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The anisotropic growth morphology and microstructure of plutonium hydride reaction sites
... Most metals resist general corrosion by forming an oxide film (also called a passive film). However, there are some environments where oxide films are not effective in protecting metals, such as pitting corrosion of stainless steel in an electrolyte 2,3 , or hydrogen corrosion of rare metals, such as cerium 4-6 , gadolinium 7-9 , holmium 10,11 , plutonium [12][13][14] , and uranium [15][16][17][18][19][20][21][22][23][24][25][26] . These rare metals are vulnerable to hydrogen attack when exposed to the low levels hydrogen over extended periods. ...
... A. Problem description Consider a corrosion system 29 consisting of a polycrystalline uranium (U) solid phase coated with an oxide film (mainly UO 2 ) 22,48,49 in the hydrogen atmosphere, and the resulting corrosion product is β-UH 3 at the temperature (513 K) in this work. Fig. 1 illustrates the schematic diagram depicting the process of hydrogeninduced spot corrosion on the surface of U metal in the hydrogen atmosphere 13,22 . The hydrogen-induced spot corrosion on the metal surface undergoes the following four distinct stages 13,29 : ...
... Early growth: The initial precipitate gradually grows in size as a result of the reaction between the metal and hydrogen at the metal-hydride interface. This growth process is accompanied by an outward expansion strain caused by the lattice mismatch between the hydride and the metal, resulting in the formation of a bulge on the metal surface 13,18 . Additionally, it has been observed that the hydride tends to nucleate and grow at the grain boundaries (GBs) 13,20 . ...
Hydrogen-induced multi-spot corrosion on the surface of polycrystalline rare metals is a complex process, which involves the interactions between phases (metal, hydride and oxide), grain orientations, grain boundaries, and corrosion spots. To accurately simulate this process and comprehend the underlying physics, a theoretical method is required that includes the following mechanisms: i) hydrogen diffusion, ii) phase transformation, iii) elastic interactions between phases, especially, the interactions between the oxide film and the hydride, iv) elastic interactions between grains, and v) interactions between hydrogen solutes and grain boundaries. In this study, we report a multiphase-field model that incorporates all these requirements, and conduct a comprehensive study of hydrogen-induced spot corrosion on the uranium metal surface, including the investigation of the oxide film, multi-spot corrosion, grain orientation, and grain boundary in the monocrystal, bicrystal, and polycrystal systems. The results indicate that the oxide film can inhibit the growth of hydrides and plays a crucial role in determining the correct morphology of the hydride at the triple junction of phases. The elastic interaction between multiple corrosion spots causes the merging of corrosion spots and promotes the growth of hydrides. The introduction of grain orientations and grain boundaries results in a variety of intriguing intracrystalline and intergranular hydride morphologies. The model presented here is generally applicable to the hydrogen-induced multi-spot corrosion on any rare metal surface.
... Based on experimental methods, the plutonium hydrogenation process is divided into four stages: incubation period, nucleation period (aggregation period), block hydrogenation, and end period [13,14]. The incubation period is considered the key to determining the progress of the entire process, which can be divided into six stages [5]. ...
... Coatings 2024, 14, 195 ...
Based on density functional theory, a first-principles study of the adsorption behavior of hydrogen atoms on the PuO2(111) surface is carried out in this work. Models for three different surface morphologies of PuO2(111) are established. It is found that the surface with the outermost oxygen atom (sub outer Pu atom) morphology has the best stability. Based on this model, the adsorption energy, bader charge, and electronic density of the states of a hydrogen atom at different adsorption sites are calculated. Finally, we analyzed the process of hydrogen dissociation into hydrogen atoms on the surface using the cNEB method. The results indicate that the top position of the outermost oxygen atom and the bridge position of the second outermost plutonium atom are relatively stable adsorption configurations, where hydrogen atoms lose electrons and release heat, forming O-H bonds with oxygen atoms. The density of states of O-p orbital electrons will undergo significant changes, reflecting the hybridization of O-p and H-s orbital electrons, forming a stable bonding effect. The dissociation of hydrogen molecules into two hydrogen atoms adsorbed on the top of oxygen atoms requires crossing an energy barrier of 1.06 eV. The decrease in total energy indicates that hydrogen tends to exist on the PuO2(111) surface in a hydrogen atom state. The research results lay the foundation for theoretically exploring the hydrogenation corrosion mechanism of the PuO2(111) surface, providing theoretical support for exploring the corrosion aging of plutonium oxide, predicting the material properties of plutonium oxide under extreme and special environments.
... The hydride growth on the metallic surface is a significant form of pitting corrosion, which can cause the destruction of materials. Hydride corrosion often occurs on the surface of rare earth metals, such as cerium [1][2][3], gadolinium [4][5][6], holmium [7,8], uranium [9][10][11], and Plutonium [12][13][14] etc, which can form stable metal-hydride phases [15][16][17][18] when they are exposed to low level hydrogen environment for a long time. A number of transition metals, for example, titanium [19,20], zirconium [21][22][23], are also known to form similar surface-breaking hydrides by the reaction of hydrogen and metals. ...
... The hydride corrosion kinetics on metallic surface generally involves the interaction between passive film, metal and hydride [9,11,12]. Composed of metal oxide, the passive film covers the metallic interface, which not only affects adsorption/chemisorption of the hydrogen but generally acts as a diffusion barrier to hydrogen [24]. Therefore, the damage of the passive film often makes the metallic surface partially exposed to the environment, resulting in a faster hydrogen adsorption and diffusion rate [13]. ...
The hydride growth on metallic surface can cause material failure, which is a significant form of pitting corrosion. A new multiphase-field model is constructed to study the hydride corrosion kinetics, in which three order parameters are introduced to represent the passive film, hydride and metal phase, respectively. Coupling with hydrogen concentration field and elastic strain field, this model not only presents the growth of hydride and the rupture of passive film, but also reveals the hydrogen diffusion mechanism and the effect of strain energy on pitting corrosion process. The simulation shows the semi-ellipsoidal cerium hydride forms and grows near the passive film/cerium interface. During this process, the passive film on the upper side of the hydride is also hydrogenated or peeled off, resulting in faster hydrogen transport, which in turn promotes the growth of hydride. The formation of cerium hydride causes volume expansion, and the strain energy is mainly distributed around the hydride, which inhibits its growth. The present study contributes to understanding the formation mechanism of hydride corrosion at mesoscale, especially the pitting corrosion kinetics of rare earth metals.
... When plutonium interacts with H 2 , H 2 O and hydrogen containing impurities (such as rubber, plastic, etc.) in environmental atmosphere, it is easy to form plutonium hydrides. 8 Due to the very active chemical properties of plutonium and the zero activation energy in the hydrogenation process, the reaction is completed in an instant, and once the hydride is formed, it will act as a catalyst to accelerate its corrosion rate in room temperature air. ...
... 17 When there is PuH x on the surface of plutonium, it will accelerate the aging of plutonium and play a catalytic role. 8 The growth and cracking process of H 2 on the surface and inside of plutonium was revealed by SEM. 18 Because it is difficult to carry out the research of plutonium hydride experimentally, the theoretical research reports on the plutonium hydrogen system have been increasing in the past decade, e.g., the structure, electronic structure and dehydrogenation behavior of plutonium hydrides. ...
The density functional theory (DFT) and DFT plus correction for on-site Coulomb interaction (DFT+U) method were performed to investigate the adsorption and dissociation of H2 on PuH2 (100), (110) and (111) surfaces. Overall, the H2 molecule can be adsorbed on the PuH2 surface without spontaneous dissociation. The calculated H–H bond lengths (RH–H) are all elongated to different degrees, and the RH–H at different adsorption sites is about 0.84–4.21% longer than in the gas phase. We found that the dissociation of H2 on the (110) surface is a spontaneous exothermic process, and a total energy of 0.60 eV is released in the whole process. The smaller barriers corroborate that the migration of an H atom on the PuH2 surface is possible, and even spontaneous diffusion may occur. The spontaneous migration of a hydrogen atom adsorbed on the (110) surface from the surface to the interior promotes the conversion of PuH2 to PuH3, which may be the fundamental driving force of hydrogenation corrosion. Our results provide useful information to explain the mechanism of hydrogenation corrosion on the PuH2 surface.
... For clarity the fitted peak positions are shown Figure 7b-c) for a line profile, shown in Figure 7a), across the interface between the metal and the oxide. The peak position of the P3 edge, Figure 7b), shifting from 16.6eV (oxide) to 17.5eV (metal) shows a distinct difference in the environment of the uranium metallic and ceramic and is in line with the observation of others [10,11]. This is mimicked in the uranium O4,5 edges measured at 111.4eV (oxide) and 112.2eV (metal) [12]. ...
... This evidence then suggests a hypothesis where the uranium is largely consumed from the side, i.e. for instance from a grain boundary. Hydriding of metal at grain boundaries is not a novel idea and examples have been identified in terms of initiation at grain boundaries [14,15,16] and also propagation of reaction fronts in the related plutoniumhydrogen reaction [17,18]. ...
This paper reports experiments investigating the reaction of H$_{2}$ with uranium metal-oxide bilayers. The bilayers consist of $\leq$ 100 nm of epitaxial $\alpha$-U (grown on a Nb buffer deposited on sapphire) with a UO$_{2}$ overlayer of thicknesses of between 20 and 80 nm. The oxides were made either by depositing via reactive magnetron sputtering, or allowing the uranium metal to oxidise in air at room temperature. The bilayers were exposed to hydrogen, with sample temperatures between 80 and 200 C, and monitored via in-situ x-ray diffraction and complimentary experiments conducted using Scanning Transmission Electron Microscopy - Electron Energy Loss Spectroscopy (STEM-EELS). Small partial pressures of H$_{2}$ caused rapid consumption of the U metal and lead to changes in the intensity and position of the diffraction peaks from both the UO$_{2}$ overlayers and the U metal. There is an orientational dependence in the rate of U consumption. From changes in the lattice parameter we deduce that hydrogen enters both the oxide and metal layers, contracting the oxide and expanding the metal. The air-grown oxide overlayers appear to hinder the H$_{2}$-reaction up to a threshold dose, but then on heating from 80 to 140 C the consumption is more rapid than for the as-deposited overlayers. STEM-EELS establishes that the U-hydride layer lies at the oxide-metal interface, and that the initial formation is at defects or grain boundaries, and involves the formation of amorphous and/or nanocrystalline UH$_{3}$. This explains why no diffraction peaks from UH$_{3}$ are observed. {\textcopyright British Crown Owned Copyright 2017/AWE}
... A particular example is that of the reaction of hydrogen with plutonium metal in storage environments [4,[10][11][12][13][14]. This reaction is thought to be governed exclusively by the barrier diffusion process [15] in the earliest stages of the reaction, such as during the "induction phase" [16,17]. The reaction of hydrogen with plutonium can produce hazardous plutonium hydride [18][19][20][21][22]. Thus, understanding the relationship between PuO 2 defects and the transport of hydrogen through this oxide allows for a better prediction of how much plutonium hydride might be produced in storage. ...
We have examined a range of point defects, Frenkel pairs, Schottky defects, and hydrogen-related defects in the PuO2 system (supercells of 96 and 768 atoms) using the ONETEP linear-scaling density functional theory code. Vacancy point defects related to oxygen are found to be more stable than those related to plutonium. The oxygen in the octahedral interstitial is higher in the formation energy than the plutonium in the same octahedral site, although the difference is less than 1 eV. We were also able to identify a stable peroxide species (1.57–2.67 eV) with a O-O distance of 1.46 Å. Of the Frenkel defects we studied, we found that the oxygen is more stable than the plutonium, whereas the Schottky stability changes as a function of supercell size. Finally, we examined a number of likely hydrogen sites in the PuO2 lattice: octahedral interstitial, oxygen edge, hydroxyl, oxygen vacancy, and plutonium vacancy. We report hydrogen which exists as a hydride at oxygen and plutonium vacancies to be relatively high in energy (2.69–3.81 and 13.71–15.54 eV, respectively). The hydrogen was found to exist as a radical at the octahedral interstitial site (2.43–3.38 eV) and which is somewhat higher formation energy than other studies find. We find that the hydrogen at the oxygen edge (as a H+ cation) and at the oxygen cube corner (as a hydroxyl) are both lower in energy (1.14–1.40 and 1.17–1.56 eV, respectively) as opposed to hydrogen in the octahedral interstitial site but again higher than found by other studies. We discuss the data in the context of potential hydrogen transport pathways and how that might be modified by radiation damage.
... The presence of oxide lm on the surface of plutonium leads to four stages of hydrogen corrosion of plutonium: induction, nucleation or acceleration, bulk hydriding, and termination. [9][10][11] The induction period mainly involves the interaction between the hydrogen and oxide layer, which is time-consuming and controllable. 12 The induction period is key to the entire hydrogenation corrosion process. ...
The interface is a region in the crystal that significantly changes various characteristics. There must be an interface between oxides of different valence states in the surface oxide layer of plutonium. In this work, a first principles approach based on DFT was used to study the hydrogen distribution and diffusion at the PuO2/α-Pu2O3 interface systematically. Our research reveals that at the interface, hydrogen can be captured by the O atoms of PuO2 and by the oxygen vacancies (OVs) of α-Pu2O3, and the capture of OVs is more energetically advantageous. On the PuO2 side, the cost of H atom diffusion towards the interface gradually increases. On the α-Pu2O3 side, the cost of H atoms diffusing inward from the interface gradually increases. OVs that already contain H atoms are more conducive to capturing H atoms. The formation of the interface has little effect on the hydrogen capture ability of O in PuO2, but it will reduce the capture ability of OVs in α-Pu2O3. Overall, the formation of interfaces has no disruptive impact on the behavior of hydrogen in the two plutonium oxides. This is closely related to the fact that α-Pu2O3 originates from PuO2 under anaerobic conditions. The difference in hydrogen behavior comes from the changes in the atomic environment and ion valence state caused by the OVs. This work supports further understanding of the behavior of hydrogen in plutonium oxides and provides a reference for further research on plutonium corrosion prevention.
... [2][3][4][5][6][7][8] During long-term storage, plutonium metal can produce free hydrogen from materials containing hydrogen through (1) plutonium reacts with outgassed water vapor (Pu + 2H 2 O → PuO 2 + 2H 2 ) and (2) radioactive decomposition of organic packaging. 1,9 The high chemical activity of plutonium also makes its surface often covered with an oxide layer. [10][11][12][13] Hydrogenation corrosion can catalyze the speedy conversion of solid metals into oxide powders. ...
Studying the effect of coexistence of CO2 and H2 on plutonium hydriding is of great significance for nuclear safety storage and disposal. In this work, we studied the microscopic adsorption morphology of CO2 and H2 molecules with low and high coverage on stoichiometric PuO2 (111) and (110) surfaces. The adsorption energy results showed that both CO2 and H2 have relatively strong reactivity with the (110) surface. The CO2 molecule may be dominant in competitive adsorption with H2. The influences of the coexistence of CO2 and H2 on the adsorption and dissociation behavior for H2 on stoichiometric and defective surfaces were further researched. The CO2 adsorption configuration on the defect surface reveals that the O atom attempts to “heal” the oxygen vacancy. The results show that the presence of CO2 can weaken the interface interaction between H2 and the surface, and increase the H2 dissociation energy barrier on the surface from about 0.518 eV to about 0.791 eV. The electronic properties and work function show that the adsorbed CO2 hinders the electron interaction between H2 and surface resulting in the blocking of hydrogen adsorption and dissociation, which may be the reason for inhibiting the hydrogenation of plutonium. Our study could provide new insights into the CO2 effect on the hydriding process of active metals.
... [7][8][9][10] The hydriding corrosion of Pu is catastrophic, because Pu-hydride can violently accelerate oxidation to pyrophoricity of Pu. [5][6] According to the diffusion barrier model, Pu-hydriding process can be divided into four periods: induction, nucleation or acceleration, bulk hydriding and termination. [11][12][13] The sequence of main steps in the induction period is usually proposed as: (i) H2 physisorption and dissociative chemisorption on Pu-oxide surface; ...
The in-depth understanding of hydrogen permeation through plutonium-oxide overlayers (mainly PuO$_{2}$ and $\alpha$-Pu$_{2}$O$_{3}$) is the prerequisite to evaluate the complex hydriding induction period of Pu. In this work, hydrogen absorption, diffusion and dissolution in $\alpha$-Pu$_{2}$O$_{3}$ are investigated by the first-principles calculations and $\textit{ab initio}$ thermodynamic method based on DFT+U and DFT-D3 schemes. Our researches reveal that H and H$_{2}$ absorption in porous $\alpha$-Pu$_{2}$O$_{3}$ are endothermic, and H atoms prefer recombining to H$_{2}$ molecules rather than bonding with $\alpha$-Pu$_{2}$O$_{3}$. The individual H and H$_{2}$ diffusion are both feasible, and the main diffusion barriers are 0.75 eV and 0.68 eV, respectively. Generally, H will recombine first as H$_{2}$ and then migration, which is opposite to H$_{2}$ first dissociation, then diffusion in PuO$_{2}$. Due to endothermic absorption and diffusion, hydrogen dissolution in $\alpha$-Pu$_{2}$O$_{3}$ needs such impetuses as pressure P$_{H2}$ and temperature. At 300K, H$_{2}$ starts to dissolve when P$_{H2}$ reaches to 3.46$\mathtt{\times}$10$^{5}$ Pa. Then the polymolecular and (H+H$_{2}$) mixed dissolution will successively appear when increasing P$_{H2}$, which also can be promoted by temperature. This work has presented a fundamental picture of hydrogen behaviors in $\alpha$-Pu$_{2}$O$_{3}$, which shall be reasonably described by the ideal solid solution model.
Hydrides of actinides, their magnetic, electronic, transport, and thermodynamic properties are discussed within a general framework of H impact on bonding, characterized by volume expansion, affecting mainly the 5f states, and a charge transfer towards H, which influences mostly the 6d and 7s states. These general mechanisms have diverse impact on individual actinides, depending on the degree of localization of their 5f states. Hydrogenation of uranium yields UH2 and UH3, binary hydrides that are strongly magnetic due to a 5f band narrowing and reduction of the 5f-6d hybridization. Pu hydrides become magnetic as well, mainly as a result of the stabilization of the magnetic 5f5 state and elimination of the admixture of the non-magnetic 5f6 component. Ab-initio computational analyses, which for example suggest that the ferromagnetism of β-UH3 is rather intricate involving two non-collinear sublattices, are corroborated by spectroscopic studies of sputter-deposited thin films, yielding a clean surface and offering a variability of compositions. It is found that valence-band photoelectron spectra cannot be compared directly with the 5fn ground-state density of states. Being affected by electron correlations in the excited final states, they rather reflect the atomic (n-1) multiplets. Similar tendencies can be identified also in hydrides of binary and ternary intermetallic compounds. H absorption can be used as a tool for fine tuning of electronic structure around a quantum critical point. A new direction is represented by actinide polyhydrides with a potential for high-temperature superconductivity.
The interfacial reaction of highly active plutonium hydride in humid circumstance is of great interest in nuclear safe handling and storage, but it is poorly understood so far. In this paper, we first studied the O2 adsorption on (110) and (111) surfaces of PuH2 by first-principles DFT + U method. The results show that there are dissociative and non-dissociative adsorption of oxygen on the surfaces. We analyze the vibrational frequencies of non-dissociative oxygen adsorbed on the surfaces. It is found that the corresponding frequency of oxygen with bond length of 1.330–1.340 Å is 1094.8–1098.2 cm⁻¹. The corresponding frequency of oxygen with bond length of 1.448–1.500 Å is 726.3–905.2 cm⁻¹. It shows that non-dissociative oxygen could be considered as superoxide (O2⁻) or peroxide (O2²⁻) species. In order to expound the atomistic evolution process of oxidized surface exposed to moist air or corrosive solution, the interactions between H2O molecules and the strongest oxygen adsorption structures were further explored. The results indicate that H2O molecules could dissociate into OH groups and H atoms, then they were captured to create Pu–O and H–O bonds. This work could provide new insights into the adsorption morphology of oxygen on hydride surface and the interaction between oxide/hydride interface and water.
Hydrogen-induced pitting corrosion of uranium is a common phenomenon, which damages the integrity and durability of this widely and expensive nuclear material. Its numerical simulation is still a challenge due to many complex mechanisms, especially solid–solid phase transformation and mechanical interaction, leading to the anisotropic growth of hydride and inducing some bulges on the surface of the uranium matrix. We apply a phase-field model to simulate hydrogen-induced pitting corrosion in α-uranium (α-U) at 673 K. In the model, the elastic strain energy is introduced to approximate the mechanical interaction between the matrix and hydride, and the free boundary condition based on the finite element method is adopted to describe the surface bulges. In our simulation, the elliptical pit morphology and its kinetics are consistent with experiment results. An analysis of displacements and stresses inside the materials reveals that the surface bulge is caused by the compression of the matrix and the dilation of the hydride. Moreover, the compressive and tensile stress concentrations are observed around the edge and top of surface bulge respectively. This phenomenon is due to the dilation and growth of hydride on the matrix. Our study provides important simulating evidence and physical interpretations for the growth of pit morphology and failure of α-U materials. This work also shows the potential of the phase-field model in the simulation of hydrogen-induced pitting corrosion.
The hydriding corrosion is harmful to the long-term storage of plutonium (Pu). Since the defect-free PuO2 overlayer has been proved to resistance hydrogen attack, Pu hydriding is mainly related to such penetrating defects as the grain boundaries (GBs) of PuO2. We perform comprehensive DFT + U-D3 calculation and tensile-test simulation to investigate the incorporation and dissolution behaviors of H in PuO2 GB, and the induced hydrogen damage. The exothermic incorporation and dissolution of H confirm the capture effect of PuO2 GB, which are contrary to endothermic behaviors of H in PuO2 bulk. The predicted saturation solubility of H demonstrates that GB is the active site that promote H dissolution, which is one of the important factors to Pu hydriding. The dissolved H can further weaken the Pu–O bonds in GB and facilitate the intergranular fracture of PuO2 overlayer. Our study provides key mechanistic insights towards interpreting of Pu hydriding corrosion, which is critical for predictive modeling of the induction time.
A brief overview of the impact of presence of hydrogen in metallic systems on magnetic properties is given. Respective differences between the 4 f, 5 f, and 3d system are emphasized using examples of materials with similar crystal structure. The analysis is based on general aspects of H bonding in metallic environment, H expanding volume and polar bonding of H. Electronegativity is used as a convenient indicator quantifying the polar bonding character.
The reactions of cerium in hydrogen mixed with different concentrations of CO2 were conducted by using a pressure-volume-temperature (PVT) method combined with an in situ hot-stage microscope. CO2 has a great suppression effect on the hydriding reaction of cerium. The existence of CO2 leads to longer induction time, less pitting, and much slower hydriding rate. It is found that the induction time of hydriding reaction grows exponentially as a function of CO2 concentrations. Besides, the nucleation and growth of hydride spots are also suppressed with increase of CO2 concentrations. Scanning electron microscopy (SEM) and in situ optical microscope measurements reveal that the hydride spots formed in H2-CO2 mixture have a prolate hemispherical morphology while the normal hydride spots formed in pure H2 show an oblate hemispherical morphology. Thermal desorption results indicate CO2 preferentially adsorbs on the active sites of CeO2 surface resulting in block of hydrogen dissociation, which is accountable for the observed hydriding inhibition phenomenon. Our studies provide new insights into the CO2 effect on the hydriding process of active metals.
The reaction between water and plutonium hydride plays a significant role in the oxidation and corrosion rate of plutonium materials. The adsorption and dissociation of water molecule on PuH2 (110) surface have been studied by first-principles GGA+U method. To search the more favorable adsorption sites and adsorption types of H2O in terms of energy, the adsorption energies of H2O on different active sites were compared. The results reveal that the adsorption of H2O molecule on the surface of PuH2 (110) basically presents chemical adsorption, while the adsorption direction of H2O molecule parallel to the PuH2 (110) surface appears to be preferentially adsorbed on the top of Pu atom. We analyze the vibrational frequencies of H2O molecule adsorbed on the surface. It is found that there is a red shift phenomenon under the action of surface atoms. Two possible dissociation pathways of H2O molecule on PuH2 (110) surface were studied by the climbing-image nudged elastic band (CI-NEB) method. The calculation result shows that the more favorable dissociation energy barrier is about 0.406 eV. This indicates that the dissociation of H2O molecule on the surface of PuH2 (110) is favorable, even at room temperature.
The in-depth investigation of hydrogen behaviors in Pu-oxide overlayers (mainly PuO2 and α-Pu2O3) is critical for modeling the complex induction period of Pu hydriding. Within density functional theory (DFT) + U + D3 schemes, our systematic first-principles calculations and ab initio thermodynamic evaluations reveal that the hydrogen incorporation, dissolution behaviors, and diffusion mechanism in PuO2 are quite different from those in α-Pu2O3, among which the highly endothermic incorporation and dissolution of hydrogen are the primary hydrogen resistance mechanism of PuO2. Since its difficult recombination, atomic H is the preferred existence state in PuO2, but H will recombine spontaneously in α-Pu2O3. In PuO2, H diffusion is always clinging to O anions, whereas in α-Pu2O3, H2 prefers to migrate along O vacancies with higher barriers. H dissolution in intact PuO2 is very difficult, which can only be driven by extremely high pressure PH2 and temperature. Based on a series of theoretical studies, we conclude that the main interactions between hydrogen and Pu-oxide overlayers are not involved with chemical reactions, and intact PuO2 can effectively inhibit hydrogen permeation.
The in-depth understanding of hydrogen permeation through plutonium-oxide overlayers is the prerequisite to evaluate the complex hydriding induction period of Pu. In this work, the incorporation, diffusion and dissolution of hydrogen in α-Pu2O3 are investigated by the first-principles calculations and ab initio thermodynamic method based on DFT+U and DFT-D3 schemes. Our study reveals that the hydrogen incorporation is endothermic and the separated H atoms prefer to recombine as H2 molecules rather than reacting with α-Pu2O3. The H and H2 diffusion are both feasible, generally, H will recombine first as H2 and then migrate. Both pressure PH2 and temperature T can promote the hydrogen dissolution in α-Pu2O3. The single H2 molecule incorporation and (H + H2) mixed dissolution will successively appear when increasing PH2. Compared to PuO2, this work indicates that Pu sesquioxide is hardly reduced by hydrogen, but the porous α-Pu2O3 facilitates hydrogen transport in Pu-oxide layers. We present the microscopic picture of hydrogen behavior in the defect-free α-Pu2O3, which could shed some light on the study of the hydriding induction period of Pu.
The high-pressure processes of plutonium hydrides are usually involved in many practical situations, such as the volume expansion during the hydride formation, the pressure of helium bubbles due to the decay of plutonium, the stress due to the formation of the oxide film. These cases could lead to the high-pressure phase transition of plutonium hydrides; and thereby affecting the properties of the material. The crystal structure and bonding properties of plutonium hydrides (PuH1−10) under atmospheric pressure and high pressure are investigated by using the first-principle method in combination with a structure searching technique. Our results show that the predicted lattice structures of plutonium hydrides are stable over the given pressure range. A novel stable stoichiometry, PuH with space group , is thermodynamically, dynamically, and mechanically stable in the pressures range of 62−188 GPa, which may be synthesized through the pressure-induced disproportionation of PuH2. In particular, our theoretically predict results indicating that PuH3 undergoes pressure-induced phase transitions with the following sequence of phases: P63cm →Pnma→ R-3m→Cmcm, and the corresponding transition pressures are computed to be 4, 76 and 150 GPa, respectively. Interestingly, the most striking feature of PuH3 is the pressure-induced transition from insulating to metallic forms. Analysis of the electric structures, charge density differences and electronic localization functions indicates that all phases act mainly as metallic with ionic bonding at high pressure.
In order to capture the occupation numbers as well as the dual behaviors for Pu 5f electrons in two typical plutonium hydrides (PuH2 and PuH3), we perform a first principles calculation on electronic properties of these two systems using a many-body method by combination of density functional theory (DFT) with dynamical mean field theory (DMFT) including the on-site Coulomb repulsion between Pu 5f states and spin-orbit coupling (SOC) effect. Results demonstrate that both PuH2 and PuH3 have dual 5f states, i.e., the localized and itinerant regimes, with average 5f occupations of 4.968 and 4.989, respectively. Spectrum function shows that plutonium hydrides will transform from metallic (PuH2) to semi-conducting (PuH3) states with the increasing of H content, while Pu 5f j = 5/2 and j = 7/2 manifolds both exhibit the insulating behaviors. Finally, the density of states (DOS), hybridization function as well as momentum-resolved electronic spectrum function are also discussed.
Hydriding reaction kinetics of cerium-lanthanum alloy was studied by the pressure-volume-temperature (PVT)method combined with in-situ microscope. The hydride nucleation and the H 2 consumption rate approximately follow an exponential model at the initial time. Furthermore, the H 2 consumption rate and the hydride nucleation were accelerated by the doped lanthanum (1–10%). The growth rate of the hydride sites approximately follows a linear law and it is not affected by the lanthanum content. An approximate parabola relation between the induction time and lanthanum content is evident. The hydriding reaction rate is accelerated by the doped lanthanum, which manifest as the accelerated H 2 consumption rate, decreased induction time and accelerated nucleation. The results of XRD, Raman spectroscopy and Auger electron spectroscopy (AES)show that the oxide layer is constituted of CeO 2 layer and nonstoichiometric transition layer, and the doped lanthanum can increase the concentration of the oxygen vacancies. The oxygen vacancies increase the active sites to accelerate the dissociation of absorbed hydrogen molecule and offers additional short-circuit diffusion paths to accelerate the transport of hydrogen atom in the oxide layer.
We perform a first principle calculation on different magnetic configurations of PuH3 system to describe this system at electronic level using local density approximation and generalized gradient approximation (GGA) within the framework of density functional theory inclusion of spin polarization (SP), on-site Coulomb repulsion U and spin–orbit coupling (SOC) effect, which are essential for strongly correlated system. The results indicate that Coulomb repulsion and SOC effects are necessary to correctly capture the electronic, magnetic and lattice properties of PuH3, however, SP has a negligible effect on lattice parameter calculations. SP + GGA + U + SOC (5.3420 Å) and GGA + U + SOC calculations (5.3438 Å) on ferromagnetic order (FM) are in good agreement with experimental value (5.34 Å) and full-potential linearized augmented plane wave calculation (5.343 Å). FM magnetic order and total magnetic moment for SP + GGA + U + SOC method are also consistent with experimental and calculation results.
One of the important research contents on hydrogen corrosion of plutonium is the determination of the complex crystal structures of plutonium hydrides and the bonding interactions between plutonium and hydrogen. However, it is very difficult to carry out the structural characterization of plutonium hydrides due to their high activity, high toxicity and radioactivity. In this work, the crystal structures, lattice vibrations and bonding properties of plutonium hydrides under ambient pressure are investigated by means of the density functional theory (DFT) + U approach. Results show that the PuH3 exists many competition phase structures. After considering spin polarization, strong correlation (U), and spin-orbit coupling (SOC) effects on the total energy and lattice dynamics stability, it is found that PuH3 at ambient pressure is more likely to be hexagonal P63cm or trigonal P3c1 structure, instead of the usual supposed structures of hexagonal P63/mmc structure (LaF3-type) and face centered cubic (BiF3-type). The calculated electronic structures clearly indicate that P63cm (P3c1) PuH3 is a semiconductor with a small band gap about 0.87 eV (0.85eV). The Pu-H bonds in Pu hydrides are dominated by the ionic interactions.
Plutonium (Pu) can react with hydrogen to form complicated continuous solid solutions with unusual chemical and physical properties. The PBE0 hybrid density functional under the framework of full-potential linearized augmented plane wave plus local orbitals is employed to investigate the structural, magnetic, lattice vibrations, and thermodynamic properties of face-centered cubic plutonium hydride (PuH2+x, x = 0, 0.25, 0.5, 0.75, 1). The decreasing trend with increasing x of the optimized lattice parameters is in reasonable agreement with experimental findings. According to the calculated formation enthalpies of PuH2+x compounds, all PuHx for both ferromagnetic(FM) and antiferromagnetic (AFM) phase are thermodynamically favorable, and the FM phase plutonium hydride is more favorable than the AFM phases. The characteristic Raman-active and the infrared-active modes at the center (Γ point) of the first Brillouin zone were further assigned and discussed. Finally, the free energy F, internal energy E, vibration enthalpy S, and constant-volume specific heat CV of PuH2+x are calculated in the range of 0–1000 K.
The chemistry and kinetics of the pyrophoric reaction of the plutonium hydride solid solution (PuHx, 1.9 ≤ x ≤ 3) are derived from pressure-time and gas analysis data obtained after exposure of PuH2.7 to air in a closed system. The reaction is described by two sequential steps that result in reaction of all O2, partial reaction of N2, and formation of H2. Hydrogen formed by indiscriminate reaction of N2 and O2 at their 3.71:1 M ratio in air during the initial step is accommodated as PuH3 inside a product layer of Pu2O3 and PuN. H2 is formed by reaction of O2 and partial reaction of N2 with PuH3 during the second step. Both steps of reaction are described by general equations for all values of x. The rate of the first step is proportional to the square of the O2 pressure, but independent of temperature, x, and N2 pressure. The second step is a factor of ten slower than step one with its rate controlled by diffusion of O2 through a boundary layer of product H2 and unreacted N2. Rates and enthalpies of reaction are presented and anticipated effects of reactant configuration on the heat flux are discussed.
First-principles DFT + U methods are performed to calculate the formation energy and to determine the relative stability of hydrogen at the different sites of UO2 and PuO2. Twenty-one incorporation sites for hydrogen, i.e., along the pathway from its first nearest-neighboring oxygen to the octahedral interstitial site, are considered. The results indicate that hydrogen in UO2 energetically prefers to exist as a hydride ion ([(UO2)n]⁺H⁻) rather than forming a hydroxyl group ([UnO2n-1]⁺[OH]⁻). The negative formation energy of hydrogen at the octahedral interstitial site of UO2 shows that hydrogen is soluble and can oxidize uranium ion to the higher valence states. However, hydrogen in PuO2 is relatively stable in the form of [PunO2n-1]⁺[OH]⁻ with comparison to [(PuO2)n]⁺H⁻. The slightly positive formation energy of hydrogen in the form of the hydroxyl group in PuO2 reveals that hydrogen is either insoluble or just lies on the edge of solubility. The differences in the existence states of atomic hydrogen in the two dioxides are proposed to be dependent on the nature of 5f electrons of uranium and plutonium; that is, uranium 5f electrons are more delocalized and more favorable to participate in chemical bonding than plutonium 5f electrons.
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 element plutonium occupies a unique place in the history of chemistry, physics, technology, and international relations. After the initial discovery based on submicrogram amounts, it is now generated by transmutation of uranium in nuclear reactors on a large scale, and has been separated in ton quantities in large industrial facilities. The intense interest in plutonium resulted from the dual-use scenario of domestic power production and nuclear weapons – drawing energy from an atomic nucleus that can produce a factor of millions in energy output relative to chemical energy sources. Indeed, within 5 years of its original synthesis, the primary use of plutonium was for the release of nuclear energy in weapons of unprecedented power, and it seemed that the new element might lead the human race to the brink of self-annihilation. Instead, it has forced the human race to govern itself without resorting to nuclear war over the past 60 years. Plutonium evokes the entire gamut of human emotions, from good to evil, from hope to despair, from the salvation of humanity to its utter destruction. There is no other element in the periodic table that has had such a profound impact on the consciousness of mankind.
In 2005, approximately 2000 metric tons of plutonium exist throughout the world in the form of used nuclear fuel, nuclear weapons components, various nuclear inventories, legacy materials, and wastes (Albright and Kramer, 2004). This number grows every year by 70 to 75 metric tons through production in irradiated nuclear fuels (Albright and Kramer, 2004). It is clear that the large inventories of plutonium must be prudently managed for many centuries. A complex blend of global political, socioeconomic, and technological challenges must be dealt with to manage these inventories efficiently and safely.
The mortality of all 14,282 workers employed at the Sellafield plant of British Nuclear Fuels between 1947 and 1975 was studied up to the end of 1988 and cancer incidence was examined from 1971 to 1986. This updates a previous report on mortality only up to the end of 1983. Ninety-nine per cent of the workers were traced satisfactorily. Cancer mortality was 4% less than that of England and Wales [standardised mortality ratio (SMR) = 96; 95% confidence interval (CI) = 90,103] and the same as that of Cumbria (SMR = 100: Cl = 94,107). Cancer incidence was 10% less than that of England and Wales [standardised registration ratio (SRR) = 90; Cl = 83.97] and 18% less than that of Northern Region (SRR = 82; Cl = 75.88). Cancer mortality rates were significantly in excess of national rates for cancers of the pleura (nine observed, 2.6 expected; P = 0.001), thyroid (six observed, 1.8 expected; P = 0.01) and ill defined and secondary sites (53 observed, 39.2 expected; P = 0.02). There were significant deficits of cancers of the liver and gall bladder, larynx and lung. Among radiation workers there were significant positive correlations between accumulated radiation dose and mortality from cancers of ill-defined and secondary sites (10 year lag: P = 0.01) and for leukaemia (2 year lag: P = 0.009), but not for cancers of the pleura and thyroid cancer. Previous findings of such associations with multiple myeloma and bladder cancer were less strong. There was a significant excess of incident cases of cancer of the oesophagus (P = 0.01), but this was not associated with accumulated radiation dose. For cancers other than leukaemia, the dose-response risk estimates were below those of the adult atomic bomb survivors, but the 90% confidence interval included risks of zero and of 2-3 times higher. For leukaemia (12 deaths, excluding CLL), under an excess relative risk model, the risk estimate derived for the Sellafield workers was about four times higher than that for the adult atomic bomb survivors with a confidence interval ranging from a half to nearly 20 times that of the atomic bomb survivors. Overall, however, there was no excess of leukaemia among the workers compared with national rates.
This paper describes the characterisation of a cast plutonium-gallium (Pu-0.42 wt.% Ga) alloy, both in the as-cast condition as well as following an homogenising heat treatment. The alloy was subjected to density measurements, differential scanning calorimetry (DSC), dilatometry, optical microscopy, electron probe micro-analysis (EPMA), X-ray diffraction (XRD) and hardness measurements. The Ga content is insufficient to retain a wholly delta-Pu (delta-Pu) phase in the as-cast condition. However, the 250-h heat treatment at 450 degrees C is sufficient to redistribute the Ga resulting in an apparently stable d-Pu phase. DSC and dilatometry did not indicate the presence of any alpha-Pu (alpha-Pu) phase in the heat-treated alloy. XRD patterns of the alloys also showed alpha-Pu to be present, although in the case of the heat-treated alloy this may be a consequence of incomplete removal of the transformed surface layer during the electropolishing process. The stability of the delta-Pu phase in the heat-treated alloy was evaluated by cooling specimens to sub-zero temperatures. The alloy exhibited a high degree of stability when subjected to cold treatments at temperatures of between -50 degrees C and -90 degrees C. Crown Copyright
This paper describes the characterisation of a naturally aged Pu–0.27 wt.% Ga alloy 35 years of age. The alloy was subjected to bulk chemical analysis, density determination, differential scanning calorimetry (DSC), optical microscopy, electron probe micro-analysis (EPMA) and hardness measurements. Despite the Ga content being only 0.27 wt.%, it nevertheless appears to be sufficient to retain the alloy in the δ-Pu phase at ambient temperature. This was demonstrated by optical microscopy, density measurements and DSC. However, the ambient temperature stability of the δ-Pu phase is tenuous. This was demonstrated by the propensity of the alloy to undergo transformation to α-Pu when cooled to sub-ambient temperatures. Indeed, both density measurements and DSC indicate that when the alloy is cooled to −50 °C for 1 h the alloy has transformed to more than 40% α-Pu. Moreover, a comparison of the Vickers hardness (expressed as a mean pressure) with transformation pressures for Pu–Ga suggests that the alloy has undergone δ → α′ transformation during the indentation process.
A model for the initiation of hydride sites on uranium metal is described for conditions of constant hydrogen pressure. The model considers variations in hydrogen permeation through the surface oxide film due to intrinsic variations in the oxide thickness. It is proposed that thin areas of surface oxide favour enhanced hydrogen permeation through the oxide and lead to the more rapid initiation of hydride sites. The time and spatial dependence of the hydrogen concentration field in the metal underlying thin areas of oxide is calculated in terms of the local oxide film thickness, the hydrogen diffusion coefficients in the oxide and metal and the hydrogen concentration in the oxide at the gas–oxide interface. The time to precipitate hydride at any location is calculated by assuming that precipitation occurs once the hydrogen concentration in the metal attains the terminal solubility limit of the metal at the prevalent temperature. The model is compatible with the reported temperature and pressure dependence of the hydride induction time. The model can also explain observations such as precipitation of hydride at or beneath the oxide–metal interface and the arrested growth of hydride sites. Finally, an expression is derived for the number of hydride sites initiated on an entire sample surface in any given time by assuming a Gaussian oxide film thickness distribution over the entire sample surface.
The product morphology of the hydrogen reaction with plutonium near the visibly observable reaction front, which separates the hydrided zone from the unreacted metal zone, has been investigated by scanning electron microscopy (SEM). Results indicate the existence of a mixed phase of metal and metal hydride, located some 20–30 μm ahead of the visibly hydrided-zone. The mixed phase regions are often located next to a grain boundary network and exhibit rays of hydride advancing toward the unreacted metal regions. Analysis indicates that hydrogen transport and therefore the hydriding reaction are preferable along the grain boundary network and defects in the metal structure rather than through a homogeneous intragrain reaction. Product fracture and formation of small hydride particles during hydriding are likely results of such inhomogeneous growth.
Samples of cerium were exposed to hydrogen under controlled conditions causing cerium hydride sites to nucleate and grow on the surface. The hydriding rate was measured in situ, and the hydrides were characterised using secondary ion mass spectrometry, scanning electron microscopy, and optical microscopy. The results show that the hydriding rate proceeded more quickly than earlier studies. Characterisation confirmed that the hydrogen is confined to the sites. The morphology of the hydrides was confirmed to be oblate, and stressed material was observed surrounding the hydride, in a number of cases lathlike features were observed surrounding the hydride sites laterally with cracking in the surface oxide above them. It is proposed that during growth the increased lattice parameter of the CeH2 induces a lateral compressive stress around the hydride, which relieves by the ca. 16% volume collapse of the γ-Ce to α-Ce pressure induced phase transition. Cracking of the surface oxide above the laths reduces the diffusion barrier to hydrogen reaching the metal/oxide interface surrounding the hydride site and contributes to the anisotropic growth of the hydrides.
We perform first-principles calculations to investigate the structural, magnetic, electronic, and mechanical properties of face-centered cubic (fcc) PuH2 and fcc PuH3 using the full potential linearized augmented plane wave method (FP-LAPW) with the generalized gradient approximation (GGA) and the local spin density approximation (LSDA) taking account of both relativistic and strong correlation effects. The optimized lattice constant a0 = 5.371 Å for fcc PuH2 and a0 = 5.343 Å for fcc PuH3 calculated in the GGA + sp (spin polarization) + U (Hubbard parameter) + SO (spin—orbit coupling) scheme are in good agreement with the experimental data. The ground state of fcc PuH3 is found to be slightly ferromagnetic. Our results indicate that fcc PuH2 is a metal while fcc PuH3 is a semiconductor with a band gap about 0.35 eV. We note that the SO and the strong correlation between localized Pu 5f electrons are responsible for the band gap of fcc PuH3. The bonds for PuH2 have mainly covalent character while there are covalent bonds in addition to apparent ionicity bonds for PuH3. We also predict the elastic constants of fcc PuH2 and fcc PuH3, which were not observed in the previous experiments.
Infrared pyrometer measurements of reaction-site dimensions and temperature profiles during growth of single hydrogen corrosion sites after exposure of Pu to H2 are consistent with a multi-step reaction sequence for hydride formation. The observed temperature increases within the reaction sites are less than predicted by a thermal model which assumes complete hydriding within the corrosion area. Those results and observation of low average H/Pu ratios imply that initial products are mixtures of PuH2 and metal grains dislodged by grain-boundary hydriding. Such mixtures are formed by fast grain-boundary reaction and slower intragrain reaction as hydrogen corrosion advances into Pu metal. Emissivities of plutonium hydride and oxide-coated metal are reported. The differences in reaction rates induced by variation in Pu mounting orientations, where hydride products either fell free from the substrate or were retained on the surface, are also described.
The findings of recent work on PuH are described and employed in resolving several inconsistencies in the reported properties of plutonium hydride. Results of pressure measurements establish the nature of a large hysteresis effect and suggest revisions in the phase diagram. The existence of five hydride phases between the dihydride and trihydride compositions is indicated. Recently reported properties and neutron diffraction data are employed in formulating conceptual models for bonding and hydrogen accommodation in the fluorite-related hydride solid solution. Evidence for the existence of vacancy clusters in the cubic hydride is presented, and the influence of cation and anion ordering on various hydride properties is discussed.
Factors which are of primary concern in the storage of plutonium metal include: its corrosion susceptibility, its pyrophoricity, and its radiation properties. Each of these factors is reviewed with particular consideration given to that work done after 1964. Plutonium dioxide is one of the most satisfactory forms in which to store plutonium for long periods. However, adsorption of gases and vapors on the oxide can affect its storage behavior. Such adsorption can result in future pressurization of sealed storage containers. Adsorption of oxygen, water vapor, carbon dioxide, and nitrogen oxides is discussed. The radioactive nature of plutonium and plutonium compounds can also adversely affect long term storage. Radiation may decompose adsorbed contaminants as well as associated wrapping materials resulting in possible pressurization of sealed containers and degradation of the metal.
The plutonium-oxygen phase diagrams has been clarified by XPS and AES results which show that three oxygen-containing plutonium phases, PuO/sub 2/, ..cap alpha..-Pu/sub 2/O/sub 3/, and PuO/sub x/C/sub y/, are successively formed when oxide-coated plutonium metal is heated in vacuo to 500/sup 0/C. Formation of the oxide carbide is evidenced by the fact that a shift in the Pu(4f/sub 7/2/) binding energy is coincident with changes in the C(1s) photoelectron and C(KLL) Auger spectra, showing that unbound carbon is converted to carbide. The reaction conditions duplicate those at which plutonium monoxide reportedly forms and demonstrate that the NaCl-type surface phase previously identified as PuO is actually PuO/sub x/C/sub y/. The oxide carbide has a spectroscopically determined composition of PuO/sub 0.65+-0.15/C/sub 0.45+-0.15/ and reacts with CO, CO/sub 2/, and O/sub 2/ at low pressures to form PuO/sub 2/ and unbound carbon in a coherent product layer (approx. 20 nm thick) which retards further oxidation. Estimated thermodynamic data for PuO/sub x/C/sub y/ are consistent with the observed formation and oxidation reactions. The Pu(4f/sub 7/2/) binding energies for PuO/sub 2/, ..cap alpha..-Pu/sub 2/O/sub 3/, PuO/sub x/C/sub y/, and Pu are 426.1, 424.4, 423.6, and 422.2 eV, respectively.
As part of an ongoing programme to quantify those parameters which influence the early stages of the plutonium hydriding reaction, the hydrogen pressure dependence of both plutonium hydriding initiation time (It) and hydriding nucleation rate (Nr) have been determined for plutonium covered in a reproducible dioxide over-layer. The data show that initiation time is inversely proportional to hydrogen pressure, while nucleation rate is proportional to hydrogen pressure. Both observations are consistent with a model of hydriding attack in which the dioxide over-layer acts as a diffusion barrier, controlling the flow of hydrogen from the gas phase to the oxide/metal interface. The low scatter and reproducibility of the experimental data set demonstrate the importance of synthesising well controlled and characterised oxide layers prior to determining these experimental parameters.
The role of cubic Pu2O3 in the corrosion of PuO2-coated Pu by H2 was investigated. Experiments were conducted to demonstrate that nucleation of hydriding is promoted by formation of Pu2O3 sites in the oxide layer. The nucleation mechanism based on diffusion of hydrogen through the PuO2 layer was evaluated and an alternative mechanism based on formation of catalytic Pu2O3 sites via the Pu–PuO2 reaction is proposed. The possibility of active participation of other impurities and inclusions in the dioxide is also discussed.
The formation of a metal hydride is associated with a large increase of volume relative to the parent metal and therefore in large strain energies. Effects of elastic energy on the hydriding of metals are revealed in the microstructural evolution and kinetics of hydride growth on free surfaces. In the present work, we study in detail the elastic fields set up by a semi-spherical hydride particle growing at a free surface of metal with cubic symmetry, with and without an oxide layer. These systems combine geometric (structural) and material anisotropies.Three stages along the microstructural evolution on the surface of some hydride forming metals exposed to hydrogen at constant pressure were described experimentally. For these stages along hydride growth, correlations with the elastic fields are suggested as follows. (a) A hydride particle at the free surface generates regions of tensile and compressive hydrostatic stress in the surrounding matrix. This may induce a preferred nucleation of new hydrides and formation of clusters of hydrides precipitates, which is indeed observed experimentally. (b) Clustering, on the other hand, may contribute to the cease of growth due to competition on hydrogen. In addition, as the particle grows, changes in the stress fields may retard further diffusion from the surface and be another contribution to the cease of growth. (c) A growing hydride increases the stress in the oxide layer and may finely break it. Then the elastic energy per unit volume drops to its minimum value and the growth may accelerate. The formation of such “growth center” is favored for that hydride precipitate that grow alone and not in a cluster.
In examining and analyzing over three decades of research on the transformations involved in Pu–Ga alloys, it is clear that many incongruities and contradictions exist. New analysis shows that under certain processing conditions large amounts of a non-diffracting disordered state exists in Pu–Ga alloys, which I call the amorphous state. The amorphous state is certainly not an equilibrium state, but it can be formed under certain pressure conditions, and more surprisingly once formed can persist for extended times and temperatures. It is believed that the emergence of the amorphous state is closely linked to the 5f-electrons going into a bonding mode. Being a physical state, the amorphous state has distinct physical properties, meta-stabilities, and sensitivities to Ga content.The existence of an amorphous state requires innovative new tools for identification and analysis. The density and compressibility data presented in this paper is one such tool for analyzing the amorphous state, but it requires other knowledge of the materials for proper analysis. Many studies point to radiation damage producing amorphous regions. New computer modeling studies show the existence of amorphous regions and Pu-rich interstitial defects in Pu–Ga alloys.
This paper reviews typical and significant results of the recent studies for the surface processes in H2 absorption by rare earth based hydrogen storage alloys (RE–HSA). The kinematics of the hydrogen uptake by RE–HSA in the H2 gas and electrochemical reactions can drastically be influenced by spontaneous and intentional modifications of the surface of RE–HSA. This paper deals with the following prototypical cases: The formation of the oxide layer on the surface of the alloys shifts the rate controlling step of the H2 absorption from H2 dissociation on the surface with thin oxide layers to the H permeation through the layer as the layer grows. Partial substitution of Ni with Al, Co or Mn for LaNi5−x(Al,Mn,Co)x results in the change in the surface H2 processes, depending upon each substituent element. The penetration of alkaline atoms (Li, Na, K) into the surface of the alloys markedly enhances the initial activation in the H2 gas and electrochemical reactions. The fluorination treatment of the alloy surfaces effectively protects the surface from the attack of contaminations in the H2 gas.
The hydrogen pressure dependence of the initiation time (It) of the plutonium hydriding reaction has been determined over a hydrogen pressure range of 10–1000mbar for plutonium covered in a dioxide over-layer. The data show that hydriding initiation time is inversely proportional to hydrogen pressure. This observation is consistent with a model of hydriding attack in which the dioxide over-layer acts as a diffusion barrier, controlling the flow of hydrogen to the oxide/metal interface. The low scatter and reproducibility of the experimental data set illustrate the importance of synthesising a reproducible oxide layer prior to determining this experimental parameter.
Plutonium hydride with the approximate composition PuH2.7 has been obtained by direct combination of the metal with pure hydrogen at temperatures between 150° and 250°. Its chemical properties have been found to be very similar to those of the metal. By means of a thermobalance the hydride has been found to be stable in air up to 150°. X-Ray diffraction photographs showed two patterns, one of which was cubic and similar to that of plutonium dioxide, and the other hexagonal.
The volume changes, which are associated with hydride formation involve large strain energy. In the present work, Finite Element calculations of the strain energy of hydrides formed at the free surface of a metal matrix is done as function of several variables: the shape of the hydride precipitate, the elastic anisotropy of the crystals with cubic symmetry, the elastic heterogeneity, elastic–plastic transition and the effect of an oxide layer on the surface. The effect of these variables on the kinematics of the elastic strains and on the distribution of the energy between the matrix and hydrides are used to interpret the results and to deduce the preferred shapes, those having the lowest energy.The elastic energy of half-spherical hydrides at the surface is found to be minimal in most of the elastic and elastic–plastic systems considered (due to different reasons). A plate-shaped hydride with broad face parallel to the free surface may become preferred in an elastic matrix if the hydride is significantly softer than the matrix, or its broad face is parallel to a soft crystallographic plane. The existence of a thick oxide layer over the free surface increases the total energy of the system and moderates the dependence of the energy on the shape. As the hydride grows, the preference of the spherical shape is enhanced. For the case of a plastic matrix covered with an oxide layer, the preferred growth shape changes from a sphere to an elongated precipitate perpendicular to the free surface.
The rate of hydriding of plutonium powder was measured over the temperature range -41 to 350 °C and at hydrogen pressures of 0.007, 0.07 and 26.5 kPa. Two kinetic models were evaluated and discussed. Hydriding rates were independent of temperature over the entire temperature range. The hydriding rates were also independent of hydrogen pressure above 1 kPa. At lower pressures, the hydriding rates followed two different types of pressure dependence. A pressure dependence of P1/2 was found in the range 1 >P> 0.07 kPa and at low pressure (P < 0.07 kPa) the rate of hydriding was proportional to P1. The results indicate adsorption-controlled kinetics at low hydrogen pressures whereas the kinetics are reaction controlled at high pressures.
This report is the second of three parts that exhibit illustrations of inclusions in plutonium metal from inherent and tramp impurities, of intermetallic and nonmetallic constituents from alloy additions, and of the effects of thermal and mechanical treatments. This part includes illustrations of the microstructures in binary cast alloys and a few selected ternary alloys that result from measured additions of diluent elements, and of the microconstituents that are characteristic of phase fields in extended alloy systems. Microhardness data are given and the etchant used in the preparation of each sample is described.
Data are limited primarily to the production of electricity because of the increasing importance of this energy form in our society and because various fuel options are available. Following discussions of some basic principles in consideration of health effects, the available health-effects data are presented for fuel extraction, transport, processing, and electricity generation from the major fuel systems, with emphasis on the major uncertainties and controversies. (PCS)
In previous work, on cooling from 300 K to 10 K the elastic moduli for both α- and δ-Pu dropped 30%. This large change may reflect effects of 5f-electron localization. In this work, the elastic moduli at ambient temperature of several Pu–Ga alloys were measured using resonant ultrasound spectroscopy (RUS). The strong temperature dependence of the bulk and shear modulus and the temperature independence of Poisson's ratio was confirmed and the upper temperature limit for α-Pu was extended to 360 K. Measurements of the time dependence of the shear moduli of Pu and Pu–2.36 at.% Ga were determined with high precision as a function of time and temperature. Using a model for time dependence of point defects, we determined the exponential time constant at ambient temperature for such variations. The low temperature results are consistent with Fluss [1].
The need to address topics of handling, storage, and disposal of plutonium and uranium is driven by concern about hazards
posed by the element and by the worldwide quantity of civilian and military materials. The projected inventory of separated
civilian plutonium for use in fabricating mixed-oxide (MOX) reactor fuel during initial decades of this century is constant
at about 120 metric tons and a comparable amount of excess military plutonium is anticipated from reductions in nuclear weapon
stockpiles (IAEA Report, 1998). Although inventories of civilian material are in oxide form, Pu from weapons programs exists
primarily as metal. Plutonium is a radiological toxin (Voelz, 2000); its management in a safe and secure manner is essential
for protecting workers, the public, and the environment.
The microscopic mechanisms which may control the rate of the reaction of gaseous hydrogen and hydride-forming metals are reviewed. A distinction is made between the early stages of the reaction associated with the nucleation and growth of the hydrides on the surface of the reacting metal and the subsequent massive stage. For the very early stage, factors affecting the ability of hydrogen gas to penetrate surface passivation layers are considered. Different types of nucleation groups are demonstrated. For the latter, massive stage, possible morphological forms of the hydride phase development are summarized. A special case, frequently encountered in binary metal–hydrogen systems, is the contracting envelope (or shrinking core) morphology. For this case, a simple evaluation of the reaction front velocity can be deduced from the overall rate measurements and from the known geometry and dimensions of the metal sample. A detailed analysis of this hydride-front velocity dependence on sample temperature and gas pressure can then point to the controlling mechanism. Some characteristics of Arrhenius-type curves and possible deviations from linear relation are discussed. Examples for possible surface-controlled (Ce), diffusion-controlled (Th, Ti, Zr, Hf) and interface-controlled (U) reactions are presented, as well as limited bulk intermetallic hydriding reactions. Certain symptomatic aspects of the kinetic behaviour related to some of the above mechanisms are discussed.
The three principal elastic constants Cij of f.c.c. plutonium were determined by measuring the ultrasonic wave velocities along a near-〈110〉 direction in a single crystal. The elastic anisotropy is twice that known for any other f.c.c. metal. The elastic-anisotropy problem is considered using a general two-body central-force interatomic potential. Based on the Cij. polar plots of the wave velocities and Young's modulus were determined. The Cij were “averaged” to obtain quasi-isotropic elastic constants, which are compared with existing polycrystalline elastic data. The elastic Debye temperature was computed by numerically integrating the Christoffel equations.
The initial stages of the formation of uranium trihydride on the surface of uranium samples reacted with gaseous hydrogen (about 1.5 atm) were studied utilizing a hot-stage microscope. The nucleation and growth processes of the product hydride were continuously monitored with a television camera and were recorded on a videotape. The formation kinetics and the morphological characteristics of the developing hydride phase are discussed. A comparison with the kinetic results obtained in the more advanced bulk hydriding stage is made.
Reaction rates of air and oxygen with cubic plutonium hydride (PuHx, 1.9<x<3), monoxide monohydride (PuOH), and Pu metal coated with these compounds are described, along with kinetic results for the Pu+H2 reaction. Pyrophoric tendencies are not observed for PuOH, but exposure of PuHx and PuHx- (or PuOH-) coated Pu to air or O2 at room temperature result in spontaneous reactions that consume both O2 and N2. These reactions and hydriding have zero or slightly negative activation energies and pressure-dependent rates. Pyrophoric reaction of PuHx and PuHx-catalyzed corrosion of Pu depend on thermal maintenance of catalytic Pu2O3 at the gas–solid interface and are prevented by formation of a protective PuO2 layer at low temperatures and low O2 pressures. The Pu+H2 reaction is catalyzed by Pu2O3 and PuHx is produced by the Pu+H2O reaction only at conditions where Pu2O3 formation is kinetically favored. Thermal ignition of Pu near 500°C is attributed to autoreduction of the PuO2 surface to Pu2O3 at that temperature. At normal storage temperatures, formation of pyrophoric corrosion products is unlikely in open oxidant-rich systems, but surfaces that catalyze rapid Pu corrosion in air are formed during extended storage in closed systems.
The reaction between uranium and water vapour has been well investigated, however discrepancies exist between the described kinetic laws, pressure dependence of the reaction rate constant and activation energies. Here this problem is looked at by examining the influence of impurities in the form of carbide inclusions on the reaction. Samples of uranium containing 600 ppm carbon were analysed during and after exposure to water vapour at 19 mbar pressure, in an environmental scanning electron microscope (ESEM) system. After water exposure, samples were analysed using secondary ion mass spectrometry (SIMS), focused ion beam (FIB) imaging and sectioning and transmission electron microscopy (TEM) with X-ray diffraction (micro-XRD). The results of the current study indicate that carbide particles on the surface of uranium readily react with water vapour to form voluminous UO(3) · xH(2)O growths at rates significantly faster than that of the metal. The observation may also have implications for previous experimental studies of uranium-water interactions, where the presence of differing levels of undetected carbide may partly account for the discrepancies observed between datasets.
Backscatter electron image of a cross section through a surface protrusion reveals hydride had formed within the micro cracks intersecting the surface
- Fig
Fig. 5. Backscatter electron image of a cross section through a surface protrusion reveals hydride had formed within the micro cracks intersecting the surface.
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