Diffusion Coefficients Of Cs Research Articles

A sensitive improved method for analyzing diffusion coefficients of Cs in compacted bentonite with different lengths

Abstract Based on the one-dimensional diffusion theory, the diffusion parameters were obtained from numerical fitting by the Cyclic Initial Value (CIV) program written by MATLAB. Taking the through-diffusion experimental of cesium (stable isotope 133Cs) as an example, on the premise of ensuring accuracy, fitting calculation was used to obtain the diffusion equilibrium time of Cs in different lengths bentonite column. The fitting results of diffusion test for tritium water (HTO) and Cs are both very well. The calculation results of the equilibrium time for Cs diffusion show that the equilibrium time obtained by CIV is less than the experimental period in both groundwater (GW) and seawater (SW). In GW environment, when the sensitivity is at the maximum setting value, the diffusion coefficient of Cs in 1.5 cm bentonite column could be calculated in a shorter period of time. Compared with the experimental period, the time was shortened by 110 days. The main purpose is to verify the feasibility of CIV through the experimental data of Cs in different column lengths. The CIV program can also be used to fit and calculate the experimental data of other ongoing diffusion experiments and get the equilibrium time required for diffusion experiments. It shows that the proposed models offer the advantages of saving experimental time and reducing experimental waste.

Diffusion of fission products in nuclear graphite: A review

• A literature review of the metallic FPs diffusion in nuclear graphite is presented. • The diffusion theories applied in the diffusivity determination are summarized. • Measurements of the diffusion coefficient of metallic FPs are discussed. • The variables affecting the diffusion coefficients of Cs, Sr, and Ag are discussed. Nuclear graphite in high-temperature gas-cooled reactors (HTGRs) acts as a moderator, a structural material, as well as an accommodation for fission products (FPs). Given the release of FPs under normal and accident conditions, the diffusion of FPs in nuclear graphite should be elucidated for the estimation of source term and reactor safety. However, the existing experimental data of diffusion are enriched in data of the nuclear graphite types (i.e., H451 and A3), that may not apply to next-generation nuclear graphite. The proposed diffusion theory and experimental systems are conducive to developinging a novel experimental method for the more accurate collection of the diffusion data and an insight into FPs diffusion in graphite. For this reason, this paper reviews the information regarding the diffusion of metallic FPs (e.g., Cs, Sr and Ag) in nuclear graphite. The diffusion theory adopted to calculate the diffusion coefficient is summarized. Emphasis is placed on the measurements for determination the diffusion coefficient and the discussion of detection limits, accuracy and experimental methods. Given the diffusion coefficient data, the variables of diffusion are addressed. The mentioned results lay a basis for gaining insights into the diffusion theory and can be exploited by employing with a valid measurement method to determine the diffusion coefficients of FPs in various nuclear graphite materials under a particular range of conditions.

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.

Structure and dynamics of Cs+ in kaolinite: Insights from molecular dynamics simulations

In the present study, the adsorption and diffusion behaviors of Cs+ in kaolinite are investigated by classical molecular dynamics (MD) simulations with the effects of concentration, temperature, and competing cations. The obtained results turn out that Cs+ adsorbs preferentially on siloxane surface in the form of inner- and outer-sphere surface complexes. The adsorption percent of Cs+ diminishes with increasing concentration. Elevating temperature facilitates the adsorption of Cs+, while introduction of foreign cations weakens the interaction between siloxane surface and Cs+. It is indicated that the diffusion coefficient of Cs+ increases remarkably as the temperature rises. Nevertheless, varying concentration and competing cations have only slight effects on diffusion coefficient of Cs+. The results presented here can provide valuable insights into the interaction mechanism between Cs+ and kaolinite basal surfaces.

Apparent diffusion coefficients of Cs and I in hardened cement paste using mineral admixtures

Apparent diffusion coefficients of Cs and I in hardened cement paste using mineral admixtures

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.

Diffusion of cesium and iodine in compressed IG-110 graphite compacts

Nuclear graphite grade IG-110 is currently used in the High Temperature Engineering Test Reactor (HTTR) in Japan for certain permanent and replaceable core components, and is a material of interest in general. Therefore, transport parameters for fission products in this material are needed. Measurement of diffusion through pressed compacts of IG-110 graphite is experimentally attractive because they are easy to prepare with homogeneous distributions of fission product surrogates. In this work, we measured diffusion coefficients for Cs and I in pressed compacts made from IG-110 powder in the 1079–1290 K temperature range, and compared them to those obtained in as-received IG-110.

ICP-MS measurement of diffusion coefficients of Cs in NBG-18 graphite

Graphite is used in the HGTR/VHTR as moderator and it also functions as a barrier to fission product release. Therefore, an elucidation of transport of fission products in reactor-grade graphite is required. We have measured diffusion coefficients of Cs in graphite NBG-18 using the release method, wherein we infused spheres of NBG-18 with Cs and measured the release rates in the temperature range of 1090–1395 K. We have obtained:DCs,NBG−18=(1.0×10−7m2/s)exp(−1.23×105J/molRT)These seem to be the first reported values of Cs diffusion coefficients in NBG-18. The values are lower than those reported for other graphites in the literature.

Measurement of cesium diffusion coefficients in graphite IG-110

An understanding of the transport of fission products in High Temperature Gas-Cooled Reactors (HTGRs) is needed for operational safety as well as source term estimations. We have measured diffusion coefficients of Cs in IG-110 by using the release method, wherein we infused small graphite spheres with Cs and measured the release rates using ICP-MS. Diffusion behavior was investigated in the temperature range of 1100–1300K. We have obtained: DCs=(1.0×10-7m2/s)exp-1.1×105J/molRT and, compared our results with those available in the literature.

Sorption kinetics of cesium on ZrO2 and ZrO2-SiO2-TiO2 microspheres

ZrO2 and ZrO2-SiO2-TiO2 (with 1:1:1 mol ratio) microspheres have been prepared by the sol-gel method and characterized to study the sorption kinetics of cesium. The effect of pH on the adsorption capacity was determined for both type sorbents and the maximum adsorption percentage of cesium was observed at pH 10. To elucidate the kinetics of the ion-exchange reaction on pure and ternary oxide microspheres, the isotopic exchange rates of cesium ions between both types of sorbent microspheres and aqueous solution were measured radiochemically and compared with each other. The diffusion coefficients of Cs+ ion for ZrO2 and ZrO2-SiO2-TiO2 sorbent microspheres at pH 10 were calculated as 3.648×10−8 m2 s−1 and 6.383×10−8 m2 s−1, respectively, under particle diffusion controlled conditions.

A study on leaching behaviour of cement blocks used as matrix for fixation of Cs activity along with ferric

Study on the cementation of the regenerated activity (from spent resin using ferric as regenerant) containing ferric in cement matrix showed that compressive strength and leaching behaviour are better when the ferric strength was < 5 N. The diffusion coefficient of Cs from the cement matrix was found to be in the range 2.4 × 10−5 cm2/day and 5.9 × 10−5 cm2/day with ferric solutions of strength in the range 0.5–3 N. When bentonite and vermiculite were included in the cement matrix, the diffusion coefficient of Cs was found to be in the range 6.2 × 10−7 cm2/day to 1.26 × 10−5 cm2/day with ferric strength in the same range.

Sorption of Radiocaesium onto Powdered and Compacted Rectorite Studied by Batch and Capillary Methods

The sorption of radiocaesium onto Na-rectorite as a function of pH value under ambient conditions was studied by batch and capillary methods. The results indicated that the sorption of Cs+ions was dependent on the ionic strength and, to a lesser extent, on the pH value. The distribution coefficients for Cs+ion sorption onto powdered rectorite (as determined via the batch technique) were much higher than those of Cs+ions onto compacted rectorite (as determined by capillary test), thereby suggesting that the sorption of Cs+ions onto rectorite was strongly dependent on the density of rectorite, i.e. the interlaminary space of the compacted rectorite contributes significantly on Cs+ion sorption. The distribution coefficients calculated from theoretical calculations, experimental measurements and the diffusion coefficients of Cs+ions were compared and discussed in detail. The sorption and diffusion mechanisms in compacted rectorite were also discussed.

Kinetic and thermodynamic studies of cesium(I) sorption on hydrous silica

Batch sorption experiments of cesium, Cs+, on SiO2 . xH2O (silica gel) have been conducted with variable times of equilibration, amounts of silica gel (0.10-1.00 g), cesium concentrations (5.00 . 10-5-2.40 . 10-3M), ionic strengths (0.20-1.40M NaClO4), pH (2.50-7.70), and temperatures (273-333 K). The diffusion coefficient of Cs+ ion was calculated to be (9.19±0.86) . 10-11 m2 . s-1 under particle diffusion-controlled conditions. The sorption rate was (3.94±0.65) . 10-3 s-1 at 298 K, pH 7.70±0.05 in 0.20M NaClO4. The sorption data fits the Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherms. Cesium sorption on 0.20 g silica gel decreased with ionic strength from (40.42±0.34)% in 0.20M NaClO4 to (6.35±0.40)% in 1.40M NaClO4, at pH(initial) 8.20±0.05. A gradual decrease in pH with increased ionic strength is consistent with a cation-exchange mechanism. Sorption of Cs+ on silica gel decreased with increased temperature, indicating an exothermic enthalpy. The presence of anions such as fluoride, carbonate, phosphate and oxalate in the aqueous medium did not influence the cesium sorption profile.

Cesium incorporation and diffusion in cancrinite, sodalite, zeolite, and allophane

At the US Department of Energy’s Hanford site, high level nuclear waste has leaked from underground storage tanks. The waste consists of hyperalkaline solutions, which upon contact with the sediments, caused dissolution of silicate minerals and precipitation of new aluminosilicate minerals. Cancrinite, sodalite, LTA zeolite, and allophane have been identified as the new mineral phases in laboratory simulations. Cesium, the major radionuclide in the waste solutions, may be incorporated into the structural framework of the precipitates. The objectives of this study were to determine the resistance of incorporated Cs + to ion exchange and the mobility and diffusion coefficient of Cs in the minerals. The minerals were synthesized in solutions mimicking the tank waste and were washed with deionized water. Two sets of experiments were conducted to test the resistance of Cs + to ion exchange. In the first set, Cs + was exchanged three times at 80 °C by 0.5 N Na +, K +, or Ca 2+. The Cs remaining in minerals was quantified after acid digestion. In the second set, we studied the Cs + desorption kinetics using 0.1 M Na + as the ion exchanger. Cesium concentration in the solution phase was measured as a function of time for 23 days. Cesium incorporated in sodalite and cancrinite was far more difficult to replace than that in LTA zeolite or allophane. Most of the incorporated Cs + (94–99%) in LTA zeolite and allophane was readily exchangeable with Na + or K +; less than 20% of Cs + in sodalite and <55% of Cs + in cancrinite could be exchanged. The fraction of desorbed Cs + was also affected by the exchanging ions; the ion with lowest dehydration energy (K +) was the most effective in replacing Cs +. The ions had to partially dehydrate to access the cages of the minerals. The results of the desorption kinetics experiments showed that Cs + desorbed quickly from LTA zeolite and the Cs + diffusion coefficient was close to that in solution; i.e., about 10 −9 m 2/s. Solid-state NMR analysis also provided evidence for the high mobility of Cs + in LTA zeolite. Cesium desorption from cancrinite, sodalite, and allophane, however, was slow, suggesting that Cs + was trapped in cages and channels of these minerals. Effective diffusion coefficients for Cs + in cancrinite and sodalite were near 10 −14 m 2/s.

Electrodeposition of cesium at mercury electrodes in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide room-temperature ionic liquid

The electrochemistry of cesium was investigated at mercury electrodes in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Bu 3MeN +Tf 2N −) room-temperature ionic liquid (RTIL) by using cyclic staircase voltammetry, rotating disk electrode voltammetry, and chronoamperometry. The reduction of cesium ions at mercury exhibits quasireversible behavior with k 0 = 9.8 × 10 −5 cm s −1 and α = 0.36. The diffusion coefficient of Cs + in this RTIL was 1.04 × 10 −8 cm 2 s −1 at 303 K. Bulk deposition/stripping experiments conducted at a rotating mercury film electrode gave an average recovery of 97% of the electrodeposited Cs. The density, absolute viscosity, and equivalent conductance of Bu 3MeN +Tf 2N − were measured over the range of temperatures from 298 to 353 K. A polynomial equation describing the temperature dependence of the density is presented. Both the viscosity and conductance exhibited the non-Arrhenius temperature dependence typical of glass-forming liquids. The ideal glass transition temperature and the activation energies for viscosity and conductance were obtained by fitting the Vogel–Tammann–Fulcher (VTF) equation to the experimental data for these transport properties.

Sorption kinetics of cesium on hydrous titanium dioxide

Summary Two types of hydrous titanium dioxide possessing different surface properties were prepared and characterized to study the sorption kinetics of cesium. The effect of pH on the adsorption capacity were determined in both type sorbents and the maximum adsorption percentage of cesium were observed at pH 12. To elucidate the kinetics of ion-exchange reaction on hydrous titanium dioxide, the isotopic exchange rates of cesium ions between hydrous titanium dioxides and aqueous solutions were measured radiochemically and compared with each other. The diffusion coefficients of Cs+ ion for Type1 and Type2 titanium dioxides at pH 12 were calculated as 2.79×10-11m2s-1 and 1.52×10-11m2s-1, respectively, under particle diffusion controlled conditions.

Sorption and diffusion behavior of Cs and Sr on Jih-Hsing bentonite

Sorption and diffusion are important processes for the transport of radionuclides through buffer materials such as bentonite. In this study, the sorption and diffusion behaviors of Cs and Sr on Jih-Hsing bentonite are investigated using batch and through-diffusion techniques. The distribution coefficients ( K d s) of Cs and Sr from batch experiments are approximately 1200 ml/g and 800 ml/g, respectively. It is found that the Freundlich isotherm model could fit the sorption isotherm with an equilibrium concentration of 10 −7–10 −1 N. The calculated retardation factors ( Rds) for samples at densities of 1.8 g/cm 3, 2.0 g/cm 3 and 2.2 g/cm 3 are 5685, 7744, and 11000 for Cs, and are 3790, 5162, and 7334 for Sr. For the through-diffusion experiments on the compacted samples with the same densities, the corresponding apparent diffusion coefficients for Cs are 2.83±0.75 ×10 −13 m 2/ s , 1.97±0.02 ×10 −13 m 2/ s , and 1.91±0.12 ×10 −13 m 2/ s , respectively. The corresponding apparent diffusion coefficients for Sr are 1.33±0.13 ×10 −13 m 2/ s , 1.51±0.15 ×10 −13 m 2/ s , and 1.34±0.10 ×10 −13 m 2/ s . The Rds obtained from the diffusion experiments for sample densities of 1.8 g/cm 3, 2.0 g/cm 3 and 2.2 g/cm 3 are 1166 ± 355, 2113 ± 123, 2796 ± 171 for Cs, and 713 ± 258, 510 ± 68, 846 ± 158 for Sr. It appears that the retardation factors obtained from the diffusion experiments are about one order of magnitude lower than those derived from the batch experiments. The discrepancy and the possible explanations are discussed in the paper.

Diffusion of Ni in Compacted Sodium Bentonite

Diffusion coefficients of key nuclides in bentonite are one of the important parameters required for performance assessment of geological disposal of high-level radioactive waste (HLW). Diffusion is generally classified into two states: non-steady state and steady state. Nonsteady state diffusion represents the diffusion coefficient in a transient state, which is called apparent diffusion coefficient (Da). In performance assessment, Da is accounted as an important parameter for evaluating nuclide retardation in bentonite. Nickel-59 is produced by activation of the corrosion products in a power reactor and is one of the important radionuclides for performance assessment because of its long half-life of 7.5×104 yr. Christiansen and Torstenfelt have reported Da of Ni in bentonitew(1). However, Da for Ni in Kunigel V1R bentonite (Kunimine Industries Co., Ltd.), which is produced in Japan, has not been measured yet. In underground, such 1, 000m deep, bentonite temperature is higher than that of ground surface because of temperature gradient(2). This temperature increase may affect diffusion behaviors of nuclides in bentonite. For example, it has been reported that the apparent diffusion coefficients for Cs, Na, HTO and Sr in bentonite become larger with increasing temperature(3)-(7). However, studies for temperature dependence on diffusion of nuclides in compacted bentonite are quite limited. In this study, the apparent diffusion coefficients for Ni in Kunigel V1 were measured as functions of density of the bentonite and temperature, then the activation energy for diffusion was calculated from the temperature dependence of the Da.

Diffusion of Cs and Sr in Compacted Bentonites Under Reducing Conditions and in the Presence of Corrosion Products of Iron

ABSTRACTIn high-level waste repositories, a carbon steel overpack will be corroded by consuming oxygen trapped in the repository after closure. Iron corrosion products are expected to interfere with migration of radionuclides by filling the pore in bentonite and sorbing radionuclides. In this study the apparent diffusion coefficients of cesium and strontium were measured in compacted Na-bentonites (Kunigel VI® and Kunipia F®, JAPAN) contacted with carbon steel and its corrosion products under reducing conditions or without carbon steel under oxidizing conditions for comparison. The apparent diffusion coefficients of cesium with and without corrosion product were 2.2 to 13 × 10−12m2/s. The apparent diffusion coefficients of strontium with and without corrosion product were 3.1 to 25 × 10−12m2/s. There were significant effects of dry density (0.8 to 2.0 g/cm3) and montmorillonite contents (50% for Kunigel V1 or 100% for Kunipia F). The presence of corrosion product decreased the apparent diffusion coefficients of Cs in both bentonites and that of Sr in Kunigel V1, especially at low dry density. This may be due to corrosion product filling the pore in the bentonite, decreasing the free pore size and density for diffusion.

Historical statistics of the United States: Colonial times to 1970

An understanding of the transport of fission products in High Temperature Gas-Cooled Reactors (HTGRs) is needed for operational safety as well as source term estimations. We have measured diffusion coefficients of Cs in IG-110 by using the release method, wherein we infused small graphite spheres with Cs and measured the release rates using ICP-MS. Diffusion behavior was investigated in the temperature range of 1100–1300 K. We have obtained: DCs=(1.0×10-7m2/s)exp-1.1×105J/molRT and, compared our results with those available in the literature.