UF6 Gas Research Articles

Numerical study of uranium hexafluoride desublimation process based on Eulerian two-phase model

This paper presents a numerical study on the condensation process of uranium hexafluoride (UF6) in the supply and extraction system of a uranium enrichment plant. The numerical study was conducted using a standard UF6 receiving container with a nominal diameter of 127 mm and a volume of 8L under constant temperature boundary conditions. A phase change heat and mass transfer model for the desublimation process of uranium hexafluoride gas was established, and this model was combined with the Euler two fluid model to predict the distribution of key parameters such as temperature, pressure, and volume fraction of each phase during the UF6 cooling process, as well as the gas flow characteristics inside the container. In addition, this study also investigated the influence of various factors on the cooling rate. The research results indicated that UF6 gas initially condensed on the container wall and gradually diffused inward, with the highest solid density observed on the wall. Furthermore, increasing the inlet gas temperature, inlet mass flow rate, and reducing the cooling temperature, initial pressure inside the container could all enhance the desublimation rate to various degrees. The optimization plan proposed based on the above influencing factors could increase the yield of UF6 solid by 64.20% in 2 h, compared to the basic operating conditions.

Coupled Computational Fluid Dynamics and Computational Thermodynamics Simulations for Chemically Reacting Flows with Phase Transformations: A Fluoride Volatility Process Example

This study presents the development of a novel computational capability for chemically reacting flows with phase transformation that can be applied to complex multi-component systems. This is done by coupling Computational Fluid Dynamic (CFD) using OpenFOAM and Computational Thermodynamics (CT) using Thermochimica and the Joint Research Centre Molten Salt Database (JRCMSD). To illustrate this capability, the Fluorite Volatility Process (FVP) is modelled because it is the first step to clean up irradiated fuel salt in the Molten Salt Fast Reactor design and needs to be better understood. Local chemical equilibrium is assumed, which means that chemical kinetics is solely governed by mass transport. FVP recovers nuclear fuel UF4(liq) by injection of fluorine gas (F2), forming UF6(gas). The vaporization rate of UF4 is investigated on four different molten salt systems, using different dynamic viscosities and temperatures. The vaporization rate was shown to be clearly dependent on both thermochemical and thermophysical properties. Higher viscosities promoted larger bubbles, which delayed the chemical reaction. Results were compared to already available experimental data, showing good agreement. The applicability of coupling CFD with Thermochimica showed to be promising and expands interpretations on multiphysics systems and will be later applied to more complex scenarios.

Uranium tetrafluoride production at pilot scale using a mercury electrode cell

This work shows the technical feasibility to obtain uranium tetrafluoride through an electrochemical mercury cell. This technique represents a custom scaling-up methodology from our previous studies to obtain UF4 using the dropping mercury electrode cell. The UF4 products were obtained from natural UF6 gas, which was hydrolyzed to obtain a 50 g/L UO2F2 solution. The electrolysis cell was made using a mercury reservoir, to reach UF4 production rates of 1 Kg UF4/day. This custom design allowed a stable UF4 production thanks to the mercury cathode, which do not permit the accumulation of solid products in its surface. The cell was tested using current densities from 5.000 to 17.500 A/m2 and temperatures from 25 to 65 °C. The maximum current efficiency achieved under these conditions was 80%. The UF4 powders possessed spherical morphology, with diameters between 20 and 80 μm. Compared to the SnCl2 precipitation, this process did not allow preferential growth of the precipitates. This improved the compaction of the UF4 – Mg powders mixtures, with densities between 3.0 and 3.5 g/cm3. The purity of the UF4 products was over 98%.

Analytical characterization of laser induced plasmas towards uranium isotopic analysis in gaseous uranium hexafluoride

To perform direct enrichment assay on gaseous uranium hexafluoride (UF6) with laser induced breakdown spectroscopy (LIBS), the dominant spectral-line features, evolution of the signal and background of the U II 424.437 nm line, and its Stark width and shift, were studied as a function of UF6 gas pressure and pulse energy of a nanosecond Nd:YAG laser. Vapor pressure of UF6 was found to be the most important parameter for LIBS analysis of gaseous UF6. Spectral congestion with numerous U lines of high excitation potential was observed and signal-to-background ratio (SBR) was low for measurements with 80 Torr UF6. Only when both UF6 vapor pressure and laser pulse energy were low, for example, less than 20 Torr pressure and 30 mJ pulse energy, the resultant LIBS spectra from gaseous UF6 resembled those obtained from solid U samples. The experimental data also suggest that U and F atoms recombine back to UF6 after the laser pulse. The U emission was found to decay fast with a persistent background signal, degrading SBR with delay time. Systematic positive biases were found for UF6 enrichment assays performed with the 235U–238U line pair at 424.412–424.437 nm, which was confirmed to be caused by self-absorption. Even with optimization of experimental parameters and incorporation of a self-absorption term into the spectral-fitting algorithm, to reduce and compensate for self-absorption, self-absorption is still a main factor limiting accurate UF6 enrichment assay. The use of another spectral window which contains no resonance lines is a prospective solution for the self-absorption issue.

Investigation of the continuum-rarefied flow and isotope separation using a hybrid CFD-DSMC simulation for [formula omitted] in a gas centrifuge

UF6 gas flow in the total regions of a rotor with the radius of 0.1 m and the length of 1 m in axisymmetric and steady states was simulated using a new hybrid CFD-DSMC method in the OpenFOAM framework. Implicit coupled density-based scheme was performed for CFD method, and Variable Hard Sphere (VHS) and diffuse model were employed in DSMC method. Also, as an initial estimation, the local Knudsen number was applied to determine the interface location between the continuum-rarefied regions (r = 0.0855 m). Then it was modified (r = 0.084 m) to reduce the computational cost. The comparison results of pure CFD and CFD-DSMC methods illustrated that there were large differences between the flow properties in the rarefied regions. Finally, the results showed that the separation factor of 6.5% and the separation power of 9.5% were different.

Modeling and Simulation of Flow and Uranium Isotopes Separation in Gas Centrifuges Using Implicit Coupled Density-Based Solver in OpenFOAM

The performance of a gas centrifuge that is used for isotopes separation is dependent on the gas flow inside it. In this study, for modeling the UF6 gas flow, an Implicit Coupled Density-Based (ICDB) solver, was developed in OpenFOAM. To validate the ICDB solver, the gas flow within the rotor in total reflux state was compared with the analytical solution obtained by Onsager model and the numerical solution obtained by the Fluent software. The results showed that the ICDB solver had acceptable accuracy and validity. Also the computational efficiency of Roe, AUSM (Advection Upstream Splitting Method) and AUSM+ up schemes were compared. The results showed AUSM+ up scheme is efficient. Then, the uranium isotopes separation in a gas centrifuge was simulated. It was revealed that all gas flow characteristics including velocity, pressure, temperature and axial mass flux, as well as uranium isotope separation parameters including separation power and separation coefficients could well be predicted.

Risk assessment of uranium hexafluoride release from a uranium conversion facility by using a fuzzy approach

Amongst the Industrial hazards, the release of toxic materials is of great importance to risk assessment of chemical industries. Uranium Hexafluoride (UF6) under some circumstances could be a very dangerous material to human health and the environment and in the case of release may have catastrophic consequences. In this study, the probability of UF6 release from a uranium conversion facility was analyzed by utilization of fault tree analysis (FTA) method. FTA is a well-known established technique in risk analysis of potential hazards. However, some shortcomings such as lack of reliability data are always a matter of concern in FTA application. To overcome this issue, expert elicitation and fuzzy set theory was applied. Results of the study showed the probability of 5.378E-4 for UF6 gas release from the uranium conversion systems. Importance and sensitivity analyses have also been conducted to evaluate the percentage contribution of each component to the top event occurrence and to identify the weak points of the whole facility. Through the study, it was determined that the operator failure and lack of a reliable automatic shut-down system are the two main important reasons for UF6 release.

Experimental study on elemental behaviors in fluorination of nuclear spent fuel with flame reactor

Our proposed spent nuclear fuel reprocessing technology named FLUOREX is a hybrid system based on reprocessing technologies of fluorination and solvent extraction for light water reactor fuel. In the current research, we experimentally clarified solid–gas transfer behaviours of the fluorides in the FLUOREX process and identified the volatile and non-volatile compounds in the fluorination. We carried out a fluorination experiment for simulated spent nuclear fuel and solid separation from the UF6 gas stream. The distribution ratios of fission product elements in the experimental apparatus were evaluated. Molybdenum, Te, Nb, and Ru were volatilized by fluorination and they accompanied the UF6 gas. However, 22.9% of the Ru and 3.4% of the Nb were retained as solids in the experimental apparatus, contrary to the fact that their partial pressures in the experiment were lower than their vapor pressures. Rubidium, Sr, Zr, Ce, and Nd were completely recovered as solid fluorides, and these results agreed with the prediction based on boiling points of their fluorides. Antimony was completely recovered as a solid; nevertheless, the boiling point of antimony pentafluoride was lower than the process temperature, and that was attributed to the formation of a non-volatile antimony oxyfluoride.

Certification of uranium hexafluoride reference materials for isotopic composition

The IRMM-019 to IRMM-029 series of uranium hexafluoride materials is certified for the isotopic composition. After conversion into uranyl nitrate solution, certification and homogeneity measurements were performed by thermal ionization mass spectrometry. Analyses were performed by Modified Total Evaporation and for some materials the major isotope amount ratio n(235U)/n(238U) was measured using a n(233U)/n(236U) double spike. Measurements were confirmed by UF6 gas source mass spectrometry. Major isotope amount ratios were certified with relative expanded uncertainties (k = 2) of 0.015–0.030 % and the minor isotope amount ratios n(234U)/n(238U) and n(236U)/n(238U) were certified with relative expanded uncertainties of 0.02–3 %.

Uranium hexafluoride (UF6) gas source mass spectrometry for certification of reference materials and nuclear safeguard measurements at IRMM

Uranium hexafluoride gas source mass spectrometry at IRMM is based on two foundations, firstly the operation of a UF6 gas source mass spectrometer (GSMS) and secondly the preparation of primary UF6 reference materials, which were converted from gravimetrically prepared mixtures of highly enriched oxides of 235U and 238U. Recently a new GSMS for uranium isotopic measurements using UF6 gas, the “URANUS” from Thermo Fisher, was installed at IRMM, which also allows measurements of the so-called “minor” isotope ratios n(234U)/n(238U) and n(236U)/n(238U). In this paper the design and the implementation of measurement techniques for the new URANUS GSMS are described. This includes the “single standard” and the “double standard” (DS) method as well as the newly developed “memory corrected double standard” method (MCDS). This required a detailed investigation of memory effects within the GSMS instrument, in particular regarding the dependence of memory effects on the isotope ratios of samples and standards. The results of this study led to new recommendations for the selection of the standards for a given sample and for suitable measurement procedures. The measurement performance for the “major” isotope ratio n(235U)/n(238U) as well as the “minor” isotope ratios n(234U)/n(238U) and n(236U)/n(238U) is presented and compared with other mass spectrometric techniques. With the installation and validation of the new URANUS GSMS instrument IRMM has established two new complementary techniques for measuring the full isotopic composition of uranium samples. UF6 GSMS in combination with the MCDS method is considered the preferred technique for samples in the UF6 form and for smaller uncertainties for measurements of the major ratio n(235U)/n(238U), while thermal ionization mass spectrometry (TIMS), in combination with the “modified total evaporation” (MTE) method as well as ion counting and high abundance sensitivity for the detection of 236U, provides a superior measurement performance for the minor isotope ratios n(234U)/n(238U) and n(236U)/n(238U).

The adsorption characteristics of uranium hexafluoride onto activated carbon in vacuum conditions

The adsorption characteristics of uranium hexafluoride (UF6) on activated carbon have been investigated under vacuum conditions for absolute pressure range of 10–35mbar. The experiments were carried out for three different types of activated carbon. Evaluation of the experimental data in terms of adsorption kinetic showed that UF6 gas adsorption by three types of activated carbon followed the pseudo-second-order-rate equation very well. The experimental isotherm data were analyzed using the Langmuir, Freundlich, and Henry equations. The results showed that the Langmuir isotherm model fitted the equilibrium data better than the other ones. At a temperature of 20°C, the maximum adsorption capacity of the first, second, and third activated carbon were 3387, 2198, and 2771 (mg/g), respectively. Adsorption studies were conducted at different temperatures to obtain the thermodynamic parameters. The results depicted the exothermic nature of adsorption and the process was feasible and spontaneous.

Experimental determination of the thickness of aluminum cascade pipes in the presence of UF6 gas during enrichment measurements

We present a method of determining the wall thickness of a pipe in a Gas Centrifuge Enrichment Plant (GCEP) when an empty pipe measurement is not feasible. Our method uses an X-ray tube for transmission measurements and a lanthanum bromide (LaBr3) scintillation detector on the opposite side of the pipe. Two filters, molybdenum (K-edge 20.0keV) and palladium (K-edge 24.35keV) are used to transform the bremsstrahlung spectra produced by the X-ray tube into more useful, sharply peaked, spectra. The maximum energies of the peaks are determined by the K-edges of the filters. The attenuation properties of the uranium hexafluoride (UF6) gas allow us to determine wall thickness by looking at the ratio of selected regions of interest (ROIs) of the Mo and Pd transmitted spectra. While the attenuation factor at these two transmission energies in the UF6 gas is nearly equal, attenuation in the aluminum pipe wall at these two energies differs by a factor of about 60. This difference allows measurement of attenuation in the pipe independent of attenuation in the UF6 gas. Feasibility studies were performed using analytical calculations, and filter thicknesses were optimized. In order to experimentally validate our attenuation measurement method, a UF6 source with variable enrichment and pipe thickness was built. We describe the experimental procedure used to verify our previous calculations and present recent results.

Adsorption of Molybdenum Hexafluoride on Magnesium Difluoride for Uranium Purification in FLUOREX Reprocessing

The nuclear fuel reprocessing method FLUOREX is a hybrid system based on fluoride volatility and using solvent extraction. Spent nuclear fuel is fluorinated, and most of the uranium is recovered as UF6 gas. UF6 contains some volatile fission product (FP) fluorides, so we considered their elimination from UF6 by adsorbing them on fluoride adsorbents. We experimentally examined the adsorption of MoF6 on MgF2 adsorbent; MoF6 is present as a volatile FP fluoride in UF6 produced by the fluorination of spent nuclear fuel. The adsorption isotherm of MoF6 adsorption on MgF2 was obtained at MoF6 pressures from 10−4 to 50 kPa. The saturated adsorption amount was 1:3 ± 0:4 mg/m2 at MoF6 pressures from 10−4 to 1 kPa. At MoF6 pressure of about 10−3 kPa, the saturated adsorption amount had no dependence on adsorption temperatures from 398 to 463 K. We deduced that MoF6 was adsorbed as a monomolecule layer on the MgF2 surface at MoF6 pressures from 10−4 to 1 kPa, and the MoF6 partial pressure in UF6 could be decreased below 1 × 10−4 kPa, which is the specific MoF6 concentration for the reenrichment process.

Adsorption of Molybdenum Hexafluoride on Magnesium Difluoride for Uranium Purification in FLUOREX Reprocessing

The nuclear fuel reprocessing method FLUOREX is a hybrid system based on fluoride volatility and using solvent extraction. Spent nuclear fuel is fluorinated, and most of the uranium is recovered as UF6 gas. UF6 contains some volatile fission product (FP) fluorides, so we considered their elimination from UF6 by adsorbing them on fluoride adsorbents. We experimentally examined the adsorption of MoF6 on MgF2 adsorbent; MoF6 is present as a volatile FP fluoride in UF6 produced by the fluorination of spent nuclear fuel. The adsorption isotherm of MoF6 adsorption on MgF2 was obtained at MoF6 pressures from 10−4 to 50 kPa. The saturated adsorption amount was 1:3 ± 0:4 mg/m2 at MoF6 pressures from 10−4 to 1 kPa. At MoF6 pressure of about 10−3 kPa, the saturated adsorption amount had no dependence on adsorption temperatures from 398 to 463 K. We deduced that MoF6 was adsorbed as a monomolecule layer on the MgF2 surface at MoF6 pressures from 10−4 to 1 kPa, and the MoF6 partial pressure in UF6 could be decreased below 1 × 10−4 kPa, which is the specific MoF6 concentration for the reenrichment process.

Evaluating the status of uranium isotope ratio measurements using an inter-laboratory comparison campaign

The REIMEP 18 (Regular European Inter-laboratory Measurement Evaluation Programme) campaign for the measurement isotopic ratios of uranium in nitric acid solution was completed in December 2006. The task for all participating laboratories was to measure the uranium isotopic composition of four uranium samples ranging from depleted to slightly enriched uranium. With 71 participating laboratories REIMEP 18 has become the largest nuclear isotopic measurement campaign organized by IRMM so far. Participation in this kind of measurement campaign is an integral part of the external quality control required for nuclear safeguards laboratories worldwide. For the first time also a significant number of academic laboratories, mainly from the geochemistry area was included. Certification measurements were carried out at IRMM using state-of-the-art mass spectrometric methodology. A MAT511 UF 6-gas source mass spectrometer (GSMS) was used to determine the n( 235U)/ n( 238U) ratios and a TRITON thermal-ionization mass-spectrometer (TIMS) for the minor isotope ratios n( 234U)/ n( 238U) and n( 236U)/ n( 238U). Verification measurements on ampouled samples were performed successfully prior to sample shipping and showed good agreement with the certified ratios. The results of the REIMEP 18 campaign confirm in general the excellent capability of nuclear safeguards and scientific laboratories in measuring isotopic abundances of uranium, although some problems were discovered for the measurements of the minor isotope ratios n( 234U)/ n( 238U) and n( 236U)/ n( 238U) and the calculation of measurement uncertainties for isotope ratios in general. This paper describes the outcome of the REIMEP 18 campaign. It includes a graphical evaluation and discussion of the results, an evaluation of the applied measurement and calibration techniques and a discussion of conclusions and actions to be taken.

Electron cyclotron resonance charge breeder (invited)

Initially developed for radioactive ion beam production, the electron cyclotron resource (ECR) charge breeder shows that the beam injection of a primary beam inside an ECR ion source is a very general process for beam production. In this article we will review the latest results obtained on the ISN Grenoble test bench for the production of clockwise (CW) or pulsed metallic ion beams with the so-called “1+/n+” method. New results are given for CW operation where the efficiency is particularly optimized for application to multicharged radioactive ion production (for example, 3.5% for Zn1+→Zn9+, 4.2% for Pb1+→Pb24+, 5% for Rb1+→Rb15+). Different ion sources have been used to study the variation of the efficiency as a function of the energy of the 1+ primary beam. Charge state distributions are especially measured for Pb and Rb ions. A new mode of operation, the electron cyclotron resonance ion trap breeder/buncher, which permits the trapping and the bunching of the n+ ion beam is demonstrated and experimentally verified. The injection of a 400 nAeRb1+ ion beam leads to a 11.5 μAe peak current of the Rb15+ ion beam extracted during the first ms. The temporal evolution of the cumulated particle transformation and trapping efficiency is measured in the case of Rb15+. This new method shows that it is possible to modify the time structure of the injected beam, just by trapping the ions inside the plasma and re-extracting them by using the “afterglow pulsed mode.” It will also be shown that it is possible to produce “multi-charged metallic ions” by using a primary source for the dissociation of a complex molecule and then by injecting the low charge state beam inside the multicharged electron cyclotron resonance ion source. A preliminary example will be presented for the production of U26+ by injection of U3+ obtained from dissociation of UF6 gas.

Activation Energy for Isotope Exchange Reaction between 235UF5 Nanoparticles and 238UF6 Gas

The temperature dependence of the rates for two kinetic processes involved in the isotopic exchange reaction between 235U-enriched UF5 nanoparticles and natural UF6 gas was experimentally examined using our kinetic model that includes the fraction of high-reactivity molecules of UF5 on the outermost layer of the particles. The activation energies obtained from the Arrhenius plot for the fast process and the slow process were similar, 36.1 and 39.3 kJ/mol, respectively. This indicates that the two rate processes are mainly caused by the difference in the preexponential factor of the Arrhenius equation. It was suggested that such a difference in the reactivity of the UF5 molecules was caused by the difference in the adsorption configurations of UF6 molecules on the surface of UF5 molecules due to the steric hindrance of neighboring fluorine atoms.

Influence of fluoridation on the strength of superplastic Zn-21AI-2Cu alloy deformed in a saline medium

The corrosion behaviour of Y2O3 (yttria) at 900°C in flowing UF6 (uranium hexafluoride) was investigated under a pressure of 1.33 × 10−3 Pa. A test loop was used to study the chemical reaction of sintered and hot-pressed yttria samples with flowing UF6 gas in an alumina reaction tube installed in an electrically heated horizontal 1200°C type furnace. A weight increase was observed after each exposure testing. Scanning electron microscopy (SEM) examination of the exposed samples identified two layers of reaction products. Significant cracking and porosity were observed in the outer layer after the experiment while the inner layer thickness decreased with increasing the exposure time. Thermodynamical analysis using Facility for the Analysis of Chemical Thermodynamics (FACT) software packages has been performed and relevant interface reactions have been suggested. Pilling–Bedworth Ratios for both layers were calculated and found to be in agreement with the morphology of the layers. Electron microprobe (EMP) and X-ray diffraction analysis (XRD) revealed the formation of UO2 and YF3 in the outer layer and a possible formation of YOF in the inner layer. The formation of the outer layer was primarily due to oxygen–fluorine exchange reactions between yttria and uranium fluorides while the inner layer was a direct consequence of the fluorine inward diffusion and reaction with yttria.

In situ observation of UF 5 nanoparticle growth in a low-pressure mixed-flow reactor

nanoparticles equipped with an in situ size-monitoring system, a LPDMA (low-pressure differential mobility analyzer), was developed to experimentally investigate the nanoparticle growth mechanism. The concentration of photoproduced UF5 molecules was controlled by changing three factors: (I) the concentration of the feed UF6 gas, (II) the laser pulse energy of the irradiation, and (III) the repetition rate of the laser pulses. The dependence of the volumetric average diameter of the photoproduced particles on the UF5 nascent concentration in all three cases was found to be very similar. The result strongly suggests that the reactor functions as a mixed-flow reactor under a complete mixing condition. The particle size measured by the LPDMA was found to be in the range of 6 to 11 nm, and it was approximately proportional to the power 0.3 of the initial concentration of photoproduced UF5 molecules.

A Note on the Inhibition of the Isotope Exchange Reaction between 235U-enriched UF5 Nanoparticles and Natural UF6 Gas.

A Note on the Inhibition of the Isotope Exchange Reaction between 235U-enriched UF5 Nanoparticles and Natural UF6 Gas.