Unalloyed Plutonium Research Articles

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.

Effect of self-irradiation on the parameters of the α → β transformation of unalloyed plutonium

Effect of self-irradiation on the parameters of the α → β transformation of unalloyed plutonium have been studied with the aid of measurements of the electrical resistance. It was established that the self-irradiation leads to the stabilization of the α phase of plutonium. The observed effect of stabilization is insignificant and is determined, apparently, mainly by the kinetics of growth of the embryos of critical dimension in the initial stage of transformation. The phase recrystallization with heating of the sample to ∼170°C completely removes the effect of stabilization. Supposedly, the kinetics of growth of critical embryos is mainly affected by the atoms of radiogenic helium, which are located in the interstices of the crystal lattice of plutonium.

The solubility of hydrogen and deuterium in alloyed, unalloyed and impure plutonium metal

Pressure-Composition-Temperature (PCT) data are presented for the plutonium-hydrogen (Pu-H) and plutonium-deuterium (Pu-D) systems in the solubility region up to terminal solubility (precipitation of PuH2). The heats of solution for PuHS and PuDS are determined from PCT data in the ranges 350-625°C for gallium alloyed Pu and 400-575°C for unalloyed Pu. The solubility of high purity plutonium alloyed with 2 at.% gallium is compared to high purity unalloyed plutonium. Significant differences are found in hydrogen solubility for unalloyed Pu versus gallium alloyed Pu. Differences in hydrogen solubility due to an apparent phase change are observable in the alloyed and unalloyed solubilities. The effect of iron impurities on Pu-Ga alloyed Pu is shown via hydrogen solubility data as preventing complete homogenization.

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.

Measurement of the heat of α → β transformation of plutonium under pulsed volumetric heating of samples in Yaguar pulsed nuclear reactor

The paper describes the procedure and results of measurement of one of the basic properties of unalloyed plutonium, namely, the heat of α → β transformation. The measurements are performed under conditions of unsteady-state volumetric heating of plutonium in the process of neutron irradiation of samples in a pulsed nuclear reactor (PNR). The experiments are aimed at determining the preassigned degree of phase transformation (PT) of plutonium by the variation of electric resistance of sample measured at one and the same temperature of ∼100 °C in the state of α phase prior to neutron irradiation and in the state of metastable β phase after neutron irradiation, completion of α → β transformation, and cooling down to this temperature of measurement. The desired degree of PT is attained in the process of pulsed volumetric heating of plutonium under neutron irradiation of sample in the channel of the core of Yaguar PNR. The value of fission energy of plutonium nuclei released in the sample is determined by measuring the absolute heating of the working medium of the monitor-detector made of plutonium dioxide and the relative activity of indicators made of uranium-235 oxide and located on the working sample and on the monitor-detector. The results of measurement of energy input and of respective variation of electric resistance due to heating and to partial or complete α → β transformation of plutonium in a sequence of experiments with neutron irradiation of sample in the PNR are used for constructing the dependence of variation of electric resistance of sample on energy input. This dependence is linear in the range of values of measured parameters corresponding to α → β transformation of plutonium. The slope of straight line reflects the ratio of variation of electric resistance in the case of phase transformation to the heat of α → β transformation. It follows from the experimentally measured slope and value of variation of electric resistance in the case of complete phase transformation that the heat of α → β transformation is ΔHαβ = 3.35 ± 0.1 kJ/mol, which agrees well with the adopted reference value of ΔHtr = 3.45 ± 0.08 kJ/mol.

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 solutions, unlike unalloyed plutonium. The dissolution of 0.5 and Ga alloys exposed to , 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 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.

Toward a Deeper Understanding of Plutonium

Plutonium is a very complex element lying near the middle of the actinide series. On the lower atomic number side of Pu is the element neptunium; its 5f electrons are highly delocalized or itinerant, participating in metallic-like bonding. The electrons in americium, the element to the right of Pu, are localized and do not participant significantly in the bonding. Plutonium is located directly on this rather abrupt transition. In the low-temperature {alpha} phase ground state, the five 5f electrons are mostly delocalized leading to a highly dense monoclinic crystal structure. Increases in temperature take the unalloyed plutonium through a series of five solid-state allotropic phase transformations before melting. One of the high temperature phases, the close-packed face centered cubic {delta} phase, is the least dense of all the phases, including the liquid. Alloying the Pu with Group IIIA elements such as aluminum or gallium retains the {delta} phase in a metastable state at ambient conditions. Ultimately, this metastable {delta} phase will decompose via a eutectoid transformation to {alpha} + Pu{sub 3}Ga. These low solute-containing {delta}-phase Pu alloys are also metastable with respect to low temperature excursions or increases in pressure and will transform to a monoclinic crystal structure at lowmore » temperatures via an isothermal martensitic phase transformation or at slightly elevated pressure. The delocalized to localized 5f electron bonding transition that occurs in the light actinides surrounding Pu gives rise to a plethora of unique and anomalous properties but also severely complicates the modeling and simulation. The development of theories and models that are sufficiently sensitive to capture the details of this transition and capable of elucidating the fundamental properties of plutonium and plutonium alloys is currently a grand challenge in actinide science. Recent advances in electronic structure theory, semi-empirical interatomic potentials, and raw computing power have enabled remarkable progress in our abilities to model many of the anomalous properties of Pu. This special issue of the Journal of Computer-Aided Materials Design highlights a number of these advances in the area of the aging of plutonium. This aging is a long-term process due to the slow radioactive decay with a long half-life of 24400 years for the major isotope of plutonium. The challenge then is to predict the changes in properties of plutonium and its alloys from experimental results of plutonium aged only for a few decades and from theory and computational models that are build on a thorough, first-principle understanding of all the complex phenomena displayed by this material. We hope that progress and success of this enterprise will guide other endeavors in Computer-Aided Materials Design and prediction of materials performance.« less

A TTT-diagram for α → β transformation of unalloyed plutonium

The paper offers a variant of the generalized TTT-diagram (temperature–time–transition) for the α → β transformation of unalloyed plutonium under isothermal conditions. The authors constructed it using published information on plutonium phase transformation kinetics under isothermal conditions and α → β transformation parameters under the conditions of pulsed electric heating. The authors also present their results—parameters of the beginning of the α → β transformation in isothermal conditions and in linear-through-volume heating by neutrons from pulsed nuclear reactors exploited at RFNC-VNIITF. Using a number of typical TTT-diagrams as examples, the authors demonstrate characteristic features in the dependence of the phase transformation incubation time on plutonium temperature in isothermal conditions (for example, in samples hold in oil) and in electric heating conditions. They identify segments where, in the authors’ view, phase transformation conditions were not isothermal and justify the use of the most authentic experimental data including those obtained by the authors for isothermal conditions for the construction of the generalized TTT-diagram. To justify the use of their own results obtained for the uniform-through-volume heating of samples by neutrons, the authors used known theoretical results for non-isothermal first-kind transition kinetics. Theoretical results suggest that for certain conditions of the first-kind transition, it is possible to relate the temperature dependences of the incubation time for non-isothermal and isothermal transformations. So, the authors propose a generalized TTT-diagram for a range of plutonium temperatures corresponding to the incubation times of unalloyed plutonium phase transformation between 10 −5 and 10 5 s.

Reaction kinetics of plutonium with oxygen, water and humid air: moisture enhancement of the corrosion rate

Kinetic measurements for the reactions of unalloyed plutonium with water vapor and oxygen in the 35 to 400°C range show that moisture enhancement of the corrosion rate is confined to a temperature regime below 200°C. In combination with kinetic data from literature sources and measurements with air at 25°C, the data define a rate-temperature envelope in which the corrosion reaction is accelerated by increasing water pressure. The lower boundary of the envelope is defined by an Arrhenius relationship based on rates measured in the moisture-independent regime above 200°C and in the moisture-dependent regime at lower temperatures. As defined by experimental data and the equilibrium vapor pressure of water, the upper envelope boundary indicates that moisture dependence is confined to the −25 to 200°C range with a rate maximum near 110°C. Activation energies within the envelope vary systematically with water pressure from 17.9 kcal mol −1 in dry (less than 1 ppm H 2O) air to 34.4 kcal mol −1 in water-saturated air. The corrosion rate is independent of oxygen pressure and has a square-root dependence on water pressure. The chemistry of moisture-enhanced corrosion is defined by results showing that plutonium oxide catalyzes the combination of hydrogen and oxygen. Hydrogen formed by the inherently rapid reaction of water with plutonium catalytically recombines with oxygen at the gas-oxide interface of the reacting metal. Kinetic anomalies attributed to reaction of β-phase plutonium in earlier studies are correlated with the presence of trace amounts of moisture in the experimental systems. Accelerated corrosion in moist argon is discussed and effects of alloying and humidity on the corrosion rate are compared.

Nucleation of the α → β and β → α transformations in unalloyed plutonium

Nucleation and growth of the α → β and the β → α allotropic transformations in unalloyed plutonium have been studied by differential scanning calorimetry for different-sized specimens cycled many times through the transformations. For the β → α reaction the nucleation rate is very low and apparently random. In general, the smaller the specimen, the greater the difficulty for nucleation. The rate of growth of the alpha phase is found to increase with decreasing temperature. The growth rate is 1.6 mm/min at 30°C. For the reverse reaction, α → β, the specimens transform simultaneously independent of size. Also, the reaction nucleates at a lower temperature but grows more slowly on the first cycle. These nucleation effects are considered to be primarily due to residual stresses caused by prior transformations. The value measured for the heat of the transformation, 13.6 ± 0.2 J/g, is lower than previously reported, and this is shown to be because the last 5 to 10% of the β → α reaction requires up to 1000 min and occurs as a continuation of the reaction that is too slow to be measured by the calorimeter.

Differential scanning calorimetry study of the effects of heating rate on the α → β transformation in unalloyed plutonium

The α → β transformation in plutonium is studied as a function of heating rate by differential scanning calorimetry over the range 0.01 °C min −1 to 10 °C min −1. Consistent with the fact that the transformation is controlled by nucleation and growth, the onset temperature and the temperature range of the reaction are shown to depend markedly upon heating rate. By investigating heating rates up to a factor of 100 lower than usual we show: 1. (1) Earlier calorimetric measurements report onset temperatures for the transformation which are too high. 2. (2) The Kissinger analysis, which predicts the scanning rate parameter to vary linearly with reciprocal temperature, does not fit the results of this study. 3. (3) With increased scanning rate, the transformation occurs at higher temperatures, and the growth rate of the β phase at a constant fraction transformed is proportional to this temperature increase.

Characterization of the plutonium-water reaction II: Formation of a binary oxide containing Pu(VI)

Characterization of products formed by reacting unalloyed plutonium with water vapor at 15 Torr pressure shows that Pu 2O 3, PuO 2 and a previously unreported binary oxide containing Pu(VI) form on the metal between 200 and 350 °C. Although the reaction produces a uniformly adherent product layer with a thickness of about 5 μm, localized pits of plutonium hydride cover 15–20% of the oxide-metal interface beneath the layer. Analytical results show that the non-equilibrium product contains layers of three oxide phases at all temperatures. Cubic α-Pu 2O 3 appearing at the metal interface is covered by a layer of dioxide. A third oxide formed at the gas-solid interface is identified by X-ray diffraction and X-ray photoelectron spectroscopy as the mixed-valence phase Pu(IV) 3− x Pu(VI) x O 6+ x . Diffraction data suggest that this oxide crystallizes in a fluorite-related NpH 2+δ or Sm 3H 7(Ca 2YF 7)-type structure. Formation of the higher oxide is supported by X-ray photoelectron spectra, indicating the presence of the Pu(VI) oxidation state. Evaluation of the results yields an oxide composition corresponding to a value of x near 0.5. Estimated thermodynamic data suggests that the oxide is a stable phase in the Pu-O system; experimental observations show that the product is preserved during exposure to air and ultrahigh vacuum. Although formation of PuO 2.2 is expected to be a slow process at low temperatures, reaction of PuO 2 with moisture adsorbed on its surface may generate large quantities of hydrogen if dioxide is stored for an extended period.

Acoustic emission from low temperature phase transformations in plutonium

Acoustic emission has been measured during the α ι β phase transformations in unalloyed plutonium and during the δ → α phase transformation in plutonium-gallium alloys. No acoustic emission was detected during the α ι β transformations, which indicates that these transformations are unlikely to be martensitic. Acoustic emission was produced by the δ → α transformation. The amount of acoustic emission produced per unit volume of material transforming increased with decreasing gallium content and was reduced by prior thermal cycling.

The chemical thermodynamics of nuclear materials VIII. The high-temperature heat capacity of unalloyed plutonium metal

The heat capacity of pure plutonium metal has been determined from 330 to 700 K by adiabatic calorimetry. This includes measurements on the α-, β-, γ-, and δ-phases. A large contribution to the heat capacity, especially in the δ-phase, is due to the electronic heat capacity. A negative anharmonic heat capacity is found for the α-phase. The enthalpies and temperatures of the transitions between these phases were also determined. With the use of thermodynamic quantities on the δ′-, ϵ-, and (l)-phases available from the literature, the thermal functions for pure plutonium metal were calculated to 1000 K.

An analysis of the kinetics of the β → α phase transformation in plutonium

Although a great deal of information has been published on the isothermal transformation characteristics of the β → α reaction in plutonium, the kinetics of the process have hitherto not been analysed in detail. Recently, a model has been proposed to account for the veining phenomenon in unalloyed plutonium. This model required for its success, that the mechanism of the β → α transformation changed in the vicinity of normal ambient temperature. It was proposed that the higher temperature reaction was one requiring thermally activated diffusion, while the lower temperature reaction was diffusionless. In order to test the validity of the model, all the suitable isothermal transformation data pertaining to the β → α phase change have been analysed. These data have been considered in the context of the form of the TTT curves which they give, the activation energy of the transformation and the temperature dependence of the exponent n derived from the equation: log log[ 1 (1 − y) ] = n log t + const , where y = fraction transformed at time = t. The analysis of the available evidence confirms that the mechanism of the transformation changes in the vicinity of normal ambient temperature. However, the kinetic evidence alone is insufficient to enable the nature of the two reaction mechanisms to be unambiguously determined.

The veining phenomenon in unalloyed plutonium

Abstract An investigation has been made of the veining phenomenon in unalloyed plutonium. The surface markings, or veins, which are sometimes seen on α-phase plutonium samples, arise as a result of the β → α transformation. As far as is known, this veining is unrivalled in its scale and form as compared with the solid state surface transformation effects shown by any other metal. The phenomenon has been explained by the application of the Le Chatelier principle to the phase change. In this instance, the large (10%) volume contraction associated with the β → α reaction and the anisotropy of the monoclinic α-phase structure, account for the fact that the veins are so prominent in plutonium. On the basis of the proposed model, the veins can only form at temperatures where the transformation mechanism is non-martensitic.

Bright field grain boundary etching techniques for alpha and delta-phase plutonium

A method for producing a uniform cathodically etched structure on plutonium-alloy specimens mounted in conventional plastic or epoxy resin mounting compounds is described. This technique has been developed because of the advantage of using a dryetching technique in the closed environment of a glove box. In addition, an electromechanical-etching technique is described which gives a bright field grain boundary etch with unalloyed plutonium and many of its alloys. In this method, specimens are electroetched and concurrently polished on a rotating wheel. One electrolyte has been found satisfactory for all the materials investigated, though for optimum results the etching voltage needs to be varied depending upon the specimen composition.

Resistivite electrique des solutions solides d'alliages Pu-Ga en phase δ a tres basses temperatures (4,2–300° K)

The electric resistivity variations of eight roomtemperature delta phase Pu-Ga alloys is studied versus temperature between 5°K and 300°K. After a succinct description of the experimental device and of the characteristics of the samples, the results are expressed as ρ = f(T) curves for each concentration and ( ρ = f (concentration) curves at different temperatures. As for α unalloyed plutonium, the resistivity of each δ phase Pu-Ga alloy displays a maximum and, as for Pu-Al and Pu-Ce alloys, this maximum is closely related to the concentration. From 3 to 10 at %, Pu-Ga alloys do not undertake any phase transformation when they are cooled to 5°K. On the other hand, those with a low concentration (1 and 1.7 at %) transform partially to α phase below 300°K. The a phase obtained is kept under a métastable state at 300°K.

Heat content and heat capacity of a Pu/1 wt % Ga delta-stabilized alloy at elevated temperatures

The heat content and heat capacity of a nominalpurity Pu/1 wt% Ga δ-stabilized alloy were determined between 0 and 685 °C using an ice calorimeter. The experimental curve for enthalpy as a function of temperature exhibited, in the δ region (0 to 510 °C), a positive deviation from linearity. Consequently, the calculated specific heat at constant pressure, C p , increased with increasing temperature from a value of 7.63 cal/g · atom · °C at 0 °C to a value of 10.36 cal/g · atom · °C at 500 °C. Enthalpy as a function of temperature in the narrow ε region (584 to 640 °C) was linear, yielding a constant specific heat of 8.10 cal/g · atom · °C; this value is comparable to the specific heat of unalloyed plutonium in the ε phase. It is felt that the relatively high heat capacities of both the δ and ε phases are a result of a high electronic specific heat contribution. The decrease in going from the δ to the ε phase may be a result of a negative anharmonic contribution for the ε phase. The increase with temperature within the δ field may be due to a steep density-of-states curve and enhanced interband transitions.

The atmospheric oxidation of unalloyed plutonium at low humidities

The rate of oxidation at 90° C of unalloyed plutonium over eleven different desiccants has been measured. The results confirm the sigmoidal shape of the oxidation rate versus relative humidity curve and place the position of the “knee”, which indicates the level below which the humidity must be reduced to obtain a significant decrease in oxidation rate, at about 2%. The vapour pressure (of water) above the desiccants used has been determined at 90° C.