Pu-Ga Research Articles

Ab initio study of gallium stabilized δ-plutonium alloys and hydrogen–vacancy complexes

All-electron density functional theory was used to investigate δ-plutonium (δ-Pu) alloyed with gallium (Ga) impurities at 3.125, 6.25, 9.375 atomic (at)% Ga concentrations. The results indicated that the lowest energy structure is anti-ferromagnetic, independent of the Ga concentration. At higher Ga concentrations (>3.125 at%), the position of the Ga atoms are separated by four nearest neighbor Pu–Pu shells. The results also showed that the lattice constant contracts with increasing Ga concentration, which is in agreement with experimental data. Furthermore with increasing Ga concentration, the face-centered-cubic structure becomes more stably coupled with increasing short-range disorder. The formation energies show that the alloying process is exothermic, with an energy range of −0.028 to −0.099 eV/atom. The analyses of the partial density of states indicated that the Pu–Ga interactions are dominated by Pu 6d and Ga 4p hybridizations, as well as Ga 4s–4p hybridizations. Finally, the computed formation energies for vacancy and hydrogen–vacancy complexes within the 3.125 at% Ga cell were 1.12 eV (endothermic) and −3.88 eV (exothermic), respectively. In addition, the hydrogen atom prefers to interact much more strongly to the Pu atom than the Ga atom in the hydrogen–vacancy complex.

Grüneisen analysis of thermodynamic properties of δ-phase Pu–Ga alloys

We present a Grüneisen analysis of the intriguing temperature dependence of the thermal expansion and bulk modulus of delta phase plutonium alloys. This analysis exploits the thermodynamic relationship between these quantities and the heat capacity and reduces the description of complicated behaviour to the determination of a manageable number of numerical constants. We conclude from the analysis (1) that there is a small Schottky-like heat capacity anomaly at high temperatures and (2) that the size of this anomaly depends strongly on alloy concentration. Grüneisen analysis provides an economical description of the thermal expansion and bulk modulus of δ-Pu.

Intrinsic Nanoscience of δ Pu–Ga Alloys: Local Structure and Speciation, Collective Behavior, Nanoscale Heterogeneity, and Aging Mechanisms

δ Pu–Ga alloys and their response to self-irradiation are important scientifically because of the unique complexity of Pu and technologically because of their importance in Science Based Stockpile Stewardship. The local order and structure and the role of the Ga are crucial to understanding the phase stability and the aging effects. X-ray diffraction that gives the long-range average structure of the periodic component of the materials and pair distribution functions analysis and X-ray absorption fine structure that give the overall and the element specific local structure have been used to examine a variety of new and aged materials, including a set of high purity δ Pu1–xGax alloys with 1.7 ≤ x ≤ 6.4 atom % Ga that span the low [Ga] portion of the δ region of the phase diagram across the ∼3.3 atom % Ga metastability boundary, a ∼1.7 atom % Ga alloy that was enriched with Pu238 to accelerate the aging process, and others. We find that metastable alloys contain tens of percents of a novel, “σ”, Pu structure that we attribute to rearrangement of the Ga-depleted regions after the self-organization of the Ga to form quasi-intermetallic Pu25–35Ga. This collective and cooperative behavior involving the Ga and other defects in terms of a tendency to aggregate into domains with structures that differ from the δ host and the resulting nanoscale heterogeneity also appears to play an important role in the observation of analogous locally ordered structures in aged materials. This description of these materials and their aging is radically different from current conceptual basis derived from other experiments that are insensitive to ordering on the angstrom–nanometer length scale.

Theoretical confirmation of Ga-stabilized anti-ferromagnetism in plutonium metal

Density functional theory (DFT) for plutonium metal is shown to be consistent with recent magnetic measurements that suggest anti-ferromagnetism in Pu–Ga alloys at low temperatures. The theoretical model predicts a stabilization of the face-centered-cubic (fcc, δ) form of plutonium in an anti-ferromagnetic configuration when alloyed with gallium. The ordered magnetic phase occurs because Ga removes the mechanical instability that exists for unalloyed δ-Pu. The cause of the Ga-induced stabilization is a combination of a lowering of the band (kinetic) and electrostatic (Coulomb) energies for the cubic relative to the tetragonal phase. Similarly, gallium plays an important role in stabilizing anti-ferromagnetism in the tetragonal P4/mmm Pu3Ga compound.

Solid state phase transformation study on a series of Pu–Ga alloys containing 0.18–0.63wt.% Ga

• Solid state phase transformation behaviours of Pu–Ga alloys were investigated. • Effects of Ga content and starting condition on Pu–Ga alloy were found significant. • The nature of as cast α-phase is independent of the nominal Ga content. • The ε → δ transformation happens through two mechanisms when Ga < 0.63 wt.%. • Increase of thermal expansion coefficient of δ-phase with increasing Ga confirmed. The effects of Ga concentration and starting condition of Pu–Ga alloys on their solid-state phase transformation behaviours in a set of Pu–Ga alloys containing 0.18–0.63 wt.% Ga were studied under non-equilibrium conditions with a heating/cooling rate of 10 °C/min in the temperature range of 25–560 °C by a dilatometry method. The characteristic abrupt slope changes shown in the dilatometry tracks during heating and cooling are ascribed to the known phase transformations in the Pu–Ga alloys. The rationale for such an ascription is discussed. The study showed that, as expected, Ga concentration and the starting condition of the Pu–Ga alloy have significant effects, demonstrated by the variety of the onset and completion temperatures of a phase transformation. We find that the nature of as cast α-phase is independent of the nominal Ga content. The ε → δ transformation happens through two mechanisms when Ga < 0.63 wt.%. Furthermore, the increase of thermal expansion coefficient of δ-phase with increasing Ga is also confirmed.

Phase transformations in Pu–Ga alloy: Synergy between simulations and experiments to elucidate direct and indirect reversion competition

Reverse transformation in plutonium alloyed with a small amount of gallium (Pu–1at.% Ga) was investigated in detail by coupling CALPHAD-based simulations with experiments. As observed previously, reverse transformation in a two-phase δ+α′ alloy exhibits the existence of competition between direct reversion (α1at.%′→δ1at.%) and indirect reversion (α1at.%′→βpure Pu(+δe)→γpure Pu(+δe)→δ1at.%). The latter leads to Ga enrichment of the δ phase (δe) as well as the emergence of several phases of pure Pu. The present paper focuses on the thermodynamic and kinetic aspects of the competition between these two reversion modes, especially between direct reversion and the first step of indirect reversion, whenever competition occurs. Thermodynamic simulations indicated that both reversion modes were possible and that indirect reversion was more favorable. Specific isothermal experiments combined with simulations revealed the existence of two potential Ga diffusion paths during indirect reversion: (i) from the α′ phase to the δ phase and (ii) from a new transient β′ phase, preceding the appearance of β phase of pure Pu, to the δ phase. These newly discovered mechanisms have enabled the present authors to fully understand the competition between the reversion modes. Finally, the ratio of direct to indirect reversion was found to depend on three factors, namely heating rate, initial α′ phase fraction and Ga content. All simulations were confirmed experimentally and highlighted key aspects of reversion mechanisms.

Modelling of dissolved H in Ga stabilised δ-Pu

The behaviour of hydrogen in Ga stabilised δ-Pu has been investigated using atomistic computer simulation techniques. We have considered only the solid solution of H in Pu–Ga. H diffusivity in the undamaged material was calculated and was shown to depend on the Ga concentration of the Pu–Ga alloy. Furthermore, localised regions of high Ga concentration within the material were shown to block H diffusion pathways. These are important findings and could allow for the possibility to control H diffusion if it were possible to control the Ga configuration within the system. The interaction of H with simple point defects was also investigated and suggests that H will behave differently in cascade damaged systems compared to undamaged systems. Vacancies were observed to trap any H interstitials that enter their vicinity, while the likelihood of dissociation was very low, effectively reducing the H diffusion coefficient to zero. On the other hand, binding energy calculations show that it is energetically unfavourable for a H interstitial to be close to a Pu interstitial. No long range interaction between H and the single point defects was observed.

Phase transformations in PuGa 1at.% alloy: Study of whole reversion process following martensitic transformation

Abstract The purpose of this work was to study PuGa 1 at.% alloy behavior from martensitic transformation to the whole reversion process. The relevant phase transformations were investigated as a function of temperature via in situ X-ray diffraction and dilatometry analyses carried out at low and high temperatures. The results show that the δ-to-α′ martensitic transformation occurring at low temperatures is direct and does not involve any intermediate γ′ phase. They also reveal that the reverse transformation of the two-phase δ + α′ alloy during heating involves two competing modes, namely direct and indirect reversion. The latter is associated with a Ga diffusion process that governs the ratio between these reversion modes. More precisely, our study demonstrates that the indirect reversion process consists of a Ga-enrichment of the remaining δ phase as well as the emergence of the β and γ phases of pure plutonium. Every stage of indirect reversion has been discussed in relation to the equilibrium phase diagram of the Pu–Ga binary system.

Absolute determination of small samples of Pu and Am by calorimetry

An extensive measurement campaign has been carried in order to recalibrate and assess the performance of the small sample calorimeter (SSCAL) that was recently upgraded. The measurements have been performed in the Performance Laboratory of the Joint Research Centre's (JRC) Nuclear Security Unit in Ispra (Italy) using calibrated electric heat sources and standard reference nuclear materials. The SSCAL is a heat flow calorimeter which works by measuring the voltage generated by a heat-emitting sample across a thermal gap based on a thermopile cup technology. Results of calorimetry measurements carried out, both inside and outside a well-controlled environment of a climatic chamber, on reference Pu–Ga samples and well-characterised 241Am samples are presented and discussed. The latter samples were produced at the JRC-ITU to be used by the JRC-IRMM for various cross-section measurements (total, neutron capture and 241Am(n,2n) 240Am).

Thermal cycling induced δ to α phase transformation via an intermediate β phase on a δ-stabilized Pu–Ga alloy

The thermal cycling induced δ to α phase transformation in a δ-stabilised Pu–Ga alloy has been investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The alloy specimen was thermally cycled from 25°C to 180°C within the DSC furnace enabling coincident collection of phase transformation information. The cycling was carried out with varied isothermal hold periods at 25°C between each DSC run. The α phase was detected in the original well-homogenised alloy and the amount of α phase was found to increase with both thermal cycling number and isothermal hold time. Nearly 8% α phase was detected after 210 cycles, with no sign that α phase in-growth had ceased or slowed. Possible mechanisms, by which thermal cycling facilitates the nucleation and in-growth of the increasing α phase content are discussed. It is hypothesised that the δ to α phase transformation proceeds via an intermediate β phase at elevated temperature. Based on this investigation, it is also suggested that thermal cycling may be used as a novel experimental method to study the meta-stable nature of δ-stabilized Pu alloys.

Point defect formation and migration in Ga stabilised δ-Pu

The energetics of point defects in Ga stabilised δ-Pu are investigated through calculations of formation energies, migration pathways and through on-the-fly kinetic Monte Carlo (OTF-KMC) calculations. Using the Modified Embedded Atom Method as the interatomic potential, defect formation energies are found to depend greatly on both the specie of the defect and the surrounding local environment. Ga vacancies are found to have considerably larger formation energies than Pu vacancies, due to the change in the nearest neighbour Pu–Ga ordering. Point vacancy and point interstitial migration involving Ga atoms or Pu atoms in regions close to substitutional Ga, results in much higher energy barriers than for migration in pure Pu. OTF-KMC calculations show that this leads to defect migration pathways confined to areas of high Pu concentration, with the degree of confinement dependent on the location and ordering of the substitutional Ga. Vacancy migration takes place only through displacing Pu atoms and split Ga interstitials become almost instantaneously substitutional, displacing a Pu atom. The results indicate that the presence of Ga within the Pu matrix has a significant effect on the recovery of point defects.

Thermal expansion and transformation behavior of cerium and plutonium alloys: an application of the Aptekar–Ponyatovsky regular solution model

In this paper we apply the Aptekar–Ponyatovsky (AP) regular solution thermodynamic model to the analysis of experimental data for the coefficient of thermal expansion (CTE) and determine the AP model parameters for unalloyed cerium metal, Ce–Th–La alloys, and Pu–Ga alloys. We find that the high temperature CTE of cerium metal follows the predictions of the AP model based on low temperature, high pressure data. For Ce–Th–La alloys we use the AP parameters to track the suppression of the first-order γ–α cerium transition. We show the AP model accounts for the negative CTE observed for Pu–Ga alloys and is equivalent to an earlier invar model. Finally, we apply the AP parameters obtained for Pu–Ga alloys to rationalize the observed δ–α transformation pressures of these alloys. We show that the anomalous values of the Grüneisen and Grüneisen–Anderson parameters are important features of the thermal properties of plutonium. A strong analogy between the properties of plutonium and cerium is confirmed.

Research on long-time self-irradiation of alloy δ-Pu 242–Ga

Self-irradiation of Pu–Ga alloy was studied by neutron diffraction, which provided information about the crystalline structure and yielded mean-square atom displacements 〈 u 2〉 from the Debye–Waller factor. The measurements were performed at room temperature using the sample based on the isotope Pu 242 with low neutron absorption cross-section to which the short-living isotope Pu 238 (1.4%) was added to accelerate self-irradiation. This composition of the sample sped up the aging processes by the factor of four and allowed us to obtain the maximum equivalent time of 23.5 years. Fcc structure remains the same throughout this interval. The changes in 〈 u 2〉 due to static displacements occur in two stages, viz. a relatively rapid growth (roughly by 50%) over the first 5–6 equivalent years and slow decline during 6–23 equivalent years making a close approach to the initial values. The latter stage has not been described in the literature. It can be explained by the confluence of point defects into helium bubbles and dislocation loops accumulated over time.

In situ X-ray diffraction study of the δ to α′ isothermal martensitic transformation kinetics in a Pu–Ga alloy

The sequence of phase formation and the isothermal kinetics of the δ to α′ phase transformation in a Pu–1.9 at.% Ga alloy are investigated with in situ X-ray diffraction at the Advanced Photon Source. It has been proposed that the formation of a γ′ intermediate phase may be responsible for the unusual double-C curve kinetics reported for this isothermal martensitic transformation. While a γ′ intermediate phase was not observed at either −120 °C or −155 °C, an unexplained shoulder on the δ (1 1 1) peak was detected at −155 °C. The isothermal kinetics of the δ → α′ transformation appear to fit a Johnson–Mehl–Avrami–Kolmogorov equation, but the exponent is not described by existing JMAK theory. The double-C curve kinetics of the δ to α′ transformation remain unresolved in plutonium science, and understanding this behavior is key to predicting phase stability in aging Pu–Ga alloys.

Martensitic interfaces and transformation crystallography in Pu–Ga alloys

The face-centered cubic (δ) to monoclinic (α) martensitic transformation in Pu–Ga is unusual because it produces a relative large change of volume up to 20%. Moreover, the transformation crystallography is not predicted satisfactorily by the classical phenomenological theories of martensite crystallography, which is based on the hypothesis that the habit plane is an invariant plane of the total shape change. In this study we use the recently developed topological model where the habit plane, i.e., the interface between the parent and martensite phases, is envisaged as a semi-coherent configuration with coherent terraces reticulated by a network of disconnections or transformation dislocations, and defects producing lattice-invariant deformation. The authors show explicitly that this network not only accommodates the coherency strains, so that no long-range strain field arises, but also produces diffusionless transformation by motion of the disconnections across the interface. It was show that the predicted habit plane inclination with respect to the parent and martensite phases in a Pu–Ga alloy is in good agreement with published experimental observations when the lattice-invariant deformation is \( (201)_{\alpha } /[10\overline{2} ]_{\alpha } \) twinning, as observed experimentally.

He bubble coarsening by migration and coalescence in annealed Pu–Ga alloys

The α-decay of plutonium in Pu–Ga alloys continually generates inert He atoms within the lattice of the Pu–Ga matrix. In naturally aged Pu specimens, those He atoms form into bubbles, He-filled vacancy clusters, with a characteristic size distribution centered near 1.4 nm. Upon annealing, the He bubbles are subject to temperature induced changes in the thermodynamic parameters governing their size, which results in a coarsening of the bubble distribution yielding a lower bubble density but larger average bubble sizes. Herein, we investigate, by means of transmission electron microscopy, the coarsening of He bubbles with several annealing treatments. Micrographs subsequent to the annealing treatments as well as in situ observations implicate migration and coalescence as the mechanism for He bubble coarsening with annealing.

Electron-density distribution and physical properties of plutonium–gallium alloys: Ab initio cluster calculations

A cluster model based on ab initio density-functional theory was used to model gallium-stabilized δ-plutonium alloys, and to calculate the electron-density distribution, its pressure dependence, bond lengths, elastic properties (second order and third order), and inelastic properties for Pu 12Ga (7.7 at% Ga) and Pu 18Ga (5.3 at% Ga). The electron distribution was found to contain localized, semi localized, and delocalized contributions, with the second possessing covalent character. Two of plutonium’s 8 valence electrons were found to be itinerant, consistent with a recent prediction based on an electrostatic model, with the electron configuration for plutonium being 7s 0.57p 0.55f 1 (itinerant) and 6d 15f 5 (localized), and that for gallium being 4s 14p 2. Applied hydrostatic pressure shifts the charge density toward a more localized Pu(d)-based distribution. The onset of the pressure-induced δ-Pu to α-Pu phase change is accompanied by a ∼0.2 electron increase in the localized population that may serve as a driving force for the phase change. Interior bonding within the Pu 12Ga subunits is stronger than that of the surrounding plutonium lattice, and the Pu–Ga bonds therein relax in a direction opposite to lattice strain. This study predicts covalency in metallic plutonium, both in the Pu–Ga bonding and in the Pu–Pu bonding.

Phase transformation in δ Pu alloys at low temperature: In situ dilatometric study

The purpose of this work is to precisely study the martensitic transformation in a plutonium-gallium alloy. Thus, the thermodynamics and kinetics of the δ→α'+δ phase transformation in a Pu-Ga alloy were studied under isochronal and isothermal conditions. The activation energy of the δ→α'+δ phase transformation at a constant cooling rate (0.5 K.min−1) was determined by using Kissinger and Ozawa models. The average value of the activation energy was found to be at −56 kJ.mol−1. Dilatometry measurement was also used to trace 'in situ' the entire transformation for several temperatures. The kinetics of the δ→α'+δ transformation were modelled under isothermal conditions in the theoretical frame of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. It is proposed that the transformation consists of three stages. The α' transformation begins with a nucleation of pre-existing embryos. Then, both nucleation and rapid growth of α' occurs simultaneously and finally, the plates width expend. Apparent activation energies for nucleation and growth transformation were determined from the temperature dependence of the constant K at respectively −34 kJ.mol−1 and −60 kJ.mol−1. Adler et al. [1] investigated also the thermodynamics and the kinetics of the martensitic transformation in Pu alloys. These nucleation energies were found by modelling of heterogeneous martensitic nucleation via strain interaction with observed superdislocation-like nucleation sites in PuGa alloys. The values obtain by this model was very close to those we find. Investigations in steels alloys indicate that these energies are of the same order for nucleation near dislocation. Then, it could be indicating a strong relationship between these dislocations and martensitic nucleation sites.

Studying structural phase transitions with X-ray thermal diffuse scattering

X-ray thermal diffuse scattering is a powerful probe of lattice dynamics. It is especially effective in the investigation of structural phase transitions. In this article, we review and compare recent experimental studies of the behaviour of phonons in connection with the structural phase transitions in TiSe2, SrTiO3 and Pu–Ga. A complete phonon softening is observed for the second-order transitions in TiSe2 and SrTiO3. By contrast, no phonon softening is detected for the first-order transition in Pu–Ga, which is unusual in comparison to other related systems.

The Aqueous Corrosion Behavior of Plutonium Metal and Plutonium-Gallium Alloys Exposed to Aqueous Nitrate and Chloride Solutions

Previous work in this laboratory examined the polarization behavior of high-purity plutonium metal exposed to various solutions, however, the polarization behavior of plutonium-gallium alloys has not been studied. The objective of this work is to establish the effect of gallium on the electrochemical behavior of plutonium metal exposed to nitric acid, sodium nitrate, and sodium chloride solutions. Results indicated that plutonium-gallium alloys were not passive in HNO3 solutions, unlike unalloyed plutonium. The dissolution of 0.5 and 1 wt % Ga alloys exposed to HNO3, and all materials exposed to NaCl, was found to be controlled by the presence of a corrosion product film. Relative oxide thickness measurements of unalloyed Pu exposed to HNO3 were obtained. Chloride was found to aggressively attack Pu and its alloys with increasing chloride concentration resulting in increased corrosion. Nitrate ions were shown to have a weak effect on the corrosion behavior of Pu, as compared to protons and chloride ions.