Stage Separation Factor Research Articles

Double Cascades for Purification of Reprocessed Uranium Hexafluoride from 232, 234, 236U Isotopes

The paper offers a double-cascade scheme for reducing the concentration of 232, 234, 236U isotopes in reprocessed uranium hexafluoride. The greatest decrease of the ratio between the masses of 236U and 235U is provided in the product of the first ordinary cascade enriched by 235U at the concentration of less than 20%. For this purpose, a special mode of stages operation is determined. Enrichment by 232, 234U is performed in the second ordinary cascade, which is fed by the product of the first cascade. After being purified from 232, 234U, the waste flow is diluted till the concentration of 235U is less than 5%. This paper describes the methodology for calculating the parameters of cascades with the stage separation factors correlating with gas centrifuges. This methodology served as a basis for a computational experiment. It is demonstrated that the output gained after the dilution meets the requirements of the American Society for Testing and Materials C996-20 specification for the commercial grade of low-enriched uranium hexafluoride in terms of 232, 234U isotopes. The content of 236U in it is several times less than during the direct enrichment of reprocessed uranium hexafluoride.

Оптимизация двойных каскадов алгоритмом пчелиной колонии для очистки регенерированного гексафторида урана от изотопов232, 234, 236U

We propose a double cascade scheme for reducing the concentration of232, 234, 236U isotopes in regenerated uranium hexafluoride. In the product of the first ordinary cascade the greatest decrease in the236U /235U mass ratio with enrichment in235U to a concentration of less than 20% is provided. To ensure the required concentrations, a special mode of operation of the stages. In the second ordinary cascade, which is fed by the product of the first, enrichment in isotopes232,234U is performed. The waste flow, purified from232, 234U, is diluted to a concentration of235U less than 5%. A method for calculating the parameters of cascades with stage separation factors corresponding to gas centrifuges is presented. A computational experiment was carried out on its basis. It was shown that the product obtained after dilution with respect to isotopes232, 234U meets the requirements of the ASTM C996-20 specification for the low-enriched commercial grade of uranium hexafluoride. The content of236U in it is several times lower than in the case of direct enrichment of regenerated uranium hexafluoride.

Separation of lithium isotopes by crown ether-room temperature ionic liquid-anisole friendly solvent system

The solvent extraction separation of lithium isotopes has become of growing interest due to the need for 6Li and 7Li isotopes in the nuclear industry. A one-step synthesis of room temperature ionic liquid with low viscosity 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][NTf2]) (with reaction yields of 87.47%) was used by the ultrasonic method. The addition of ionic liquid [EMIm][NTf2] in the system significantly improved the extraction efficiency and separation factor compared to the traditional organic solvent anisole for extraction separation of lithium isotopes. The distribution ratio increased from 0.00145 (with the single stage separation factor of 1.024 ± 0.001) to 0.134 (with the single stage separation factor of 1.034 ± 0.001). Furthermore, a multi-stage extraction process was intended to enhance lithium isotope separation performance. Each stage of stripping can be considered as an extraction separation equilibrium process, in which the strip liquor from the load organic phase is used for separation in the next stage as the “original aqueous solution”. The abundance of 6Li+ gradually improved with an increase in the extraction stages. The relative abundance of 6Li+ increased by 1.492% at the 20-stage extraction separation (from 7.53% to 9.022%). This new and simple approach (stripping cross-flow multistage method (SCFM)) was to achieve large enrichment of lighter isotopes of lithium (6Li+) by combined extraction and stripping process. In summary, the present study offers the possibility for the enrichment of 6Li+ during lithium isotope extraction separation.

Inversion of boron isotope effect during chemical exchange in extraction systems

In the processes of boron isotopic exchange in liquid-liquid systems using boric acid as the main working substance, the lighter boron isotope is concentrating in the organic phase, which takes place both in the case of phase exchange (tributyl phosphate, TBP) and at the chemical (trioctylamine, TOA) isotope exchange. That is, boron-10, as a rule, predominantly passes into the organic phase. In contrast to the previously observed effects, our studies for several extraction systems using secondary amines (diethyl, dipropyl, and dibutylamine as the organic phase) demonstrated the inversion of the boron isotope effect when a more light isotope 10B is concentrated in the aqueous phase. At the same time, the level of the values of the single stage separation factor of boron isotopes turned out to be quite high and amounted to 1.020 - 1.028. The paper discusses the possible causes of the discovered phenomenon, and also considers the potential for practical application of such an effect.

Lithium isotope separation using 4′-acetylbenzo-15-crown-5 and 1-butyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl] imide in the synergistic extraction system

A novel synergistic extraction system was studied for the separation of lithium isotopes using lithium salts solution as aqueous phase and dichloromethane containing 4′-acetylbenzo-15-crown-5 and [BMIm][NTf2] as organic phase with solvent extraction method. The key factors such as anions of lithium salts, concentrations of LiNTf2, initial pH values of aqueous phase, solvents, volume ratio of dichloromethane and [BMIm][NTf2] were investigated in this study. Results showed that the maximum extraction rate of lithium was 68.36% and the maximum single stage separation factor of 6Li and 7Li was 1.056 ± 0.002. The formed complex in organic phase was studied by the slope analysis method, which indicated that the complex was composed of Li-ion and 4′-actylbenzo-15-crown-5 by 1:1. The thermodynamic parameters of the 6Li and 7Li exchange reaction were calculated by data obtained on temperature during experiments, which explained that the exchange behavior of 6Li and 7Li in this system was a spontaneous exothermic reaction. In addition, this system was able to efficiently separate lithium isotopes at lower temperature.

Separation of boron isotopes by chemical exchange in liquid-liquid systems

The currently used technologies for the production of boron isotopes for nuclear power and nuclear medicine are based on the application of toxic boron halides. The report is devoted to alternative processes for enrichment of 10B and 11B by chemical exchange in extraction systems using boric acid, that is, without of boron halides. The results of studying phase and isotope equilibrium in two liquid-liquid systems are considered: an aqueous solution of boric acid – an organic phase in the form of tributyl phosphate or a solution of trioctylamine in o-xylene. The obtained values of the separation factor for boron isotopes characterize the predominantly phase isotope effect in the first system, and in the second system the effect of chemical isotope exchange. Accordingly, the values of the single stage separation factor are 1.0067 ± 0.0035 and 1.025 ± 0.005 and are close to similar values for rectification of boron trifluoride and chemical exchange in the anisole process of boron isotope separation. The practical use of the studied extraction systems may allow the separation of boron isotopes in the horizontal cascades of centrifugal extractors in the absence of an environmental hazard.

Tritium separation performance of adsorption/exchange distillation tower packed with structured packing

Separation of tritium from tritiated water is analyzed based on the theoretical plate model comparatively for the three cases among a water distillation tower, the Girdler-Spevack (G-S) bithermal exchange process and the combined electrolysis chemical exchange (CECE) process. The McCabe-Thiele diagrams to design large-scale detritiation systems are drawn for each, and tritium concentration profiles in each system are compared. It is clarified how detritiation behavior in the distillation tower is enhanced by an increase in the separation factor. The number of theoretical stages and internal flow rates to achieve detritiation of the bottom-to-top concentration ratio of xB/xD = 100 in cases of the G-S and CECE processes, are estimated based on the equilibrium stage separation factor reported in the past.A water distillation tower of 108 mm in diameter and 1000 mm in height packed with structured packing coated with Zeolite 13X are experimentally tested for detritiation of a large amount of wastewater to be exhausted from nuclear reactors. Separation performance between HTO and H2O under reduced pressure is experimentally verified in the water distillation tower. Enhancement of the xB/xD ratio is experimentally proved under the total reflux condition as a function of evaporation rate. Activation on surfaces of adsorbent coated on the structured packings enhances the stage separation factor. The enhancement ratio is affected by liquid-gas dispersion in the column and flow instability in the packed tower.

Transient Behavior in Water Distillation Tower for Tritium Separation and Its Long-Time Operation Test Results

Transient separation behavior in tritiated water distillation tower packed with materials having ability to adsorb water is investigated analytically and experimentally for nuclear reactor safety. Analytical equations based on the stage model are set up for simulation of the transient behavior of tritium (T) separation from wastewater. It is found that a dimensionless time defined in terms of the inside vapor flow rate and the liquid holdups in tower, reboiler and condenser can correlate variations over time to achieve a steady-state T concentration. However, when mixing with different T concentrations at a feed point occurs, the transition time period becomes longer than expected. Effects of the reflux ratio, the stage separation factor and the total stage number on the transient and steady-state T concentrations are numerically calculated. Variations over time to achieve each steady-state value are compared with experimental data using a small-scale tower. Long time distillation experiment for one month has been completed, and it is confirmed that a distillation column packed with ceramic Raschig rings coated with zeolite 13X adsorbent is hardly affected by water corrosion.

Lithium isotope separation by liquid-liquid extraction using ionic liquid system containing dibenzo-14-crown-4

In this paper, a novel liquid-liquid extraction system was investigated for the selective separation of lithium isotopes using ionic liquid (IL) and dichloroethane as extraction solvent and dibenzo-14-crown-4 (DB14C4) as extractant. Results showed that the separation performance of DB14C4 has been improved after the ILs were involved in the extraction process. The maximum single stage separation factor α for 6Li/7Li obtained in the present study was 1.021. The light lithium isotope 6Li and the heavy lithium isotope 7Li were concentrated in organic phase and solution phase, respectively. The order of the single stage separation factor obtained in different lithium salts was LiI>LiBr>LiClO4>LiCl. From the temperature dependence data, the thermodynamic functions values of the lithium isotope exchange reaction were calculated.

Preparation and Characterization of Polysulfone-graft-4′-aminobenzo-15-crown-5-ether for Lithium Isotope Separation

A novel polymer polysulfone (PSF)-graft-4′-aminobenzo-15-crown-5-ether (AB15C5) (PSF-g-AB15C5) for lithium isotope separation was prepared from PSF and AB15C5 as starting materials via nucleophilic substitution reaction. The chemical structure and properties of PSF-g-AB15C5 polymers were characterized by FT-IR, 1H NMR, XPS, and TGA. The polymers obtained were used for lithium isotope separation by solid–liquid extraction. The effects of the immobilization amount of crown ether grafting on PSF, the type of counteranion of lithium salt, and the kind of solvent on the single stage separation factor were explored. Results showed that the single stage separation factor was only 1.002 ± 0.002 for AB15C5 in the traditional liquid–liquid extraction system of H2O–LiCl/CHCl3–AB15C5, whereas the single stage separation factor increased from 1.003 ± 0.001 to 1.015 ± 0.002 with the increase of the immobilization amount of crown ether from 0.23 to 0.79 mmol g–1 on PSF-g-AB15C5 polymers in the solid–liquid extraction system of CH3OH–LiCl/PSF-g-AB15C5 polymers. The order of the single stage separation factor obtained in different lithium salts was LiI > LiBr > LiClO4 > LiCl. Further, the maximum value of single stage separation factor was 1.031 ± 0.002 for the extraction system of CH3NO2–LiCl/PSF-g-AB15C5 polymers. Moreover, 6Li and 7Li were enriched in the polymer phase and the solution phase, respectively.

Determination of physicochemical parameters of 28Si isotope enrichment in countercurrent column

The main physicochemical parameters of silicon isotopes separation process by chemical exchange method with thermal phase reversal in systems SiF4-SiF4 complex with aliphatic alcohols were determined by direct and indirect (calculated) ways. Experimentally, the single stage separation factor α were obtained using the single equilibration method, the height equivalent to a theoretical stage (HETS) was determinated after obtaining of steady state of exchange columns work with the length of 1 and 2.5 m accordingly. Indirectly, α and HETS values were calculated using the initial periods of exchange columns operation. The conclusion about the applicability of determination by indirect method of main parameters of exchange system without steady state obtaining of exchange column was done.

Effects of Separation Processes on Minor Uranium Isotopes in Enrichment Cascades

In natural uranium, there are trace amounts of the “minor isotope” 234U in addition to the more abundant 235U and 238U. Uranium irradiated in a reactor and separated from the spent reactor fuel by reprocessing will contain additional minor isotopes. In uranium enrichment, cascades of separating units are tuned to produce the desired level of the isotope 235U, but the levels of other isotopes will also change. Measurement of the assays of the 235U in the feed, product and withdrawal streams of each stage allow the calculation of the stage separation factor for the 235U. The separation factors for the other isotopes may be calculated. In this aritcle, two methods of modeling the separation factors in a way that represents the physics of two enrichment processes, gas centrifuge and gaseous diffusion, are explored. This technique could be a valuable tool in nuclear forensics. For example, it could be important to know the origin of a sample of highly enriched uranium, found either in intercepted material or in explosive debris. Analysis that could point to the enrichment plant where the uranium was produced could immediately identify weaknesses in safeguards and physical security systems.

Ideal and Optimum Cascades

In gaseous diffusion and gas centrifuge uranium enrichment plants, separating units are connected in cascades. The stage separation factor in gaseous diffusion is close to unity, while in a gas centrifuge it is much larger. Ideal cascades for separating binary mixtures have been designed with no mixing of the species. A new concept, particularly applicable to cascades with large stage separation factors, is called the “optimum” cascade. These cascades allow mixing, but the value of the total flow as found in some cases is less than in corresponding ideal cascades. In this paper, ideal and optimum cascades are discussed and compared.

A Calculation Procedure for Designing Ideal Centrifugal Separation Cascades

A numerical method is presented for determining flow rates in centrifugal isotope separation cascades that satisfy the required “ideal cascade” conditions. This method is an iterative method, which in the iteration process uses the q-iteration method to give the concentration and distributions of components under a specified hydraulic status for a cascade; and then, based on the concentration distributions, solves the algebraic equations describing the hydraulic status of the cascade to improve the specified hydraulic status towards the required “ideal cascade” conditions. The material losses in the cascade pipes and centrifuges are taken into account. Numerical simulations are performed by taking an MARC cascade with variant stage separation factors as an example. The results from four cases with and without material losses are compared. The results demonstrate that the method works very well, and show that when the material losses are nonexistent or very small, the concentration distributions can exactly satisfy the “ideal cascade” condition; but when the material losses are large, the “ideal cascade” condition can not be satisfied, but the method can produce a solution that allows the condition to be approximately satisfied.

Adsorption and isotope effects by ion exchange with 1-aza-12-crown-4 bonded Merrifield peptide resin

Magnesium isotope effects were investigated by chemical ion exchange with synthesized 1-aza-12-crown-4 bonded Merrifield peptide resin using elution chromatography. The capacity of azacrown ion exchanger was 0.89 meq/g dry resin. The heavier isotopes of magnesium were enriched in the resin phase, while the lighter isotopes were enriched in the solution phase. The hydration effect is less than the complexation and isotope mass effects. The single stage separation factor was determined according to the method of Glueckauf from the elution curve and isotopic assays. The separation factors of 24Mg 2+– 25Mg 2+, 24Mg 2+– 26Mg 2+, and 25Mg 2+– 26Mg 2+ were 1.012, 1.023, and 1.011, respectively.

Distillation Cascades Based on Differential Vaporization and Differential Condensation Stages. II. Four- and Six-Stage DCDV Cascades and Hybrid Cascades Containing DV Reboilers and DC Condensers

The conceptual design for N-stage cascades based on the differential condensation (DC), and differential vaporization (DV) processes is presented. Four and six stage “no-mix” and constant recycle DCDV cascades have been modeled to separate an arbitrary binary feed. The study suggests that cascades based on the differential processes could be significantly more efficient than cascades based on equilibrium stages (ordinary distillation). In no-mix DCDV cascades, all stage separation factors are equal and significantly greater than the equilibrium stage separation factors (i.e., relative volatility). In constant recycle DCDV cascades, the DV reboiler and DC (partial) condenser stages have stage separation factors much higher than the relative volatility and are significantly greater than the separation factors for the interior DCDV stages, although the stage separation factors for the interior stages are still higher than the relative volatility. Constant recycle hybrid cascades containing DV reboilers and DC condensers coupled with equilibrium interior stages are more efficient than constant recycle ordinary distillation cascades containing the same number of stages, especially for systems with higher relative volatility that require fewer stages. For systems with lower relative volatility (greater number of stages) the difference in performance between the hybrid and ordinary distillation cascades is less pronounced.

Separation of lithium isotopes by elution chromatography with an AB18C6 bonded Merrifield peptide resin

Cation exchange chromatographic separation of lithium isotopes was carried out with an 4'-aminobenzo-18-crown-6(AB18C6) bonded Merrifield peptide resin. This resin has a capacity of 2.25 meq/g dry resin. Upon column elution chromatography, a single stage separation factor of 1.0095, was obtained by the Glueckauf theory from the elution curve and isotopic assays. The heavier isotope, 7Li, was concentrated in the resin phase, while the lighter one, 6Li, concentrated in the solution phase.

DISTILLATION CASCADES BASED ON DIFFERENTIAL VAPORIZATION AND DIFFERENTIAL CONDENSATION STAGES. I. TWO-STAGE DCDV CASCADES

Distillation cascades based on differential condensation (DC) and differential vaporization (DV) stages are considered as an alternative to ordinary (equilibrium stage) distillation for the separation of binary mixtures. In the differential condensation–differential vaporization (DCDV) cascades, stage separation factors can be much higher than for cascades based on equilibrium stages, and as a result, significantly fewer stages and, possibly, lower recycle ratios may be required compared with conventional distillation. This paper presents the theory behind the DV and DC stages and explores the use of the two-stage cascade design (a DV reboiler and DC condenser), to recover oxygen for use in hypersonic flight. Part II will consider N-stage cascades that could possibly be used to separate most systems now separated by ordinary distillation.

Syntheses and separating properties of triazacrown and AM18C6 bonded Merrifield peptide resins

Separation of lithium and magnesium isotopes by cation exchange elution chromatography was carried out with a synthesized 1,13,16-trioxa-4,7,10-triazacyclooctadecane (N 3O 3)-4,7,10-trimerrifield peptide resin and with a 2 ′-aminomethyl-18-crown-6 (AM18C6) bonded Merrifield peptide resin. The resins have a capacity of 0.1 and 2.3 meq/g dry resin. A single stage separation factor of lithium isotopes, 1.018 was obtained by the Glueckauf theory from the elution curve and isotopic assays. The heavier isotope, 7Li was concentrated in the resin phase, while the lighter isotope, 6Li concentrated in the solution phase. On the other hand, the heavier isotopes of magnesium were concentrated in the solution phase, while the lighter isotopes were concentrated in the resin phase. The separation factors of 24Mg– 25Mg, 24Mg– 26Mg, and 25Mg– 26Mg isotope pair fractionations were 1.012, 1.022, and 1.012, respectively.

Synthesis and Separating Properties of Tris(Merrifield Peptide Resin)

A triazacrown ion exchanger, 1,13,16-trioxa-4,7,10-triazacyclooctadecane (TOTA)-4,7,10- tris(Merrifield peptide resin) was synthesized. This resin has a capacity of 0.1 meq/g dry resin. The resin was used for the elution chromatographic separation of lithium isotopes. Upon column chromatography [0.2 cm I. D. x 35 cm height] using 3.0 M NH4Cl solution as an eluent, the single stage separation factor of 1.027 was obtained. The heavier isotope, 7Li+, was enriched in the resin phase, while the lighter isotope, 6Li+, was enriched in the solution phase.