by estrogen. In contrast to the stimulatory effect of estradiol on P450scc activity, estrogen did not alter either the P450scc mRNA levels or the activities of 3-hydroxy-3-methylglutaryl coenzyme-A reductase and cholesterol ester hydrolase-rate-limiting enzymes for the de novo and deesterification pathways, respectively, for cholesterol formation in syncytiotrophoblasts in culture. Collectively, these results indicate that estrogen regulates the P450scc component of the progesterone biosynthetic pathway, which we suggest signals functional/biochemical differentiation of syncytiotrophoblasts during primate pregnancy.

Regulation of human placental trophoblast low-density lipoprotein uptake in vitro by estrogen.

In the present study, human trophoblast cells were studied in culture to determine the effect of estrogen on low-density lipoprotein (LDL) uptake and progesterone formation. Cytotrophoblasts were obtained from term human placentas and incubated for 72 h with 10% FBS to stimulate formation of syncytia. Syncytiotrophoblasts were then incubated for an additional 48 h with estradiol and/or LDL protein. Estradiol plus LDL stimulated progesterone to a level that was 133% greater (P < 0.05) than control (314 +/- 69 ng/mg protein) or LDL alone, suggesting that estrogen stimulated progesterone formation via an increase in LDL uptake and utilization. To examine this possibility, trophoblast cells were cultured with estradiol for 48 h as above, then incubated for 12 h with 6-200 micrograms/mL [125I]LDL. Mean (+/- SE) specific uptake of [125I]LDL (ng/mg cell protein) was approximately 50% greater (P < 0.01) with estradiol (638 +/- 23) than with vehicle (429 +/- 54). Scatchard analysis demonstrated that the dissociation constant for LDL uptake was similar in the presence (2.9 +/- 0.4 x 10(-6)M) and absence (2.8 +/- 0.6 x 10(-8)M) of estradiol, indicating that estrogen increased LDL receptor number without affecting affinity. LDL uptake was increased (P < 0.05) by incubating trophoblast with as little as 0.10 ng/mL estradiol (approximately 10(-9) M). We conclude that estrogen regulates placental trophoblast cell uptake of LDL and, thus, the availability of cholesterol for progesterone bio-synthesis during human pregnancy.

Postsynthetic regulation of insulin-like growth factor-binding protein-3 by MCF-7 human breast cancer cells in culture.

Breast cancer cells are exposed to insulin-like growth factors (IGFs) which stimulate their proliferation, and to IFG-binding proteins (IGFBPs) which sequester and modulate IGF action. The primary circulatory IGFBP is IGFBP-3. In the present study, cultured MCF-7 breast cancer regulated clearance of IGFBP-3 via both cell association and proteolysis. Exogenously added IGFBP-3 was significantly cleared from the medium over time yielding the formation of smaller sized immunodetected fragments. Clearance was inhibited by IGF-I and -II. In contrast, clearance was not affected by growth factors and an IGF-analog having mitogenic activity but not binding to IGFBPs. In fact, activity of the IGFs and analogs paralleled their degree of binding to the IGFBP, suggesting that the IGF-binding altered IGFBP-3 making it less susceptible to clearance. Qualitatively similar results were obtained when these experiments were conducted using cell-free conditioned medium, thus suggesting the presence of secreted protease(s). However, level of proteolytic activity was much less than that found in the presence of cells. Clearance of rhIGFBP-3 also involved binding to the cell. Disappearance of rhIGFBP-3 was shown to be attenuated by heparin, which blocks cell surface binding sites. In contrast, compounds which block internalization did not inhibit IGFBP-3 clearance.

Follicular atresia in pigs: measurement and physiology.

The physiological regulation of follicular atresia was investigated during the early luteal phase after ovulation and during altrenogest-synchronized preovulatory maturation in pigs (gilts). Apoptosis in dispersed granulosa cells was determined by flow cytometry. Apoptotic (A0) cells contain low, subdiploid amounts of DNA fluorescence. Follicles were classified biochemically as atretic or nonatretic based on the percentage of A0 (% A0) cells, atretic with > or = 10%, and nonatretic with < 10% A0 granulosa cells. The % A0 granulosa cells/follicle ranged from .02 to 89. Follicles containing debris in their isolated granulosa cells were classified as morphologically atretic. The morphological and biochemical criteria of atresia were in agreement for 224 of 248 follicles. Internucleosomal DNA cleavage, the hallmark of apoptosis, was determined by autoradiographic analysis of [32P]3'-end labeled DNA from granulosa cells. Densitometric analysis showed that optical density of [32P]3'-end labeled DNA fragments in the .18 to 20 kbp size range was correlated with the % A0 cells (R > .9, n = 22, P < .001). During altrenogest-synchronized preovulatory maturation, < 5% of large (> 6 mm in diameter) follicles were atretic. Among medium-sized follicles (3 to 6 mm) on d 1 and 3 of preovulatory maturation, only 17% were atretic, in contrast with d 5 when 87% were atretic. During the early luteal phase, atretic follicles/pig increased from 6% on d 5 to 50% on d 7 after estrus. Follicular fluid estradiol-17 beta concentration was greater (P < .001) in nonatretic than in atretic follicles on d 5 and 6 after estrus, but by d 7 estradiol-17 beta had decreased to a mean < 1 ng/mL in nonatretic and atretic follicles. The increase in apoptosis in granulosa cells and loss of estradiol-17 beta production in vivo indicated a high incidence of atresia among the first group of follicles grown after ovulation in pigs. These results indicate that apoptotic cell death was involved in degeneration of granulosa cells and atresia during two different stages of follicular development.

Changes in insulin-like growth factor-binding protein-2 and -3 in follicular fluid during atresia of follicles grown after ovulation in pigs.

In pigs, medium-sized follicles (3-6 mm in diameter), lost during preovulatory maturation, are rapidly replenished after ovulation, increasing tenfold in number between day 3 and day 8 of the oestrous cycle. The incidence of atresia in medium-sized follicles was previously found to increase between day 5 and day 7 after the onset of oestrus (day 0). This study was conducted to determine whether changes in concentrations of insulin-like growth factor binding proteins (IGFBPs) in pig follicular fluid from medium-sized follicles were related to increased incidence of atresia. Individual follicles were isolated on days 5, 6 and 7 after oestrus from eleven pigs. Follicles were defined as atretic when they contained > or = 10% apoptotic granulosa cells (cells containing subdiploid amounts of DNA fluorescence determined by flow cytometry). Concentrations of IGFBP-2 and -3 were determined by ligand blot analysis with 125I-labelled insulin-like growth factor II and quantified by densitometry. The mean percentage of apoptotic cells per follicle increased (P < or = 0.01) from 3.6 to 35.0 and the mean concentration of IGFBP-2 increased (P < or = 0.001) 3.3-fold between day 5 and day 7 after oestrus. The concentration of IGFBP-3 did not differ significantly among days. Mean oestradiol concentration in follicular fluid decreased (P < or = 0.05) from 52.3 to 0.9 ng ml-1 between day 5 and day 7. Androstenedione concentrations in follicular fluid also decreased (P < or = 0.01) between day 5 and day 7, but progesterone did not differ significantly among days.(ABSTRACT TRUNCATED AT 250 WORDS)