Diffusion Of Cesium Research Articles

Preparation of pollucite ceramic matrices as 137Cs ionizing radiation source by spark plasma sintering

The paper proposes an original one-stage method using spark plasma sintering technology (SPS) of manufacturing the ionizing radiation source (IRS) of closed type with a non-dispersible ceramic core based on pollucite obtained using aluminosilicate raw materials saturated (24.3 wt%) with cesium, and doped with 10–40 wt% CsCl. According to the XRD, EDX, AAS and DTA-TG data, optimal conditions for hydrothermal synthesis zeolite NaA were established, the consolidation kinetics and phase transformations into ceramic pollucite under SPS conditions were studied. The ceramics had a relative density of 99.8 % and mechanical strength up to 732 MPa, a low rate of cesium leaching 10−7 g/cm2 day and thermal resistance in air up to 1000 °C. It was confirmed by SEM and EDX that the destruction and deformation of pollucite ceramics in the composition with CsCl and steel at the boundary of their contact in the design of the IRS product was absent and diffusion of cesium outside the ceramics did not occur. The achieved cesium content in the ceramic core of the resulting the ionizing radiation source, which meets high quality and regulatory requirements, is 44.3 wt%. The results of the study may be promising for the manufacture of industrial products.

Diffusion of tritium, technetium, cesium, and uranium from phosphate glass leachates in pore solution of clay materials

The through diffusion method at room temperature was used to study the migration of radionuclides (3H, 99Tc, 137Cs, 233U) in compacted samples of clay materials during pore diffusion from a model leachate of the RW phosphate matrix with a total salt content of about 400 mg/L. Based on the results of the experiments, the effective diffusion coeffcients and coeffcients of the sorption distribution of radionuclides for the studied barrier clay materials were determined. Regularities of the diffusion transfer of tritium, cesium and uranium depending on the structure, mineral composition of clay materials and the content of radionuclides in the pore solution are revealed. Numerical modeles are proposed for calculating the effective pore di usion coeffcients of these radionuclides. There are still insuffcient data to analyze the factors affecting the diffusion of technetium.

Diffusion of Tritium, Technetium, Cesium and Uranium from Phosphate Glass Leachates in a Pore Solution of Clay Materials

Diffusion of Tritium, Technetium, Cesium and Uranium from Phosphate Glass Leachates in a Pore Solution of Clay Materials

Solidification of cesium containing magnetic zeolite sorbent by spark plasma sintering

The possibility of effective solidification of spent sorbents containing radioactive cesium by spark plasma sintering (SPS) technology for their safe and long-term disposal was studied. The kinetics of consolidation spent sorbent based on magnetic zeolite after adsorption saturation with cesium (∼23 wt %) under SPS conditions in the range of 700–950 °C was carried out. The structural, elemental and phase composition changes during high-rate (minutes) heating of zeolite powder saturated with Cs+ ions was studied by SEM, EDX and XRD methods. As a result, ceramic samples with mineral-like structure of Fe-substituted pollucite CsFeSi2O6 were successfully obtained. It was determined that the optimal temperature of SPS consolidation was 900 °C, at which the formation of CsFeSi2O6 ceramics with a relative density of 91.2%, compressive strength of 497.6 MPa, Vickers microhardness of 1019 HV, rate of cesium leaching of 2.33 × 10−8 g/cm2 × day was reached. It was found that the mechanism of cesium diffusion from the volume of ceramics corresponded to flushing from the available surface of the sample, the lowest diffusion coefficient was 2.71 × 10−16 cm2/s, as well as the lowest penetration depth of the solution into the volume of ceramics was 1.04 × 10−5 cm. The adsorbents prepared by SPS up to 900 °C retained the magnetization, and at higher temperature the magnetization decreased, which was caused by phase and structural changes of ceramics. The characteristics of ceramic obtained from spent adsorbent complied with the requirements of GOST R 50926–96 and ISO 6961:1982 for solidified Cs-containing radioactive waste.

Scaling effect on cesium diffusion in compacted MX-80 bentonite for buffer materials in HLW repository

Abstract In this study, radionuclide behavior in high-level radioactive waste (HLW) disposal repositories is complicated because of the spatial heterogeneity of porous media, coupled flow-transport mechanisms, and multiple chemical reaction processes. Discrepancies in the diffusion behavior of a non-sorbing tracer (HTO) and a reactive tracer (137Cs) in porous media have long been recognized but are not yet fully understood, which hinders effective assessment of the capabilities of buffer materials. This paper was dedicated to exploring and explaining the discrepancies in the transport behavior of non-sorbing and reactive tracers through laboratory experiments and an investigation of contributing mechanisms. Our results showed that for a bentonite sample of the same thickness, 137Cs has smaller apparent and less effective diffusion coefficients than those for HTO. These discrepancies can be attributed to the negative surface electric effects, atomic properties, and chemical reactions. In the case of bentonite samples with different thicknesses (0.5, 0.75, 2.0, 2.5 cm), the apparent and effective diffusion coefficients show an increasing trend with bentonite thickness. According to the experimental data and fitting results, the apparent and effective diffusion coefficients are highly related to bentonite thickness. Thus, scaling effects on transport parameters were proposed to explain the results, which were attributed to the nonuniform distribution of the pore space in the bentonite sample. The scale effect behavior of radionuclide was quantified through a regression analysis. The results can be used to improve buffer designs for radionuclides diffusion.

Diffusion of cesium in oxidized and unoxidized IG-110 nuclear graphite

Time-release diffusion measurements of cesium have been conducted over the temperature range 1073 K – 1973 K on oxidized and unoxidized IG-110 graphite. Four cesium concentrations were tested to investigate the concentration dependence of the diffusion coefficient. Two levels of oxidation were tested and compared to unoxidized concentration-matched sets to explore the effects of graphite oxidation. The results demonstrate that cesium diffusion coefficient in unoxidized IG-110 graphite is independent of concentration within the range 34 – 163 µg Cs/ggraphite. Above this, the effective cesium diffusion coefficient changes with concentration. The diffusion coefficient was increased by a factor of 2–12 in the oxidized set with 7.8% mass loss. These results can be used to aid predictive modeling of cesium diffusion in HTGR cores.

Understanding the Formation Process of Perovskite Layers Grown by Chemical Vapour Deposition

This work aims at extending the understanding of the formation processes of (Cs0.07FA0.93)PbI3 perovskite layers deposited by a two-step vapour method. In a first step, an inorganic CsI/PbI2 precursor stack is deposited by thermal evaporation (TE). A chemical vapour deposition (CVD) is then used to convert the precursor into the perovskite layer by reaction with a chemical vapour of formamidinium iodide (FAI). Here we show how crystallinity and morphology of the TE precursor layer are both influenced not only by the substrate surface properties but also by the thermal treatment in the initial phase of the CVD process. Furthermore, we provide insights on the evolution of perovskite film formation and show how a uniform elemental composition is achieved by the diffusion of cesium through PbI2 during the CVD conversion reaction.

Hydrothermal synthesis and spark plasma sintering of NaY zeolite as solid-state matrices for cesium-137 immobilization

Hydrothermal synthesis of NaY-type zeolite was carried out and the effect of temperature on the phase composition, crystal structure, textural characteristics, particle size and morphology, as well as sorption properties to Cs+ ions was studied. Solid-state matrices based on NaY zeolite the Faujasite structure containing 26.1 wt% cesium were obtained by spark plasma sintering (SPS) with high values of compressive strength (to 132.9 MPa) and Vickers microhardness to HV~ 698, Fracture toughness (K1c) ~ 1.26 MPa m1/2. The kinetics of ceramic matrices consolidation, phase composition and morphology using dilatometric studies, XRD, and SEM were studied. The thermogravimetric analysis shown the high thermal stability of the obtained samples up to 1300 °C. The high hydrolytic stability of CsAlSiO4 ceramic was proven (leaching rate of 2.33 ×10−8 g·cm−2·day−1 and cesium diffusion coefficient De 1.41 ×10−13), which exceeds the requirements of GOST R 50926–96 and ISO 6961:1982 for solid-state matrices.

Synthesis of amorphous KAlSi3O8 for cesium radionuclide immobilization into solid matrices using spark plasma sintering technique

An effective sorption material for cesium radionuclides immobilization in highly safe and reliable solid-state matrices was proposed. Prepared aluminosilicate (КAlSi3O8) adsorbent had amorphous mesoporous structure and Cs+ ions sorption capacity of ∼3.7 mmol/g. The physical-chemical characteristics of (Cs, К)AlSi3O8 sample saturated with Cs + ions were studied using XRD, FT-IR, SEM-EDX, and DTA-TG methods. Firstly, solid-state aluminosilicate matrices were obtained using spark plasma sintering (SPS) technology with high values of relative density (up to 99.9%), compressive strength (31.3–79.2 MPa), and Vickers microhardness (0.9–5.3 GPa). The sample obtained at 1000 °C had a low value of Cs+ leaching from matrices (RCs within the range of 10-7 g cm-2·day-1) and cesium diffusion coefficient (De 9.07 × 10-14 cm2/s). It was shown that prepared aluminosilicate cesium matrices comply with regulatory requirements of GOST R 50926-96 and ANSI/ANS 16.1.

Cs diffusion mechanisms in UO2 investigated by SIMS, TEM, and atomistic simulations.

Experimental investigations and atomistic simulations are combined to study the cesium diffusion processes at high temperature in UO2. After 133Cs implantation in UO2 samples, diffusion coefficients are determined using the depth profile evolution after annealing as measured by secondary ion mass spectrometry. An activation energy of 1.8 ± 0.2eV is subsequently deduced in the 1300-1600 °C temperature range. Experimental results are compared to nudged elastic band simulations performed for different atomic paths including several types of uranium vacancy defects. Activation energies ranging from 0.49 up to 2.34eV are derived, showing the influence of the defect (both in terms of type and concentration) on the Cs diffusion process. Finally, molecular dynamics simulations are performed, allowing the identification of preferential Cs trajectories that corroborate experimental observations.

Effect of alkali activators on diffusivity of metakaolin-based geopolymers

A basic investigation into whether a geopolymer can be utilized as a part of an artificial barrier during radioactive waste disposal was conducted in this study. Geopolymers are comprised primarily alumina and silica, and they exhibit negligible leaching owing to the absence of calcium. Studies on geopolymers are limited compared to those on other cementitious materials because the physical characteristics of geopolymers vary with the production conditions. In this work, metakaolin based geopolymers were prepared, and their diffusion performance was analyzed. The results indicate that the diffusivity of cesium in a geopolymer is affected by the type of alkali activator. Sodium-activated geopolymers had higher cesium adsorption capacity than potassium-activated geopolymers. The cesium adsorption capacity also had a significant effect on the diffusivity of cesium in the geopolymers. It was shown that, in addition to the pore structure and surface area, the mobility of water affects the diffusion performance of the geopolymer.

Mechanisms of Sorption of Cesium and Strontium by Ferrocyanide-Modified Clinoptilolite at Their High Concentration in Solutions

The kinetics of cesium and strontium sorption by natural clinoptilolite and its ferrocyanide-modified analogs, NPF-Cl and NPF-Cl (Ca-form), from solutions of various compositions containing 100 mg L–1 sorbate was studied, and the texture and porosity of the sorbent samples before and after the sorption of cesium and strontium were determined. The sorption process occurs in three-stages and is limited by a heterogeneous ion exchange reaction (HIOR) on the ferrocyanide phase located on the outer surface of clinoptilolite and in its macropores, internal diffusion of cesium and strontium into sorbent granule, and precipitation of ferrocyanide phases of complex composition in small mesopores and micropores of sorbents. The HIOR and precipitation occur with involvement of solution components (sodium, calcium, and iron). Sorption of cesium and strontium is accompanied by a significant change in the texture and porosity of the sorbents.

Geopolymerisation of gasified ion-exchange resins, mechanical properties and short-term leaching studies

A combination of gasification of low and intermediate level radioactive waste (LILW) and conditioning of the resulting product within a geopolymer matrix is a potential alternative for vitrification technologies as an immobilisation method. Geopolymer matrices have demonstrated good retention capability for radionuclides in many studies and the technology has been implemented at an industrial scale in Slovakia and Czech Republic. However, the practical waste loading has been limited by the mechanical properties of the encapsulated matrix. Even a small amount of ion exchange resin (IXR) decreases the strength of the matrix and cohesion of the matrix is lost when the fraction of resin exceeds 15-20%. In this study, the potential to combine gasification as thermal treatment and various inorganic binders as encapsulation matrices was evaluated. After gasification, the mechanical properties were not similarly sensitive to the encapsulation of IXR. Gasification enabled substantially higher loading of IXR into the sample. Also, gasification of the IXR decreased matrix apparent Caesium diffusion. Very low apparent diffusion coefficients of Cs were calculated when gasified resin was encapsulated in metakaolin matrix. Theoretically, the amount of Cs within the same volume of encapsulated material could be increased by 800 times following gasification and encapsulation within an alkali-activated metakaolin (MK) binder.

Thermal properties of CsPbCl3 thin films across phase transitions

The local thermal conductivity, thermal diffusivity, and volumetric heat capacity of cesium lead chloride perovskite thin films are mapped simultaneously and with highest spatial resolution by a scanning near-field thermal microscope. Both, the 3D phase (CsPbCl3) and the 0D phase (Cs4PbCl6) are investigated. For CsPbCl3 thin films the variation of the thermal properties across the phase transitions in the range from room-temperature to 65 °C are analyzed. While the thermal conductivity at room temperature is ultra-low, a significant increase of the thermal conductivity is found for the cubic phase of CsPbCl3 (T > 46 °C). While only slight variations in the thermal conductivity are detectable for transitions from the monoclinic to the orthorhombic to the tetragonal phase, thermal diffusivity and volumetric heat capacity measurements are extremely sensitive to the amount of heat involved in the respective transition. It is shown that upon transition to the cubic phase of CsPbCl3 thin films, the relative increase of the volumetric heat capacity is significantly higher than that of the thermal conductivity. Thus, the thermal diffusivity in the cubic phase becomes notably lower in comparison to that of the respective phase at room temperature. An increase of the volumetric heat capacity had been theoretically predicted earlier but could not be confirmed in previous experimental studies. The findings of our thermal analysis are of great general importance for fundamental material research and for the thermal design of thin-film devices based on CsPbCl3 perovskites.

High-flux thin-film composite polyamide (TFCP) forward osmosis membranes for concentration of simulated cesium- and strontium-bearing effluent solution

ABSTRACTIn this study, we discuss the preparations of thin-film composite polyamide (TFCP) membranes in spiral configuration and their potential for concentration of simulated cesium and strontium-bearing effluent solution (only cesium chloride, strontium chloride and sodium chloride) by forward osmosis (FO). The membranes are prepared by in-situ polymerization technique using m-phenylene diamine (MPD) with phenol and trimesoyl chloride (TMC) system over hydrophilic sulfonated polyethersulfone support followed by heat curing in steam (temperature ~90–100°C). Prepared membranes are characterized in terms of separation performance in FO mode (de-ionized water as feed and 1.0 M NaCl solution as draw solution). When phenol additives are added into water phase, the membrane achieved high water flux of 25.6 L m−2 h−1 with a modest salt flux of 6.43 g m−2 h−1. A 2512 spiral element was rolled using indigenously developed TFCP-FO membranes and performance was evaluated in terms of volume reduction and back-diffusion of draw solution salts using simulated feed solution (500-ppm NaCl; 1-ppm SrCl2 and CsCl each) with 2(M) solutions of CaCl2 & (NH4)2CO3 as draw solution. It was found that the volume reduction of the feed solution is 61–65% after 3 h and back diffusion of cesium and strontium from draw solution to feed are found around 0.022–0.029 ppm.

Adsorption and diffusion of cesium in pyrophyllite pore: a molecular dynamics study

In the article, molecular dynamics simulations are carried out to investigate the adsorption and diffusion behaviors of Cs+ in pyrophyllite nanopore. The results of radial distribution function and coordination number indicate that the first hydration shell radius of Cs+ and hydration number are 3.05 A and 8.0~8.5, respectively. Atomic density profiles demonstrate that Cs+ adsorbed on the pyrophyllite basal in the form of outer-sphere surface complex. And the concentration and temperature have little impact on the hydrated structure and adsorption percent of Cs+. Increasing temperature will promote the thermal motion of Cs+, which leads to diffusion coefficient increased dramatically.

Modeling Transport of Cesium in Grimsel Granodiorite With Micrometer Scale Heterogeneities and Dynamic Update of Kd

AbstractTransport and retardation of cesium in Grimsel granodiorite taking into account heterogeneity of mineral and pore structure was studied using rock samples overcored from an in situ diffusion test at the Grimsel Test Site. The field test was part of the Long‐Term Diffusion (LTD) project designed to characterize retardation properties (diffusion and distribution coefficients) under in situ conditions. Results of the LTD experiment for cesium showed that in‐diffusion profiles and spatial concentration distributions were strongly influenced by the heterogeneous pore structure and mineral distribution. In order to study the effect of heterogeneity on the in‐diffusion profile and spatial concentration distribution, a Time Domain Random Walk (TDRW) method was applied along with a feature for modeling chemical sorption in geological materials. A heterogeneous mineral structure of Grimsel granodiorite was constructed using X‐ray microcomputed tomography (X‐μCT) and the map was linked to previous results for mineral specific porosities and distribution coefficients (Kd) that were determined using C‐14‐PMMA autoradiography and batch sorption experiments, respectively. After this the heterogeneous structure contains information on local porosity and Kd in 3‐D. It was found that the heterogeneity of the mineral structure on the micrometer scale affects significantly the diffusion and sorption of cesium in Grimsel granodiorite at the centimeter scale. Furthermore, the modeled in‐diffusion profiles and spatial concentration distributions show similar shape and pattern to those from the LTD experiment. It was concluded that the use of detailed structure characterization and quantitative data on heterogeneity can significantly improve the interpretation and evaluation of transport experiments.

Microtomography-based Inter-Granular Network for the simulation of radionuclide diffusion and sorption in a granitic rock

Field investigation studies, conducted in the context of safety analyses of deep geological repositories for nuclear waste, have pointed out that in fractured crystalline rocks sorbing radionuclides can diffuse surprisingly long distances deep into the intact rock matrix; i.e. much longer distances than those predicted by reactive transport models based on a homogeneous description of the properties of the rock matrix. Here, we focus on cesium diffusion and use detailed micro characterisation data, based on micro computed tomography, along with a grain-scale Inter-Granular Network model, to offer a plausible explanation for the anomalously long cesium penetration profiles observed in these in-situ experiments. The sparse distribution of chemically reactive grains (i.e. grains belonging to sorbing mineral phases) is shown to have a strong control on the diffusive patterns of sorbing radionuclides. The computed penetration profiles of cesium agree well with an analytical model based on two parallel diffusive pathways. This agreement, along with visual inspection of the spatial distribution of cesium concentration, indicates that for sorbing radionuclides the medium indeed behaves as a composite system, with most of the mass being retained close to the injection boundary and a non-negligible part diffusing faster along preferential diffusive pathways.

Characterization of cesium diffusion behavior into granite matrix using Rutherford backscattering spectrometry

The characterization of radionuclide diffusion behavior is necessary for performance assessment of granite as a geological barrier for high-level radioactive waste disposal. Rutherford backscattering spectrometry (RBS), a novel nuclear ion-beam technique, was selected in this study because it is suitable for analyzing the concentration gradients of heavy elements in a well-defined matrix and allows measuring diffusion coefficients on a micrometer scale. In this study Cs was selected to represent Cs-135 (a key radionuclide in high-level waste) diffusion in granite. The Cs energy spectrum and concentration deep profile were analyzed and the diffusion coefficient of Cs in granite for three different locations were determined, which were 2.06×10−19m2s−1, 3.58×10−19m2s−1, and 7.19×10−19m2s−1-19m2s–19m2s−1, respectively, which were of a similiar order of magnitude. Results from other studies are also compared and discussed in this paper.

Defect diffusion assisted formation of cesium metal clusters in cesium halide thin films

This manuscript describes the evolution of grain structures in cesium halide films which leads to localized surface plasmon resonance (LSPR) peaks in UV–visible absorption spectra. Thin films of cesium halide age with time as grain boundaries recede making grains spherical in shape in order to minimize the free surface energy of the system. The process is also assisted by the presence of point defects/color centers in cesium halide films. The defects migrate outwards towards the grain surface, giving cesium metal clusters at the surface, resulting in core–shell structure. Along with volume diffusion, a surface diffusion of cesium takes place towards sites of facet grains boundaries resulting in a necking phenomenon, appearing like bridges between the daughter cesium halide grains (grains appearing due to the division of a single grain). Breaking away of the daughter cesium halide grains results in nano-rods that contribute to the SPR peak seen.