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DOE OSTI2014

Investigation of the diffusion behavior of sodium in Cu(In,Ga)Se{sub 2} layers

Sodium diffusion in Cu(In,Ga)Se{sub 2} (CIGS) layers was investigated over a temperature range from 157 °C to 400 °C. The diffusion profiles were measured by secondary ion mass spectrometry. Sodium ions diffused from a sodium fluoride (NaF) layer on the CIGS surface into the CIGS layer. From Na diffusion profiles, the diffusion along grain boundaries could be distinguished from the diffusion into the grain interior. Atom-probe tomography measurements reveal that even at a low temperature of 157 °C bulk diffusion of sodium into CIGS occurs. Based on this data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation D{sub Na}{sup V} = 9.7 × 10{sup −9} exp(−0.36 eV/k{sub B}T) cm{sup 2} s{sup −1} and the fast diffusion along the grain boundaries by D{sub Na}{sup GB} = 6.5 × 10{sup −9} exp(−0.21 eV/k{sub B}T) cm{sup 2} s{sup −1}. Hence, we propose that sodium ions do not only passivate grain boundaries, but also act as dopants in the CIGS bulk.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

The interstitialcy diffusion in FCC copper: A molecular dynamics study

Damage of materials due to neutron irradiation occurs via energetic cascades caused by energetic primary knock-on atoms (PKA) created by the energetic neutron as it passes through the material. These cascades result in creation of Frenkel Pairs (interstitials and vacancies). The interstitials and vacancies diffuse and recombine to (I) nullify the damage when an interstitial recombines with a vacancy, (II) form interstitial clusters when two or more interstitials recombine, and (III) form vacancy clusters when several vacancies come together. The latter two processes result in change of material properties. Interstitial diffusion has reported time-scales of microseconds and vacancy diffusion has diffusion time-scales of the order of seconds. We have carried out molecular dynamics (MD) simulations of interstitial diffusion in crystal Cu to study the mechanism of diffusion. It is found that interstitialcy diffusion – wherein an interstitial displaces a lattice atom thereby making the lattice atom an interstitial – has time-scales of a few tens of pico-seconds. Therefore we propose that the “interstitialcy diffusion” mechanism could play a major part in the diffusive-recombinations of the Frenkel Pairs created during the cascade.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Phosphorus and boron diffusion paths in polycrystalline silicon gate of a trench-type three-dimensional metal-oxide-semiconductor field effect transistor investigated by atom probe tomography

The dopant (P and B) diffusion path in n- and p-types polycrystalline-Si gates of trench-type three-dimensional (3D) metal-oxide-semiconductor field-effect transistors (MOSFETs) were investigated using atom probe tomography, based on the annealing time dependence of the dopant distribution at 900 °C. Remarkable differences were observed between P and B diffusion behavior. In the initial stage of diffusion, P atoms diffuse into deeper regions from the implanted region along grain boundaries in the n-type polycrystalline-Si gate. With longer annealing times, segregation of P on the grain boundaries was observed; however, few P atoms were observed within the large grains or on the gate/gate oxide interface distant from grain boundaries. These results indicate that P atoms diffuse along grain boundaries much faster than through the bulk or along the gate/gate oxide interface. On the other hand, in the p-type polycrystalline-Si gate, segregation of B was observed only at the initial stage of diffusion. After further annealing, the B atoms became uniformly distributed, and no clear segregation of B was observed. Therefore, B atoms diffuse not only along the grain boundary but also through the bulk. Furthermore, B atoms diffused deeper than P atoms along the grain boundaries under the same annealing conditions. This information on the diffusion behavior of P and B is essential for optimizing annealing conditions in order to control the P and B distributions in the polycrystalline-Si gates of trench-type 3D MOSFETs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2016

Ge auto-doping and out-diffusion in InGaP grown on Ge substrate and their effects on the ordering of InGaP

We report on the Ge auto-doping and out-diffusion in InGaP epilayer with Cu-Pt ordering grown on 4-in. Ge substrate. Ge profiles determined from secondary ion mass spectrometry indicate that the Ge out-diffusion depth is within 100 nm. However, the edge of the wafer suffers from stronger Ge gas-phase auto-doping than the center, leading to ordering deterioration in the InGaP epilayer. In the edge, we observed a residual Cu-Pt ordering layer left beneath the surface, suggesting that the ordering deterioration takes place after the deposition rather than during the deposition and In/Ga inter-diffusion enhanced by Ge vapor-phase auto-doping is responsible for the deterioration. We thus propose a di-vacancy diffusion model, in which the amphoteric Ge increases the di-vacancy density, resulting in a Ge density dependent diffusion. In the model, the In/Ga inter-diffusion and Ge out-diffusion are realized by the random hopping of In/Ga host atoms and Ge atoms to di-vacancies, respectively. Simulation based on this model well fits the Ge out-diffusion profiles, suggesting its validity. By comparing the Ge diffusion coefficient obtained from the fitting and the characteristic time constant of ordering deterioration estimated from the residual ordering layer, we found that the hopping rates of Ge and the host atoms are in the same order of magnitude, indicating that di-vacancies are bound in the vicinity of Ge atoms.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2025

Water under hydrophobic confinement: entropy and diffusion

The properties of liquid water are known to change drastically in confined geometries. A most interesting and intriguing phenomenon is that the diffusion of water is found to be strongly enhanced by the proximity of a hydrophobic confining wall relative to the bulk diffusion. We report a molecular dynamics simulation using a classical water model investigating the water diffusion near a non-interacting smooth confining wall, which is assumed to imitate a hydrophobic surface, revealing a pronounced diffusion enhancement within several water layers adjacent to the wall. We present evidence that the observed diffusion enhancement can be accounted for, with a quantitative accuracy, using the universal scaling law for liquid diffusion that relates the diffusion rate to the excess entropy. These results show that the scaling law, which has so far only been used for the description of the diffusion in simple liquids, can successfully describe the diffusion in water. It is shown that the law can be used for the analysis of water dynamics under nanoscale hydrophobic confinement, which is currently a subject of intense research activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Experimental and theoretical comparison of Sb, As, and P diffusion mechanisms and doping in CdTe

Fundamental material doping challenges have limited CdTe electro-optical applications. In this work, the As atomistic diffusion mechanisms in CdTe are examined by spatially resolving dopant incorporation in both single-crystalline and polycrystalline CdTe over a range of experimental conditions. Density-functional theory calculations predict experimental activation energies and indicate As diffuses slowly through the Te sublattice and quickly along GBs similar to Sb. Because of its atomic size and associated defect chemistry, As does not have a fast interstitial diffusion component similar to P. Experiments to incorporate and activate P, As, and Sb in polycrystalline CdTe are conducted to examine if ex-situ Group V doping can overcome historic polycrystalline doping limits. The distinct P, As, and Sb diffusion characteristics create different strategies for increasing hole density. Because fast interstitial diffusion is prominent for P, less aggressive diffusion conditions followed by Cd overpressure to relocate the Group V element to the Te lattice site is effective. For larger atoms, slower diffusion through the Te sublattice requires more aggressive diffusion, however further activation is not always necessary. Based on the new physical understanding, we have obtained greater than 10^16 cm^-3 hole density in polycrystalline CdTe films by As and P diffusion.

14 SOLAR ENERGY↗

DOE OSTI2014

TU-C-12A-07: Characterization of Longitudinal Reproducibility of Quantitative Diffusion Imaging Data Acquired with Four Different Protocols Using a Phantom

Purpose: To characterize and compare the longitudinal reproducibility of diffusion imaging data acquired with four different protocols using a phantom. Methods: The Diffusive Quantitative Imaging Phantom (DQIP) was constructed using fifteen cylindrical compartments within a larger compartment, filled with deionized water doped with CuSO4 and NaCl. The smaller compartments contained arrays of hexagonal or cylindrical glass capillaries of varying inner diameters, for differing restraint of water diffusion. The sensitivity of diffusion imaging metrics to signal-to-noise ratio (SNR) was probed by doping compartments with differing ratios of deuterium oxide to H2O. A cork phantom enclosure was constructed to increase thermal stability during scanning and a cork holder was made to reproduce scanner positioning. Four different protocols of DWI (diffusion weighted imaging) and DTI (Diffusion tensor imaging) imaging were assembled on a GE Excite HDx 3.0T MRI scanner to collect imaging data over 9-10 days. Data was processed with in-house software created in Matlab to obtain fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values. Results: All DTI and DWI sequences showed good longitudinal stability of mean FA and ADC values per compartment, exhibiting low standard deviation ∼9%. A t-test was performed to compare mean FA values from the DTI clinical protocol to those of the DTI special protocol, indicating significantly different values in the majority of compartments. ANOVA performed on ADC values for all DTI and DWI sequences also showed significantly different values in a majority of compartments. Conclusion: This work has the potential for quantifying systemic variations between diffusion imaging sequences from different platforms. Characterization of DWI and DTI performance were done over four sequences with predictable results. This data suggests that the DQIP phantom may be a reliable method of monitoring day-to-day and scan-to-scan variation in diffusion imaging sequences from different platforms. Schott Glass North America and The Phantom Laboratory have donated materials and personnel time to this project.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2016

TU-AB-BRA-07: Distortion-Free 3D Diffusion MRI On An MRI-Guided Radiotherapy System for Longitudinal Tumor Response Assessment

Purpose: To develop a reliable, 3D distortion-free diffusion MRI technique for longitudinal tumor response assessment and MRI-guided adaptive radiotherapy(RT). Methods: A diffusion prepared 3D turbo spin echo readout (DP-TSE) sequence was developed and compared with the conventional diffusion-weighted single-shot echo-planar-imaging (DW-ssEPI) sequence in a commercially available diffusion phantom, and one head-and-neck and one brain cancer patient on an MRI-guided RT system (ViewRay). In phantom study, the geometric fidelity was quantified as the ratio between the left-right (RL) and anterior-posterior (AP) dimension. Ten slices were measured on DP-TSE, DW-ssEPI and standard TSE images where the later was used as the geometric reference. ADC accuracy was verified at both 0°C (reference ADC available) and room temperature with a range of diffusivity between 0.35 and 2.0*10{sup −3}mm{sup 2}/s. The ADC reproducibility was assessed based on 8 room-temperature measurements on 6 different days. In the pilot single-slice in-vivo study, CT images were used as the geometric reference, and ADC maps from both diffusion sequences were compared. Results: Distortion and susceptive-related artifact were severe in DW-ssEPI, with significantly lower RL/AP ratio (0.9579±0.0163) than DP-TSE (0.9990±0.0031) and TSE (0.9995±0.0031). ADCs from the two diffusion sequences both matched well with the vendor-provided values at 0°C; however DW-ssEPI fails to provide accurate ADC for high diffusivity vials at room temperature due to high noise level (10 times higher than DP-TSE). The DP-TSE sequence had excellent ADC reproducibility with <4% ADC variation among 8 separate measurements. In patient study, DP-TSE exhibited substantially improved geometric reliability. ROI analysis in ADC maps generated from DP-TSE and DW-ssEPI showed <5% difference where high b-value images were excluded from the latter approach due to excessive noise level. Conclusion: A diffusion MRI sequence with excellent geometric fidelity, accurate and highly reproducible ADC measurements was proposed for longitudinal tumor response assessment using an MRI-guided RT system. Yu Gao acknowledges research support from ViewRay.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2017

Ion Diffusivity through the Solid Electrolyte Interphase in Lithium-Ion Batteries

Understanding the transport properties of the solid electrolyte interface (SEI) is a critical piece in the development of lithium ion batteries (LIB) with better performance. We studied the lithium ion diffusivity in the main components of the SEI found in LIB with silicon anodes and performed classical molecular dynamics (MD) simulations on lithium fluoride (LiF), lithium oxide (Li 2 O) and lithium carbonate (Li 2 CO 3 ) in order to provide insights and to calculate the diffusion coefficients of Li-ions at temperatures in the range of 250 K to 400 K, which is within the LIB operating temperature range. We find a slight increase in the diffusivity as the temperature increases and since diffusion is noticeable at high temperatures, Li-ion diffusion in the range of 130 to 1800 K was also studied and the diffusion mechanisms involved in each SEI compound were analyzed. We observed that the predominant mechanisms of Li-ion diffusion included vacancy assisted and knock-off diffusion in LiF, direct exchange in Li 2 O, and vacancy and knock-off in Li 2 CO 3 . Moreover, we also evaluated the effect of applied electric fields in the diffusion of Li-ions at room temperature.

25 ENERGY STORAGE↗

DOE OSTI2012

Bulk and surface controlled diffusion of fission gas atoms

Fission gas retention and release impact nuclear fuel performance by, e.g., causing fuel swelling leading to mechanical interaction with the clad, increasing the plenum pressure and reducing the gap thermal conductivity. All of these processes are important to understand in order to optimize operating conditions of nuclear reactors and to simulate accident scenarios. Most fission gases have low solubility in the fuel matrix, which is especially pronounced for large fission gas atoms such as Xe and Kr, and as a result there is a significant driving force for segregation of gas atoms to extended defects such as grain boundaries or dislocations and subsequently for nucleation of gas bubbles at these sinks. Several empirical or semi-empirical models have been developed for fission gas release in nuclear fuels, e.g. . One of the most commonly used models in fuel performance codes was published by Massih and Forsberg. This model is similar to the early Booth model in that it applies an equivalent sphere to separate bulk UO 2 from grain boundaries represented by the sphere circumference. Compared to the Booth model, it also captures trapping at grain boundaries, fission gas resolution and it describes release from the boundary by applying timedependent boundary conditions to the circumference. In this work we focus on the step where fission gas atoms diffuse from the grain interior to the grain boundaries. The original Massih-Forsberg model describes this process by applying an effective diffusivity divided into three temperature regimes. In this report we present results from density functional theory calculations (DFT) that are relevant for the high (D 3 ) and intermediate (D 2 ) temperature diffusivities of fission gases. The results are validated by making a quantitative comparison to Turnbull's and Matzke's data. For the intrinsic or high temperature regime we report activation energies for both Xe and Kr diffusion in UO 2±x , which compare favorably to available experiments. This is an extension of previous work. In particular, it applies improved chemistry models for the UO 2±x nonstoichiometry and its impact on the fission gas activation energies. The derivation of these models follows the approach that used in our recent study of uranium vacancy diffusion in UO 2 . Also, based on the calculated DFT data we analyze vacancy enhanced diffusion mechanisms in the intermediate temperature regime. In addition to vacancy enhanced diffusion we investigate species transport on the (111) UO 2 surface. This is motivated by the formation of small voids partially filled with fission gas atoms (bubbles) in UO 2 under irradiation, for which surface diffusion could be the rate-limiting transport step. Diffusion of such bubbles constitutes an alternative mechanism for mass transport in these materials.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2016

Report on simulation of fission gas and fission product diffusion in UO 2

In UO 2 nuclear fuel, the retention and release of fission gas atoms such as xenon (Xe) are important for nuclear fuel performance by, for example, reducing the fuel thermal conductivity, causing fuel swelling that leads to mechanical interaction with the clad, increasing the plenum pressure and reducing the fuel–clad gap thermal conductivity. We use multi-scale simulations to determine fission gas diffusion mechanisms as well as the corresponding rates in UO 2 under both intrinsic and irradiation conditions. In addition to Xe and Kr, the fission products Zr, Ru, Ce, Y, La, Sr and Ba have been investigated. Density functional theory (DFT) calculations are used to study formation, binding and migration energies of small clusters of Xe atoms and vacancies. Empirical potential calculations enable us to determine the corresponding entropies and attempt frequencies for migration as well as investigate the properties of large clusters or small fission gas bubbles. A continuum reaction-diffusion model is developed for Xe and point defects based on the mechanisms and rates obtained from atomistic simulations. Effective fission gas diffusivities are then obtained by solving this set of equations for different chemical and irradiation conditions using the MARMOT phase field code. The predictions are compared to available experimental data. The importance of the large Xe U3O cluster (a Xe atom in a uranium + oxygen vacancy trap site with two bound uranium vacancies) is emphasized, which is a consequence of its high mobility and high binding energy. We find that the Xe U3O cluster gives Xe diffusion coefficients that are higher for intrinsic conditions than under irradiation over a wide range of temperatures. Under irradiation the fast-moving Xe U3O cluster recombines quickly with irradiation-induced interstitial U ions, while this mechanism is less important for intrinsic conditions. The net result is higher concentration of the Xe U3O cluster for intrinsic conditions than under irradiation. We speculate that differences in the irradiation conditions and their impact on the Xe U3O cluster can explain the wide range of diffusivities reported in experimental studies. However, all vacancy-mediated mechanisms underestimate the Xe diffusivity compared to the empirical radiation-enhanced rate used in most fission gas release models. We investigate the possibility that diffusion of small fission gas bubbles or extended Xe-vacancy clusters may give rise to the observed radiation-enhanced diffusion coefficient. These studies highlight the importance of U divacancies and an octahedron coordination of uranium vacancies encompassing a Xe fission gas atom. The latter cluster can migrate via a multistep mechanism with a rather low effective barrier, which together with irradiation-induced clusters of uranium vacancies, gives rise to the irradiation-enhanced diffusion coefficient observed in experiments.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2021

Theoretical Analysis of Experimental Data of Sodium Diffusion in Oxidized Molybdenum Thin Films

In this work, the diffusion process of sodium (Na) in molybdenum (Mo) thin films while it was deposited on soda lime glass (SLG) was studied. A small amount of oxygen was present in the chamber while the direct-current (DC) magnetron sputtering was used for the deposition. The substrate temperatures were varied to observe its effect. Such molybdenum films, with or without oxidations, are often used in thin film solar cells, either as back contact or as hole transport layers. Secondary ion mass spectrometry (SIMS) was used to quantify the concentration of the species. A grain diffusion mechanistic model incorporating the effect of grain and grain boundary geometrical shape and size was developed. The model was used to provide an in-depth theoretical analysis of the sodium diffusion in molybdenum thin films that lead to the measured SIMS data. It was observed that not only diffusion coefficients should be considered when analyzing diffusion processes in thin films but also the ratio of grain boundary size to grain size. Both depend on substrate temperature and directly affect the amount of diffused species in the film. The data were analyzed under the light of the film growth speed versus diffusion front speed, the effect of oxygen content, and the effect of substrate temperature on the overall diffusion process. The temperature inversely affects the ratio of grain boundary size and grain size and directly affects the diffusion coefficient, which leads to a preferable temperature at which the highest amount of alkali can be found in the film.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Perpendicular Diffusion of Solar Energetic Particles: Model Results and Implications for Electrons

The processes responsible for the effective longitudinal transport of solar energetic particles (SEPs) are still not completely understood. We address this issue by simulating SEP electron propagation using a spatially 2D transport model that includes perpendicular diffusion. By implementing, as far as possible, the most reasonable estimates of the transport (diffusion) coefficients, we compare our results, in a qualitative manner, to recent observations at energies of 55–105 keV, focusing on the longitudinal distribution of the peak intensity, the maximum anisotropy, and the onset time. By using transport coefficients that are derived from first principles, we limit the number of free parameters in the model to (i) the probability of SEPs following diffusing magnetic field lines, quantified by a∈[0,1], and (ii) the broadness of the Gaussian injection function. It is found that the model solutions are extremely sensitive to the magnitude of the perpendicular diffusion coefficient and relatively insensitive to the form of the injection function as long as a reasonable value of a = 0.2 is used. We illustrate the effects of perpendicular diffusion on the model solutions and discuss the viability of this process as a dominant mechanism by which SEPs are transported in longitude. Lastly, we try to quantity the effectiveness of perpendicular diffusion as an interplay between the magnitude of the relevant diffusion coefficient and the SEP intensity gradient driving the diffusion process. It follows that perpendicular diffusion is extremely effective early in an SEP event when large intensity gradients are present, while the effectiveness quickly decreases with time thereafter.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2023

Passive Rocket Diffuser Testing: Reacting Flow Performance of Advanced Configurations

Upper stage rocket engines are optimized for operation at the edge of space. Ground testing often requires simulation of high-altitude environments to prevent damage from off-design operation. Passive supersonic diffusers are an effective means of establishing the partial vacuum needed around the nozzle exit. The low cost of diffusers relative to active vacuum sources (ejectors, pumps, etc.) provides a perpetual incentive to improve their performance and expand the envelope of passively testable engines. Second-throat diffusers have been the de facto standard for rocket testing since the 1950s, and numerous studies have focused on parametric optimization of the topology. Yet, even the best second-throat designs may require more driving pressure than an upper stage engine can provide. To overcome conventional limits, engineers at NASA’s Stennis Space Center (SSC) adapted an obscure cold-flow diffuser topology to the extremes of immersion in supersonic combustion products. Aerodynamic experimentation was conducted at SSC’s E-3 test stand to quantify the performance of prototypical hot-fire spike diffuser hardware and enable direct comparison to prior states of the art. A LO X /GH 2 thruster was contoured to approximate the regeneratively-cooled nozzle of an upper-stage engine and fired into 28 downstream diffuser configurations: 16 second-throat, 2 centerbody, and 10 spike. Temperature and pressure were recorded along the test article walls for a range of chamber pressures spanning 3.5 - 5.0 MPa. The second-throat configurations with the lowest start and unstart pressure ratios were shown to outperform equivalent systems in literature and taken as baselines for evaluation of advanced diffuser performance. Compared to these exemplars, centerbody diffusers decreased start pressure ratio by 11% but increased unstart pressure ratio by 15%. Spike diffusers reduced start and unstart pressure ratios by 24% and 9%, respectively.

Rocket↗

DOE OSTI2013

Improved diffusion coefficients generated from Monte Carlo codes

Monte Carlo codes are becoming more widely used for reactor analysis. Some of these applications involve the generation of diffusion theory parameters including macroscopic cross sections and diffusion coefficients. Two approximations used to generate diffusion coefficients are assessed using the Monte Carlo code MC21. The first is the method of homogenization; whether to weight either fine-group transport cross sections or fine-group diffusion coefficients when collapsing to few-group diffusion coefficients. The second is a fundamental approximation made to the energy-dependent P1 equations to derive the energy-dependent diffusion equations. Standard Monte Carlo codes usually generate a flux-weighted transport cross section with no correction to the diffusion approximation. Results indicate that this causes noticeable tilting in reconstructed pin powers in simple test lattices with L2 norm error of 3.6%. This error is reduced significantly to 0.27% when weighting fine-group diffusion coefficients by the flux and applying a correction to the diffusion approximation. Noticeable tilting in reconstructed fluxes and pin powers was reduced when applying these corrections. (authors)

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

DOE OSTI2018

Effect of Weld Consumable Conditioning on the Diffusible Hydrogen and Subsequent Residual Stress and Flexural Strength of Multipass Welded P91 Steels

P91 steel weld joint was prepared using the shielded metal arc welding process and four different conditions of weld consumable that provide the different levels of diffusible hydrogen in deposited metal. In the present research, the effects of diffusible hydrogen content on the flexural strength, lower critical stress, and tensile strength of P91 steel welds were also determined with respect to different electrode conditions. To investigate the effect of diffusible hydrogen on multipass welding, top and root side flexural tests were performed. The residual stresses (axial stress and transverse stress) were also measured using the blind hole drilling method for different conditions of welding consumable. The peak value of residual stresses was measured at the center of the weld fusion zone. The maximum value of transverse stress was measured to be 355 MPa for case II (6.21 mL/100 g of diffusible hydrogen), while the maximum axial stress was about 218 MPa for case IV (12.43 mL/100 g of diffusible hydrogen). A three-dimensional finite element simulation was also performed to predict the residual stress distribution and thermal profile along the welded joint. The experimentally determined residual stresses correlated well with the numerically estimated residual stresses. The diffusible hydrogen content was not observed to have any significant effect on the residual stresses. The corrected residual stress values were also predicted by considering the plasticity-induced error. However, the flexural performance of the welded joint was affected by the diffusible hydrogen content. The top and root flexural strength was measured to be optimum for the low level of diffusible hydrogen content, and the values decreased with an increase in diffusible hydrogen content.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Study on impact of Cr and Mo on diffusion of H in 2.25Cr1Mo steel using first-principle calculations

Chrome-molybdenum steel (2.25Cr1Mo steel) is one of the main materials of a steam generator (SG) in high-temperature reactor-pebblebed modules (HTR-PM). It is essential to analyze the source term of tritium in this material, because the behavior of tritium in a SG is important for performing source term analysis in normal and accident conditions. In this article, the diffusion behavior of H atom in 2.25Cr1Mo steel was calculated to estimate the diffusivity of tritium using first-principles density functional theory. To develop and simplify the model of hydrogen diffusion in 2.25Cr1Mo steel, the impact of Cr and Mo on the diffusion of hydrogen in bcc-Fe were first calculated, all the possible diffusion paths were considered, and the minimum energy path was obtained. The diffusion activation energy and pre-exponential factor of the diffusion coefficient were obtained from Vienna Ab initio Simulation Package combined with the climbing image-nudged elastic band method. Here the results indicate that the minimum energy path for the impurity H atom is from one tetrahedral interstitial site to an adjacent tetrahedral interstitial site. The function of the diffusion coefficient of H in 2.25Cr1Mo steel with temperature T can be expressed as D = 1.486 X 10 -7 x ($\frac{-16.350kJ/mol}{RT}$) (M2/s). The diffusion coefficient of our calculation and some of the previous experiments have an excellent quantitative agreement, which indicates the reliability of our crystalline model and the practicability of the present theoretical approach. More importantly, the computational results in this work can be treated as a good screening method to collect reasonable experimental data, which will provide a good reference for tritium source term evaluation in the SG of the HTR-PM.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Long-term diffusion of U(VI) in bentonite: Dependence on density

As a contribution to the safety assessment of nuclear waste repositories, U(VI) diffusion through the potential buffer material MX-80 bentonite was investigated at three clay dry densities over six years. Synthetic MX-80 model pore water was used as background electrolyte. Speciation calculations showed that Ca 2 UO 2 (CO 3 ) 3 (aq) was the main U(VI) species. The in- and out-diffusion of U(VI) was investigated separately. U(VI) diffused about 3 mm, 1.5 mm, and 1 mm into the clay plug at ρ = 1.3, 1.6, and 1.9 g/cm 3 , respectively. No through-diffusion of the U(VI) tracer was observed. However, leaching of natural uranium contained in the clay occurred and uranium was detected in all receiving reservoirs. As expected, the effective and apparent diffusion coefficients, D e and D a , decreased with increasing dry density. The D a values for the out-diffusion of natural U(VI) were in good agreement with previously determined values. Surprisingly, D a values for the in-diffusion of U(VI) were about two orders of magnitude lower than values obtained in short-term in-diffusion experiments reported in the literature. Some potential reasons for this behavior that were evaluated are changes of the U(VI) speciation within the clay (precipitation, reduction) or changes of the clay porosity and pore connectivity with time. By applying Archie's law and the extended Archie's law, it was estimated that a significantly smaller effective porosity must be present for the long-term in-diffusion of U(VI). Finally, the results suggest that long-term studies of key transport phenomena may reveal additional processes that can directly impact long-term repository safety assessments.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗