Plutonium Alloys Research Articles

Analysis of differential scanning calorimetry data for aged plutonium.

Differential scanning calorimetry data for samples of a 52 year old plutonium alloy with 3.3 at. % Ga that were heated beyond the melting point is analyzed using transition state theory to find activation energies for the δ to ɛ and ɛ to liquid phase transitions. A Bayesian statistical method involving a Gaussian process model is used to find mean values and confidence intervals for the activation energies. The activation energy for the δ to ɛ phase transition increases by 3.3 ± 3.8% per decade, relative to the case when all age related plutonium lattice point defects have been removed through annealing. The corresponding increase in activation energy for the ɛ to liquid transition is shown to be 7.1 ± 1.8% per decade. It is postulated that the change in activation energy with age for both phase transitions is caused, in part, by the accumulation of the same type of lattice point defects associated with the observed increase in elastic bulk modulus over time.

Characterization of δ-PuGa (1 at%. Ga) Oxidation Under Dry Oxygen Atmosphere Exposure

The oxidation of δ-stabilized plutonium alloy was studied under dry oxygen exposures for temperatures varying from 100 up to 300 °C and oxygen partial pressures varying from 10–4 up to 500 mbar. The coupling of X-ray diffraction, Raman spectroscopy and FIB-SEM has allowed to show that the oxide scale is composed of an outer layer of PuO2 and an inner mixed layer of α + β-Pu2O3 platelets propagating into a metallic zone corresponding to the stable phase of unalloyed Pu. Furthermore, the analysis of Pu oxidation kinetics has displayed first a parabolic growth governed by the diffusion of interstitial oxygen. This step consists of the thickening of the Pu2O3 layer with a decrease in α-Pu2O3 ratio in favor of β-Pu2O3. Then, a second step occurs consisting of a linear growth of the PuO2-layer with the formation of thick nodules which tend to cover the whole oxide surface. Based on the results of this work, a general oxidation mechanism for δ-Pu alloy is provided.

Electrical resistivity of delta phase plutonium alloys

ABSTRACT I present an analysis of the temperature dependence of the electrical resistivity of δ-phase plutonium alloys. I find that the resistivity is mainly proportional to the electronic Kondo heat capacity, with an additional term, linear in temperature, that is correlated with the absolute low-temperature resistivity. This indicates a so-called Mooij correlation, which is often observed in disordered materials. These two terms can be understood together as different contributions to the resistivity, proportional to their respective electronic densities of states. The method presented in this paper provides a simple means of quantifying such resistivity data, and the data for a large number of δ-phase plutonium alloys have been fitted with only four parameters each.

Development of advanced machine learning models for analysis of plutonium surrogate optical emission spectra.

This work investigates and applies machine learning paradigms seldom seen in analytical spectroscopy for quantification of gallium in cerium matrices via processing of laser-plasma spectra. Ensemble regressions, support vector machine regressions, Gaussian kernel regressions, and artificial neural network techniques are trained and tested on cerium-gallium pellet spectra. A thorough hyperparameter optimization experiment is conducted initially to determine the best design features for each model. The optimized models are evaluated for sensitivity and precision using the limit of detection (LoD) and root mean-squared error of prediction (RMSEP) metrics, respectively. Gaussian kernel regression yields the superlative predictive model with an RMSEP of 0.33% and an LoD of 0.015% for quantification of Ga in a Ce matrix. This study concludes that these machine learning methods could yield robust prediction models for rapid quality control analysis of plutonium alloys.

The Taming of Plutonium: Plutonium Metallurgy and the Manhattan Project

We describe the wartime challenges associated with the rapid developments in plutonium chemistry and metallurgy that were necessary to produce the core of the Trinity Device. Beginning with microgram quantities of plutonium metal late in 1943, initial measurements showed a wide and confusing variance in density and other properties. These confusing results were the first clues to the astounding complexity of plutonium. As this complexity was revealed, it introduced new challenges for the fabrication of kilogram-scale parts. In a remarkable period from January 1944 to June 1945, Manhattan Project scientists made rapid progress in understanding plutonium chemistry and metallurgy. By early 1945, they had discovered five of the six ambient-pressure phases of unalloyed plutonium and reported the density of these phases to within a value of 0.1 g/cm3 of those accepted today. They solved the stability problem introduced by these phases with a rapid alloy development program that ultimately identified gallium as the preferred element to stabilize the δ-phase, producing a plutonium alloy still of scientific and technical interest today. We conclude with a description of postwar developments in these areas, including applications of wartime plutonium metallurgy to civilian applications in nuclear reactors. We dedicate this paper to the memory of Ed Hammel, the Manhattan Project plutonium metallurgist whose previous description and documentation of plutonium history during the war has been essential in our research.

Experience of mixed nitride uranium–plutonium fuel fabrication at the siberian chemical plant jsc site

The “Proryv” project is significant for the Russian Federation and relates to the activity that has been assigned the status of “especially important” for the Rosatom State Corporation. It is planned to develop a new generation of nuclear energy technologies based on the closed nuclear fuel cycle of fast neutron reactors with a lead coolant and mixed nitride uranium-plutonium nuclear fuel as a part of this direction (hereinafter – MNUP fuel).The first method for MNUP fuel producing was the direct synthesis of nitrides from starting metals and alloys of uranium and plutonium (hydrogenation-nitration), implemented in laboratory conditions. The disadvantage of this method is its explosion and fire hazard.A method for carbothermal reduction in the atmosphere of nitrogen from the initial oxides of uranium and plutonium is proposed for the production of MNUP fuel on an industrial scale.According to the results of the literature data analysis of considered above methods, carbothermic reduction in a nitrogen atmosphere from the starting oxides of uranium and plutonium is the most optimal for the industrial production of MNUP fuel.Based on the foregoing as part of the «Proryv» project the concept of MNUP fuel production by the carbothermic synthesis method was adopted as the main method at the site of Siberian Chemical Plant (Joint stock company). For this purpose in 2012 a set of experimental facilities (a unique platform for the pilot industrial production of fuel components (from pellets to fuel assemblies) of fast reactors) was created.During this period for the first time in the world at the set of experimental facilities full-scale nuclear fuel assemblies based on MNUP fuel composition were made, data for verifying the technological model of the processes for manufacturing MNUP fuel were prepared, using experimental models of the equipment a complex technological process with experimental scaling of the manufacturing technology of MNUP fuel was developed, an automatic algorithm sorting of tablets and systems for providing and controlling the parameters of the box atmosphere was developed, as well as construction materials of furnaces for the synthesis and sintering of nitride fuel were tested. In total, 24 nuclear fuel assemblies — more than 1000 fuel elements of various designs with different shells — were transferred for loading into the BN-600 reactor. Successfully completed irradiation of 21 nuclear fuel assemblies.

Parabolic oxidation kinetics of a plutonium alloy at room temperature

Auger electron spectroscopy (AES) was used to characterize the oxide scale formed on an ∼0.5 wt. % Ga plutonium metal alloy. Analysis of changes to the characteristic Auger transitions for Pu and O during sputter depth profiles were used in combination with previously measured sputter yields to determine the thickness of the oxide scale and provide insight into its chemistry. From this data, the parabolic kinetics for the Pu-Ga alloy were determined as a function of relative humidity, RH = 14–92%, and oxygen partial pressure, p(O2) = 31–157 Torr, at 25 °C. For each relative humidity investigated, the parabolic rate constant was found to be independent of oxygen partial pressure. Power-law fits to the relative humidity effect on the rate constant was found to have a power of n = 1.265, indicating the diffusion species which rate-limits the reaction isn’t molecular water, which subsequent dissociates at the oxide-metal interface, or hydroxyl from dissociation at the gas-surface interface.

Rapid quantitative analysis of trace elements in plutonium alloys using a handheld laser-induced breakdown spectroscopy (LIBS) device coupled with chemometrics and machine learning.

We present the first reported quantification of trace elements in plutonium via a portable laser-induced breakdown spectroscopy (LIBS) device and demonstrate the use of chemometric analysis to enhance the handheld device's sensitivity and precision. Quantification of trace elements such as iron and nickel in plutonium metal via LIBS is a challenging problem due to the complex nature of the plutonium optical emission spectra. While rapid analysis of plutonium alloys has been demonstrated using portable LIBS devices, such as the SciAps Z300, their detection limits for trace elements are severely constrained by their achievable pulse power and length, light collection optics, and detectors. In this paper, analytical methods are evaluated as a means to circumvent the detection constraints. Three chemometric methods often used in analytical spectroscopy are evaluated; principal component regression, partial least-squares regression, and artificial neural networks. These models are evaluated based on goodness-of-fit metrics, root mean-squared error, and their achievable limits of detection (LoDs). Partial least squares proved superior for determining content of iron and nickel in plutonium metal, yielding LoDs of 15 and 20 ppm, respectively. These results of identifying the undesirable trace elements in plutonium components are critical for applications such as fabricating radioisotope thermoelectric generators or nuclear fuel.

Structure and mechanical properties of Ce-La alloys containing 3-10 wt. % La

This paper describes a study of the microstructure and mechanical properties of a range of cerium-lanthanum (Ce-La) alloys with La contents of between 3 wt. % and 10 wt. %. Ce-La alloys are regarded as non-radioactive surrogates for delta-phase plutonium (δ-Pu) alloys. The Ce-La alloys were characterised using optical microscopy, energy dispersive spectroscopy and X-ray diffraction while the mechanical properties were determined using micro-hardness, ultrasonic velocity and nanoindentation measurements. The presence of La appears to impart a degree of solid solution hardening to the alloys, as indicated by the modest increase in micro-hardness with increasing La content; a small increase in elastic modulus with La content was also seen. The hardness and elastic modulus values of the Ce-La alloys in this study are similar to those of homogenized Pu-Ga alloys in the Ga-lean part of the δ-Pu phase field. Despite the propensity of the Ce-La alloys to undergo oxidation in air, the oxide thicknesses of the alloys, which were determined by energy dispersive spectroscopy, were found to be insufficient to exert a significant influence on the nanoindentation measurements.

Equation of state and strain-induced stabilization of δ-phase stabilized plutonium alloys

An in-situ high-pressure neutron diffraction experiment was conducted on a δ-phase stabilized plutonium alloy with isotope 242Pu. Upon room-temperature compression, neither of the previously reported pressure-induced transformation paths, δ → α′ or δ → γ’ → α′, was observed up to 1.2 GPa. Instead, a drastic reduction in the diffraction intensity of the δ phase was observed when the pressure was above 0.8 GPa. At the highest pressure of the experiment (1.2 GPa), the diffraction data appear to be characteristic of an amorphous state, manifested by the diminishing intensities of all diffraction lines. In addition, no evidence was found to support the transformation to a body-centered tetragonal structure (δ’), which was previously inferred from the diffraction line broadening during initial compression to 0.1 GPa. The discrepancies between the present and previous experiments suggest a substantial strain-induced stabilization of the δ-phase and is presumably attributed to the different stress states in the high-pressure environments. From the pressure - volume measurements, the determined isothermal bulk modulus for the δ-phase is in the range of 31.9 ± 1.3–34.8 ± 1.8 GPa using different pressure scales, comparable to those obtained from the resonant ultrasound spectroscopy measurements of the alloys of similar composition. The pressure-induced elastic softening is neither convinced in the present work, nor can it be resolved from the diffraction experiments if it is intrinsically weak.

The anisotropic growth morphology and microstructure of plutonium hydride reaction sites

Plutonium hydride reaction sites grown on δ-stabilised plutonium alloy have been investigated by a combination of scanning electron microscopy, ion beam milling and cross sectional polishing. The reaction sites are oblate and results indicate that this may be the consequence of failure of the native oxide over the metal surrounding the hydride sites. The interior of the hydride reaction sites has a significantly modified microstructure to that of the surrounding metal. Growth of the hydride into the plutonium metal appears to have a strong relationship with the metallurgical features, causing a discontinuous interface. Possible models for anisotropic growth and formation of a discontinuous interface are discussed.

Effects of self-irradiation in plutonium alloys

In this paper, we present updated results of self-irradiation effects on 238Pu-enriched 239Pu alloys measured by immersion density, dilatometry, and tensile tests. We obtained the self-irradiation equivalent time of nearly 200 years, nearly 100 years longer than in our previous papers. At this extended aging, we find the rate of decrease in density has slowed significantly, stabilizing around 15.73 g/cc, without signs of void swelling. The volume expansion measured at 35 °C also shows apparent saturation at less than 0.25%. Quasi-static tensile measurement still show gradual increase in the strength of plutonium alloys with age.

Impact of compressive pressure on the phase stability and transformation behaviour of δ-phase stabilised plutonium alloys

The phase stability of δ-phase stabilised alloys is typically assessed as a function of time by periodically monitoring the specimens using thermal analytical methods. The specimens are subjected to repeat thermal cycling through phase transformations prior to and following conditioning, designed to test the degree of stability of the alloys.This work examines the impact of thermal cycling on the δ-phase stability of Pu-1at% Ga and Pu-2at% Ga alloys subjected to compressive stresses. Compressive pressure in the range of 100–300MPa induced a significant percentage of α′-phase in the Pu-1at% Ga alloy specimens, whereas no detectable levels of α′-phase were recorded in specimens of the Pu-2at% Ga alloy. However, on thermally cycling the specimens, α′-phase was detected in the Pu-2at% Ga alloy as well. Furthermore, a significant increase in α′-phase was recorded as both alloys were subjected to an increasing number of thermal cycles. The α′-phase increase reached a plateau after a given number of cycles for each of the alloys. The transformation of the α′-phase back to the δ-phase in specimens of both alloys followed an indirect path via the β-phase and γ-phase. Increasing the compressive pressure up to ∼600MPa generated higher levels of α′-phase in the Pu-1at% Ga alloy specimens and resulted in significant levels of α′-phase being recorded in the Pu-2at% Ga alloy specimens. The transformation of the α′-phase back to the δ-phase appeared to follow an indirect path in the Pu-1at% Ga alloy. This work shows new evidence that in the Pu-2at% Ga alloy, the transformation followed both the indirect and direct (α′–δ phase) path.

Phase stability of α′-phase in a Pu-1at% Ga alloy following low temperature treatment

Abstract A δ-phase (fcc) stabilised plutonium alloy will retain typical metallic characteristics of ductility and malleability from ambient temperature to 450 °C. The α′-phase (monoclinic) which is hard and brittle may be induced to form within the stabilised δ-phase depending on the degree of homogeneity of the alloy and the alloying content, thermal treatment and applied pressure. It is therefore of considerable interest to understand the conditions that will allow α′-phase to form and what impact this may have on the δ-phase properties. The scope of this study was to investigate the phase stability of a plutonium 1 at% gallium alloy (Pu-0.274 wt% Ga alloy) following treatment at low temperatures. The transformation of the induced α′-phase back to the δ-phase has been reported to occur either by an indirect path via the β and γ-phases or via both direct α′–δ phase transformation and indirect transformation paths. This work reports how differential scanning calorimetry (DSC) can be advantageously used to differentiate between the different mechanisms and transformation paths. This study also demonstrates that by conducting recovery anneals, inducing isothermal transformation and optimising the heating rates for each transformation, a different mechanism and transformation path may be favoured.

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.

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.

Temperature and concentration dependences of the plutonium-americium alloy resistivity at high temperatures

A many-band model of conductivity is stated in terms of which it is shown qualitatively and quantitatively that interference scattering of conduction electrons may be the main reason for the negative temperature coefficient of resistance in actinide alloys at high temperatures (T > ΘD). The temperature run of the resistivity in such alloys is completely determined by the balance between coherent and incoherent contributions to electron-impurity-phonon scattering. The model is used to analyze the concentration and temperature dependences of the plutonium and americium alloy resistivity.

Microstructural analysis of the δ to α' phase transformation in plutonium alloys using X-ray diffraction

The partial martensitic δ→α' transformation in plutonium alloys is sensitive to chemical composition, sample thermal history, as well as crystalline defects. The present work investigates the δ-Pu phase microstructure before and after the martensitic transformation δ→δ+α'. More precisely, microstructural modifications of the host δ-phase, resulting from the stress induced by a cell volume difference of 19% between the δ and α'-phases, were analysed. Microstructural information about crystallite size and microstrain of a highly homogenized Pu-Ga alloy was extracted from x-ray diffraction patterns using a three dimension crystallite size and microstrain model. This is available in Rietveld refinement software and consists in anisotropic broadening analysis of diffraction peaks. Crystallite size doesn't significantly change with the phase transformation contrary to microstrain that is multiplied, on average, by five. Furthermore, internal normal and shear microstress are multiplied, respectively, by 2 and 13 when α'-phase appears. Last, dislocation densities, calculated from crystallite size and microstrain, are compared to TEM results available in the literature.

Probing Radiation Damage in Plutonium Alloys with Multiple Measurement Techniques

AbstractA material subjected to radiation damage will usually experience changes in its physical properties. Measuring these changes in the physical properties provides a basis to study radiation damage in a material which is important for a variety of real world applications from reactor materials to semiconducting devices. When investigating radiation damage, the relative sensitivity of any given property can vary considerably based on the concentration and type of damage present as well as external parameters such as the temperature and starting material composition. By measuring multiple physical properties, these differing sensitivities can be leveraged to provide greater insight into the different aspects of radiation damage accumulation, thereby providing a broader understanding of the mechanisms involved. In this report, self-damage from α-particle decay in Pu is investigated by measuring two different properties: magnetic susceptibility and resistivity. The results suggest that while the first annealing stage obeys second order chemical kinetics, the primary mechanism is not the recombination of vacancy-interstitial close pairs.

Theory of the residual electrical resistivity of binary actinide alloys

A system of self-consistent equations has been proposed for the coherent potential approximation of the multiband conductivity model for the case of conduction electron scattering from chaotic electric fields of ions of disordered binary alloy components at zero temperature. It has been qualitatively demonstrated that the deviation of the concentration dependence of the residual electrical resistivity of actinide alloys with multiband conductivity from the Nordheim rule is caused by the explicit dependence of the electrical resistivity of the alloy on the magnitude and sign of the real part of the Green’s function at the Fermi level. The derived system of equations for the multiband coherent potential approximation has been used to calculate the concentration dependence of the density of states and the residual electrical resistivity of the alloys of neptunium and plutonium. The results of the calculations have been compared with the available experimental data.