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At least 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

The development and application of the stirred‐reactor coupon analysis (SRCA) test method

A new technique, termed the stirred‐reactor coupon analysis (SRCA) method, has been developed to measure the rate of glass dissolution in forward‐rate conditions. Monolithic glass coupons are partially masked with an inert material before placement in a large volume of well‐mixed solution with known chemistry and temperature for a predetermined duration. After the test, the mask is removed, and the difference in step height between the protected area and the exposed corroded portions of the sample coupon is measured to determine the extent of glass dissolution. The step height is converted to a rate measurement using the test duration and glass density. Test parameters such as sample surface preparation and test duration were evaluated to determine their effects on the measured rates. Additionally, results from an interlaboratory study (ILS) consisting of 12 laboratories from 11 different institutions are presented, where each laboratory performed 12 independent tests. When removing experimental outlier data, the 95% reproducibility limits for the SRCA method has no statistical difference with previously published standardized test methods used to determine the forward rate of glass dissolution. Overall, this paper describes steps necessary to perform the test method and provides the statistical calculations to evaluate test accuracy.

chemical durability↗
DOE OSTI2018

Resorption of Ca{sub 3–} {sub x}M{sub 2x}(PO{sub 4}){sub 2} (M = Na, K) Calcium Phosphate Bioceramics in Model Solutions

The resorbability of bioceramics in the Ca{sub 3}(PO{sub 4}){sub 2}–CaNaPO{sub 4}–CaKPO{sub 4} system is evaluated in an approach involving thermodynamic assessment of solubility and investigation of the dissolution kinetics in model media, in particular in citric acid solutions. Thermodynamic calculation indicates high solubility of the Ca{sub 5}Na{sub 2}(PO{sub 4}){sub 4}, α-CaМPO{sub 4}, β-CaKPO{sub 4}, and β-СаK{sub 0.6}Na{sub 0.4}PO{sub 4} phases. Investigation of the dissolution kinetics of ceramics has made it possible to identify two distinct types of behavior of resorbable materials in weakly acidic solutions: with fast resorption kinetics in the case of the phases based on nagelschmidtite solid solutions and α-CaМPO{sub 4} disordered high-temperature solid solutions, and with a nearly constant, relatively low dissolution rate and a high solubility limit in the case of β-СaK{sub 1–} {sub x}Na{sub x}-based phases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Anodic processes in the chemical and electrochemical etching of Si crystals in acid-fluoride solutions: Pore formation mechanism

The interaction of heavily doped p- and n-type Si crystals with hydrofluoric acid in the dark with and without contact with metals having greatly differing work functions (Ag and Pd) is studied. The dependences of the dissolution rates of Si crystals in HF solutions that contain oxidizing agents with different redox potentials (FeCl{sub 3}, V{sub 2}O{sub 5} and CrO{sub 3}) on the type and level of silicon doping are determined. Analysis of the experimental data suggests that valence-band holes in silicon are not directly involved in the anodic reactions of silicon oxidation and dissolution and their generation in crystals does not limit the rate of these processes. It is also shown that the character and rate of the chemical process leading to silicon dissolution in HF-containing electrolytes are determined by the interfacial potential attained at the semiconductor–electrolyte interface. The mechanism of electrochemical pore formation in silicon crystals is discussed in terms of selfconsistent cooperative reactions of nucleophilic substitution between chemisorbed fluorine anions and coordination- saturated silicon atoms in the crystal subsurface layer. A specific feature of these reactions for silicon crystals is that vacant nonbonding d{sup 2}sp{sup 3} orbitals of Si atoms, associated with sixfold degenerate states corresponding to the Δ valley of the conduction band, are involved in the formation of intermediate complexes. According to the suggested model, the pore-formation process spontaneously develops in local regions of the interface under the action of the interfacial potential in the adsorption layer and occurs as a result of the detachment of (SiF{sub 2}){sub n} polymer chains from the crystal. Just this process leads to the preferential propagation of pores along the <100> crystallographic directions. The thermodynamic aspects of pore nucleation and the effect of the potential drop across the interface, conduction type, and free-carrier concentration in the crystal on the pore size and structure are discussed. The concepts developed in the study can consistently account for experimental facts characterizing the etching of silicon crystals with various electrical parameters under various conditions providing the anodic polarization of crystals in HF-containing solutions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Energetic Substantiation of the Formation of Ytterbium Dimers in Single Forsterite Crystals

A structural computer simulation of ytterbium-containing forsterite crystals has been carried out. Atomistic simulation is performed using the GULP 4.1 software package (General Utility Lattice Program). Different mechanisms of ytterbium dissolution in forsterite crystals are considered, and the dissolution energies of isolated defects, as well as charged and neutral clusters of various configurations, are calculated. The results of the calculation manifest that the formation of ytterbium clusters with a magnesium vacancy gives a significant gain in the dissolution energy. The formation of neutral clusters (dimers) in the M1 site (Yb{sub Mg1}ν{sub Mg1}Yb{sub Mg1}) leads to an energy reduction by 1.7 eV compared with the statistical distribution of defects. As a result of simulation, it is shown the energetic substantiation of the formation of ytterbium dimers in forsterite crystals. A model of the most energetically favorable center in the M1 site is proposed—a dimer, consisting of a pair of trivalent ytterbium ions with a magnesium vacancy between them, forming a chain parallel to the crystallographic axis c.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

All that is silver is not toxic: silver ion and particle kinetics reveals the role of silver ion aging and dosimetry on the toxicity of silver nanoparticles

When suspended in cell culture medium, nano-objects composed of soluble metals such as silver can dissolve resulting in ion formation, altered particle properties (e.g. mass, morphology, etc.), and modulated cellular dose. Cultured cells are exposed not just to nanoparticles but to a complex, dynamic mixture of altered nanoparticles, free ions, and ion-ligand complexes. Here, three different cell types (RAW 264.7 macrophages and bone marrow derived macrophages from wild-type C57BL/6J mice and Scavenger Receptor A deficient (SR-A (-/-) ) mice) were exposed to 20 and 110 nm silver nanoparticles, and RAW 264.7 cells were exposed to freshly mixed silver ions, aged silver ions (ions incubated in cell culture medium), and ions formed from nanoparticle dissolution. The In Vitro Sedimentation, Diffusion, Dissolution, and Dosimetry Model (ISD3) was used to predict dose metrics for each exposure scenario. Silver nanoparticles, freshly mixed ions, and ions from nanoparticle dissolution were toxic, while aged ions were not toxic. Macrophages from SR-A (-/-) mice did not take up 20 nm silver nanoparticles as well as wild-types but demonstrated no differences in silver levels after exposure to 110 nm nanoparticles. Dose response modeling with ISD3 predicted dose metrics suggest that amount of ions in cells and area under the curve (AUC) of ion amount in cells are the most predictive of cell viability after nanoparticle and combined nanoparticle/dissolution-formed-ions exposures, respectively. Results of this study suggest that the unbound silver cation is the ultimate toxicant, and ions formed extracellularly drive toxicity after exposure to nanoparticles. Applying computational modeling (ISD3) to better understand dose metrics for soluble nanoparticles allows for better interpretation of in vitro hazard assessments.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2016

The role of gamma radiation on zircaloy-4 corrosion - 2016-0029

A key microstructural feature of irradiated oxide films that form on Zircaloy-4 is the development of significant amounts of interconnected porosity or microcracking, particularly along oxide grain boundaries. This development disrupts the protective nature of the barrier layer, effectively resulting in an increase in the observed post-transition corrosion rates. This observation suggests that some aspect of oxide dissolution may be occurring. It is noteworthy that when exposed to photons with energies greater than that of the oxide band gap, n-type semiconducting oxides are prone to dissolution. Because zirconia (ZrO{sub 2}) is an n-type semiconductor, it is plausible that high-energy photons may play an important role in corrosion. In an operating nuclear reactor, neutron and gamma flux levels are closely interrelated because they arise, for the most part, from the same originating process (i.e., the fissioning of nuclear fuel). Thus, it is difficult to isolate the independent effects of neutron and gamma radiation on corrosion. It is, however, theoretically conceivable to assess the role that gamma radiation may have on corrosion by comparing instances where variations in radiation environment exists; specifically by comparing where variations in gamma flux levels relative to neutron flux levels within different regions of a reactor or from reactor to reactor exist. Comparisons of the corrosion rates obtained between two different reactors (the Halden test reactor and the advanced test reactor [ATR]) that have differing gamma-to-neutron ratios were performed. In addition, an examination of corrosion rate data from the ATR, in which a variation in gamma-to-neutron flux ratio exists, was also conducted. Results are presented, from both assessments, which indicate that gamma radiation plays a role in the irradiated corrosion behavior of Zircaloy-4. In addition to a discussion of experimental data, potential mechanisms associated with photon-induced oxide dissolution of n-type semiconducting oxides, such as zirconia, is also provided. (authors)

36 MATERIALS SCIENCE↗
DOE OSTI2011

Effective Permeability Change in Wellbore Cement with Carbon Dioxide Reaction

Portland cement, a common sealing material for wellbores for geological carbon sequestration was reacted with CO{sub 2} in supercritical, gaseous, and aqueous phases at various pressure and temperature conditions to simulate cement-CO{sub 2} reaction along the wellbore from carbon injection depth to the near-surface. Hydrated Portland cement columns (14 mm diameter x 90 mm length; water-to-cement ratio = 0.33) including additives such as steel coupons and Wallula basalt fragments were reacted with CO{sub 2} in the wet supercritical (the top half) and dissolved (the bottom half) phases under carbon sequestration condition with high pressure (10 MPa) and temperature (50 C) for 5 months, while small-sized hydrated Portland cement columns (7 mm diameter x 20 mm length; water-to-cement ratio = 0.38) were reacted with CO{sub 2} in dissolved phase at high pressure (10 MPa) and temperature (50 C) for 1 month or with wet CO{sub 2} in gaseous phase at low pressure (0.2 MPa) and temperature (20 C) for 3 months. XMT images reveal that the cement reacted with CO{sub 2} saturated groundwater had degradation depth of {approx}1 mm for 1 month and {approx}3.5 mm for 5 month, whereas the degradation was minor with cement exposure to supercritical CO{sub 2}. SEM-EDS analysis showed that the carbonated cement was comprised of three distinct zones; the innermost less degraded zone with Ca atom % > C atom %, the inner degraded zone with Ca atom % {approx} C atom % due to precipitation of calcite, the outer degraded zone with C atom % > Ca atom % due to dissolution of calcite and C-S-H, as well as adsorption of carbon to cement matrix. The outer degraded zone of carbonated cement was porous and fractured because of dissolution-dominated reaction by carbonic acid exposure, which resulted in the increase in BJH pore volume and BET surface area. In contrast, cement-wet CO{sub 2}(g) reaction at low P (0.2 MPa)-T (20 C) conditions for 1 to 3 months was dominated by precipitation of micron-sized calcite on the outside surface of cement, which resulted in the decrease in BJH pore volume and BET surface area. Cement carbonation and pore structure change are significantly dependent on pressure and temperature conditions as well as the phase of CO{sub 2}, which controls the balance between precipitation and dissolution in cement matrix. Geochemical modeling result suggests that ratio of solid (cement)-to-solution (carbonated water) has a significant effect on cement carbonation, thus the cement-CO{sub 2} reaction experiment needs to be conducted under realistic conditions representing the in-situ wellbore environment of carbon sequestration field site. Total porosity and air permeability for a duplicate cement column with water-to-cement ratio of 0.38 measured after oven-drying by Core Laboratories using Boyle's Law technique and steady-state method were 31% and 0.576 mD. A novel method to measure the effective liquid permeability of a cement column using X-ray micro-tomography images after injection of pressurized KI (potassium iodide) is under development by PNNL. Preliminary results indicate the permeability of a cement column with water-to-cement ratio of 0.38 is 4-8 mD. PNNL will apply the method to understand the effective permeability change of Portland cement by CO{sub 2}(g) reaction under a variety of pressure and temperature conditions to develop a more reliable well-bore leakage risk model.

36 MATERIALS SCIENCE↗
DOE OSTI2012

HB-Line Anion Exchange Purification of AFS-2 Plutonium for MOX

Non-radioactive cerium (Ce) and radioactive plutonium (Pu) anion exchange column experiments using scaled HB-Line designs were performed to investigate the feasibility of using either gadolinium nitrate (Gd) or boric acid (B as H 3 BO 3 ) as a neutron poison in the H-Canyon dissolution process. Expected typical concentrations of probable impurities were tested and the removal of these impurities by a decontamination wash was measured. Impurity concentrations are compared to two specifications - designated as Column A or Column B (most restrictive) - proposed for plutonium oxide (PuO 2 ) product shipped to the Mixed Oxide (MOX) Fuel Fabrication Facility (MFFF). Use of Gd as a neutron poison requires a larger volume of wash for the proposed Column A specification. Since boron (B) has a higher proposed specification and is more easily removed by washing, it appears to be the better candidate for use in the H-Canyon dissolution process. Some difficulty was observed in achieving the Column A specification due to the limited effectiveness that the wash step has in removing the residual B after~4 BV's wash. However a combination of the experimental 10 BV's wash results and a calculated DF from the oxalate precipitation process yields an overall DF sufficient to meet the Column A specification. For those impurities (other than B) not removed by 10 BV's of wash, the impurity is either not expected to be present in the feedstock or process, or recommendations have been provided for improvement in the analytical detection/method or validation of calculated results. In summary, boron is recommended as the appropriate neutron poison for H-Canyon dissolution and impurities are expected to meet the Column A specification limits for oxide production in HB-Line.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Energy Saving Melting and Revert Reduction (E-SMARRT): Optimization of Heat Treatments on Stainless Steel Castings for Improved Corrosion Resistance and Mechanical Properties

It is commonly believed that high alloy steel castings have inferior corrosion resistance to their wrought counterparts as a result of the increased amount of microsegregation remaining in the as-cast structure. Homogenization and dissolution heat treatments are often utilized to reduce or eliminate the residual microsegregation and dissolve the secondary phases. Detailed electron probe microanalysis (EPMA) and light optical microscopy (LOM) were utilized to correlate the amount of homogenization and dissolution present after various thermal treatments with calculated values and with the resultant corrosion resistance of the alloys.The influence of heat treatment time and temperature on the homogenization and dissolution kinetics were investigated using stainless steel alloys CN3MN and CK3MCuN. The influence of heat treatment time and temperature on the impact toughness and corrosion reistance of cast stainless steel alloys CF-3, CF-3M, CF-8, and CF-8M was also investigated.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2016

Optimization of the Processing of Mo Disks

The objective of this work is to decrease the processing time for irradiated disks of enriched Mo for the production of 99 Mo. Results are given for the dissolution of nonirradiated Mo disks, optimization of the process for large-scale dissolution of sintered disks, optimization of the removal of the main side products (Zr and Nb) from dissolved targets, and dissolution of irradiated Mo disks.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2013

Degradation of Dome Cutting Minerals in Hanford Waste - 13100

At the Hanford Tank Farms, recent changes in retrieval technology require cutting new risers in several single-shell tanks. The Hanford Tank Farm Operator is using water jet technology with abrasive silicate minerals such as garnet or olivine to cut through the concrete and rebar dome. The abrasiveness of these minerals, which become part of the high-level waste stream, may enhance the erosion of waste processing equipment. However, garnet and olivine are not thermodynamically stable in Hanford waste, slowly degrading over time. How likely these materials are to dissolve completely in the waste before the waste is processed in the Waste Treatment and Immobilization Plant can be evaluated using theoretical analysis for olivine and collected direct experimental evidence for garnet. Based on an extensive literature study, a large number of primary silicates decompose into sodalite and cancrinite when exposed to Hanford waste. Given sufficient time, the sodalite also degrades into cancrinite. Even though cancrinite has not been directly added to any Hanford tanks during process times, it is the most common silicate observed in current Hanford waste. By analogy, olivine and garnet are expected to ultimately also decompose into cancrinite. Garnet used in a concrete cutting demonstration was immersed in a simulated supernate representing the estimated composition of the liquid retrieving waste from Hanford tank 241-C-107 at both ambient and elevated temperatures. This simulant was amended with extra NaOH to determine if adding caustic would help enhance the degradation rate of garnet. The results showed that the garnet degradation rate was highest at the highest NaOH concentration and temperature. At the end of 12 weeks, however, the garnet grains were mostly intact, even when immersed in 2 molar NaOH at 80 deg. C. Cancrinite was identified as the degradation product on the surface of the garnet grains. In the case of olivine, the rate of degradation in the high-pH regimes of a waste tank is expected to depend on two main parameters: carbonate is expected to slow olivine degradation rates, whereas hydroxide is expected to enhance olivine dissolution rates. Which of these two competing dissolution drivers will have a larger impact on the dissolution rate in the specific environment of a waste tank is currently not identifiable. In general, cancrinite is much smaller and less hard than either olivine or garnet, so would be expected to be less erosive to processing equipment. Complete degradation of either garnet or olivine prior to being processed at the Waste Treatment and Immobilization Plant cannot be confirmed, however. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2012

Treatment of Uranium Slugs at the CEA Marcoule site - 12026

The decladding units on the Marcoule nuclear site in southeast France were commissioned in the early 1960's to receive spent fuel cartridge for interim storage in pools and to prepare them for dissolution. After decladding, the uranium slugs were sent to the reprocessing plant for dissolution. The units were shut down in 1997. All the operational and process equipment was dismantled and transferred to the ANDRA waste repository between 1998 and 2001. During cleanup of the G2-G3 interim storage pool all the internal components (baskets and machinery) mounted on the civil engineering structures were cut up. In November 2007, a UNGG fuel slug in its aluminum cartridge was found at the bottom of the pool. Similarly, while iron slugs (used to control plutonium-producing reactors) were being sorted, five de-cladded uranium slugs were found between 2006 and 2008. As the processing facilities had already been dismantled, it was decided to send these fuel cartridges to a laboratory for dissolution. The project was to recover and condition the cartridge and slugs and transport them safely to the laboratory. This required the design of dedicated equipment due in particular to the high dose rate and pyro-phoricity hazard. The procedure also required safety clearance by the regulatory authorities for the operations within the facility and for the transport of radioactive materials. The cartridge was dissolved in May 2009, 18 months after it was discovered. Based on the experience acquired, the retrieval of the uranium slugs was completed in March 2011 after a year of preparation. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2012

Treatment of G1 Baskets at the CEA Marcoule Site - 12027

In the dismantling program for the first-generation French reactors in accordance with the nonproliferation treaty, the CEA is in charge of cleanup and dismantling operations for the facilities at Marcoule, including the decladding units. The G1 decladding was built between 1955 and 1957 in order to de-clad spent fuel elements from the G1 plutonium-producing reactor and prepare them for dissolution. The facility was also used for interim storage of G1, G2 and G3 fuel dissolution baskets, which had been used during plant operation for transfer (from the decladding facility to the UP1 plant) and/or dissolution of spent fuel elements. One of the cleanup projects involves recovery of the baskets, which will be cut up, sorted, and conditioned in metal bins. The bins will be immobilized with cement grout, then transferred to the onsite solid waste conditioning facility (CDS) and to the repository operated by the French National Radioactive Waste Management Agency (ANDRA). The project is now in progress, after special safety permits were issued and measurement stations and dedicated tools were developed to handle all types of baskets (which differed according to their origin and use). The disposal of all the baskets is scheduled to last 2 years and will produce 55 metal waste bins. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2012

Joint EM-NE-International Study of Glass Behavior over Geologic Time Scales - 12303

Vitrification has been chosen as the best demonstrated available technology for waste immobilization worldwide. To date, the contributions of physical and chemical processes controlling the long-term glass dissolution rate in geologic disposal remain uncertain, leading to a lack of international consensus on a rate law for glass corrosion. Existing rate laws have overcome uncertainty through conservatism, but a thorough mechanistic understanding of waste form durability in geologic environments would improve public and regulator confidence. If it is possible to take credit for the true durability of the waste form in repository system evaluations, then it is possible to design the repository with less conservatism with concomitant cost savings. To gain a fundamental understanding of the dissolution rate law, six nations have joined together to formulate a joint plan for collaborative research into the mechanisms controlling the long-term corrosion of glass. This report highlights the technical program plan behind the US portion of this effort, with an emphasis on the current understanding (and limitations) of several mechanistic theories for glass corrosion. Some recent results are presented to provide an example of the ongoing research. Atom probe tomography has been used to provide a high-resolution analysis of elemental concentration gradients present at the hydrated glass / pristine glass interface in SON68 after 25.75 years of corrosion in a simulated granitic groundwater at 90 deg. C. The most valuable result of these initial studies is the success of the technique. Characterization by APT had never been previously demonstrated for glass corrosion layers. The resolution of APT is a powerful addition to the tools with which we can investigate the mechanisms dominating glass corrosion. Some other key results of this study include the observation that the elemental interfacial width between the hydrated glass and pristine glass appears to be much sharper (∼2 nm for B, Na and Al) than had been previously measured using nanoSIMS (∼240 nm). It is not clear whether the APT analysis and nanoSIMS characterizations were possibly performed on topographically unique regions, or whether nanoSIMS overestimated the elemental width. However, the APT data seems very convincing that the elemental width can be much sharper than was previously thought. This result calls into question some of the assumptions made for the diffusion-control models of glass dissolution, since such a sharp profile would not match the diffusion coefficients used to date. Other results, such as the observation of apparently layered concentration profiles, show that gel evolution is not as simple as is currently assumed in nearly every model. This task is a good example of the collaborative nature of the I-TEAM effort. Based on experimental needs and differences in expertise, scientists from DOE and CEA worked together to change the level of understanding in the field. These types of interactions are nearly ubiquitous among the tasks in the technical program plan. With the excellence of the team in place and the willingness of the participants to work together for a common understanding, the stated goal of consensus on the mechanistic basis for radionuclide release from glass is well within reach. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2015

Chemical decontamination of a primary coolant system using hydrazine based solutions - 15215

For the decontamination of primary coolant system, we have developed a new chemical decontamination process named HYBRID (Hydrazine Based Reductive metal Ion Decontamination) which is not containing organic chelates and organic acids. In the first phase to develop HYBRID process, dissolution tests of magnetite using hydrazine based chemical solutions were carried out in various conditions. We found out the effects of parameters such as the concentration of hydrazine, solution pH, temperature, and the addition of transition metal ions on the dissolution behavior. We suggested the concept of HYBRID process with the dissolution mechanism by the complex formation between hydrazine and copper ions, and the reducing power of the hydrazine copper coordination compound to the ferric ions in magnetite. In the second phase to verify the performance of HYBRID process, the chemical decontamination tests using HYBRID coupled with NP were carried out with radioactive specimen taken from the fuel test loop in which operating conditions are same as those in pressurized water reactor. The contact does rate was greatly decreased after the repetitive application of NP and HYBRID. In addition, the corrosion compatibility of the structural material and the decomposition of hydrazine to reduce the secondary waste have been investigated. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2016

An Approach to Improve the Retention of the Fission Product Ag in TRISO Fuel Particles - 18596

The silicon carbide (SiC) coating included in TRISO particles acts as the main barrier against the radioactive fission products release into the coolant circuit. However, radioactive silver (Ag{sup 110m}) was identified outside the intact particles with a very high release fraction, which could be problematic in the licensing process for modern HTR fuel. Our previous work proposed that dissolution/recrystallization of SiC by Ag could help Ag migrate at 1800 deg. C. The aim of this work is to study whether dissolution of SiC by Ag could take place and contribute to the Ag migration at operation temperature (from 1000 to 1300 deg. C). Both, experiment and thermodynamic analysis, showed that SiC dissolution by Ag was possible above 890 deg. C. But the dissolution from 1000 to 1300 deg. C is far less effective in moving Ag due to the insufficient driving force for activating self-penetration into the grain boundaries, compared with the previous work conducted at 1800 deg. C. Compared with silver alone, the silver-palladium mixture was more permeable in SiC and showed a diffusive behavior. The diffusion front was Ag rich while most palladium remained in reaction zone rather than in the diffusion front. Furthermore, thermodynamic analysis suggested that non-stoichiometric SiC with additional free silicon could impede such a reaction. Hence it is conceivable that a sacrificial layer of SiC+Si on the inner side of the main SiC coating could improve fission product retention overall without a significant increase in cost. (authors)

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Use and misuse of large-density asymptotics in the reaction-infiltration instability

Analyzing the dissolution of rocks and other porous materials is simplified by the large dis-parity between mineral and reactant concentrations. In essence, the porosity remains frozen on the time scale of the reactant transport, which can then be treated as a quasi-stationary process. This conceptual idea can be derived mathematically using asymptotic methods, which show that the length scales in the system are, to a first approximation, independent of the ratio of reactant and mineral concentrations. Nevertheless, in a growing number of papers on dissolutional instabilities, the reactant-mineral concentration ratio has been incorrectly linked to the thickness of the dissolution front. In this paper we critically review the application of asymptotic methods to the reaction-infiltration instability. In particular, we discuss the limited validity of the thin-front or ‘‘Stefan’’ limit, where the interface between dissolved and undissolved mineral is sharp.

58 GEOSCIENCES↗
DOE OSTI2017

Linear permeability evolution of expanding conduits due to feedback between flow and fast phase change: Linear Permeability Evolution of Conduit

Conduits are ubiquitous and critical pathways for many fluids relevant for geophysical processes such as magma, water, and gases. Predicting flow through conduits is challenging when the conduit geometry coevolves with the flow. We theoretically show that the permeability (k) of a conduit whose walls are eroding due to fast phase change increases linearly with time because of a self-reinforcing mechanism. This simple result is surprising given complex feedbacks between flow, transport, and phase change. The theory is congruent with previous experimental observations of fracture dissolution in calcite. Supporting computational fracture dissolution experiments showed that k only slightly increases until the dissolution front reaches the narrowest conduit constriction, after which the linear evolution of k manifests. Finally, the theory holds across multiple scales and a broad range of Peclet and Damkohler numbers and thus advances the prediction of dynamic mass fluxes through expanding conduits in various geologic and environmental settings

58 GEOSCIENCES↗