UF6 Gas Research Articles

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

(1999). A Note on the Inhibition of the Isotope Exchange Reaction between 235U-enriched UF5 Nanoparticles and Natural UF6 Gas. Journal of Nuclear Science and Technology: Vol. 36, No. 1, pp. 108-109.

Isotope exchange reaction between 235UF5 nanoparticles and 238UF6 gas

The equilibrium in the isotope exchange reaction between 235U-enriched UF5 nanoparticles and natural UF6 gas was experimentally investigated. From the relationship between the isotopic fraction of UF6 gas at equilibrium and the specific surface area of the UF5 nanoparticles, it was experimentally demonstrated for the first time that all UF5 molecules on the outermost layer of the particles participated in the reaction until the isotopic fraction of UF5 on the outermost layer became equal to that of the UF6 gas. A fast rate process during the early period of the reaction and a subsequent slow rate process were quantitatively observed. These facts contradict the previously reported assumption not supported by experimental evidence that the UF5 molecules in the underlying layer of the particle participate in the reaction. A new model based on our experimental findings was proposed. Our model suggests that there are two kinds of molecules on the outermost layer with different reactivities and all UF5 molecules on the outermost layer contribute to the reaction. The rate equations were derived from the proposed model and analytically solved. The time dependency of the isotopic fraction of the UF6 gas and UF5 particles was expressed by three kinetic parameters such as the rate constant of the high reactivity molecule, ks, the rate constant of the low reactivity molecule, ks′, and the fraction of high reactivity molecules on the outermost layer, ξ. Our model reproduced well all the experimental data determined in the present study with ks=5.5×10−20 cm3/(s molecules), ks′=4.5×10−22 cm3/(s molecules), and ξ=0.078.

Effect of 235UF6238UF6 Gas Mixing on the Kinetic Rate of the Isotope Exchange Reaction between 235U-Enriched UF5 Nanoparticles and Natural UF6 Gas.

Effect of 235UF6238UF6 Gas Mixing on the Kinetic Rate of the Isotope Exchange Reaction between 235U-Enriched UF5 Nanoparticles and Natural UF6 Gas.

Transport and equilibrium properties of UF6 gas simultaneously fitted by an effective isotropic potential with temperature-dependent parameters

All experimental data of the viscosity ( eta ), the second virial coefficient (B) and the self-diffusion ( rho D) (103 experimental points altogether) are approximated in the frame of their experimental errors by means of an effective isotropic (n-6) potential with temperature-dependent parameters. The UF6 gas at given temperature (T) is considered as one vibrationally excited state with a size averaged over all excitations. The equilibrium distance (rm) increases with temperature according to the vibrational excitation of the molecule. The potential depth well ( epsilon ) decreases with enlargement of the equilibrium distance in order to keep the long range forces unchanged. Parameters of the ground state-ground state interactions (T=0 K) are derived by means of a minimization of the sum of the squared deviations between experimental and calculated properties normalized to the experimental error. Approximate formulae for the temperature dependences rmeff(T), B(T), eta (T) and rho D(T) to be used for accurate and easy calculations are given. A comparison with available (n-6) potentials is made.

Uranium Isotope Exchange between Uranium Hexafluoride and Uranium Pentafluoride.

To aim at a better understanding of the uranium isotope exchange reaction between gaseous UF6 and solid UF5 experiments were done with natural UF6 gas and solid UF5 containing 3% 235U under different pressures of UF6. The experimental results suggest a two-process reaction with an initial rapid increase of 235UF6 in the gas phase followed by its slight and gradual increase. A rate equation based on a collision model is given for the two-process reaction which includes a primary exchange reaction on the solid surface and a secondary reaction participated by underlying UF5 molecules. An analytical solution is provided for both of 235UF6 concentration in the gas phase and 235UF5 concentration on the solid surface, which is useful for determining the parameters characterizing the exchange reaction. A numerical analysis is also made to evaluate the influence of gas samplings. A remarkable agreement is found between the particle sizes of UF5 estimated from the reaction parameter and from the direct observation ...

Gas-flow experiments in the transition region

A special gas-flow facility was designed and constructed for the purpose of accurately measuring UF6 gas flow through a variety of gas-flow restrictions. The facility was used to measure the gas flow through 15 different orifices and 20 short tubes over a nominal pressure range from 0.002 to 100 Torr. The intent of the experiments was to confirm a new theoretical approach to gas flow through short tubes in the transition range between laminar viscous flow and molecular flow. The theoretical approach previously discussed [Santeler, J. Vac. Sci. Technol. A 4, 338, 348 (1986)] (1986)] became a part of the basis for several computer programs used for calculating gas flow in vacuum systems. A number of interesting results in turbulent flow were observed during the experiments and are discussed in the paper. The results of the experiments confirmed the proposed model and were used to evaluate specific parameters of the proposed equations.

A New Calculation Method for Simulation of the Growing Particles-Gas Reaction System.

The growing particles-gas reaction is very important in the molecular laser isotope separation of uranium-235 (235U) at RIKEN, because an isotope exchange reaction between growing 235U enriched UF5 particles and natural UF6 is thought to be one of the most serious problems which causes the separation factor to deterionate. We present here a new simple calculation method to estimate the influence of this reaction system. Using an exchange reaction probability of 1.1 ×10-9 reported by G. Y. Grigore'v et al, , we found that the influence of the isotope exchange reaction between growing 235U enriched UF5 particles and natural UF6 gas was negligible to the separation factor.

An FT-IR Study of the Atmospheric Hydrolysis of Uranium Hexafluoride

The handling and processing of UF6 is accompanied by the potential for a reaction with water, usually as airborne moisture, and the subsequent risk of releasing airborne, particulate UO2F2 and gaseous hydrogen fluoride. We have studied this reaction under the simulated conditions of a “release” of UF6, whereby hot, pressurized UF6 gas suddenly escapes into ambient air. The “release” experiment simulates, on a small scale, the accidental mixing of UF6 gas with the atmosphere in the absence of wind currents. The data obtained with the use of FT-IR spectroscopy have provided information which permits a much better understanding of the behavior of the evolved hydrogen fluoride, the residual water vapor, and the airborne, particulate UO2F2 and of their relation to each other.

Separation characteristics of gas centrifuges. Approximate analysis of separation performance.

An analytical method of solving the convection-diffusion equation is presented for the centrifugal separation of a binary isotopic gas mixture under the condition that the pure axial flow velocity field is attained. In the present method, it is assumed that the radial concentration profile can be expressed in the form of multi-term expansion, and the two-dimensional partial differential equation is transformed into a set of ordinary differential equations through the multiple integration in the radial direction. Computations are made on UF6 gas in various types of centrifuges which have the ports for feed, product, and waste on the end plates. The results of the present analysis are compared with the other results of existing theoretical analysis. In spite of the approximate approach, the present method can provide a closer agreement with the two-dimensional finite difference method (FDM) proposed by Kai. Finally, it is shown that the results of the present analysis agree with the experimental data of Bea...

Fiber Optic Pulsed Laser Delivery For Remote Measurements

The results of a research program on the delivery of high-peak-power laser light via fiber optics are presented. We discuss the influence of the host medium and the optical signals on the choice of fiber materials, complemented by a consideration of the measurement environment's effects on the quality of the data. We pay close attention to the choice of input/output beam/fiber coupling optics, nonlinear processes in the core, measurement system noise, and baseline drifts. Useful discussions of pulsed laser damage to optical fibers and data for optimization of a given fiber optic laser beam delivery system are given. As an example, details are given of the optical and instrumentational aspects of a particular fiber optic system developed for remote sensing of pressures and temperatures of UF6 gas in an operating advanced gas centrifuge.

ChemInform Abstract: REACTIONS OF URANIUM HEXAFLUORIDE PHOTOLYSIS PRODUCTS

AbstractEs wird bestätigt, daß bei der UV‐Photolyse des UF6 im spektralen B‐Bandbereich die einfache Reaktion A abläuft.

Reactions of uranium hexafluoride photolysis products

This paper confirms that the ultraviolet photolysis reactions of UF6 in the B band spectral region is simple bond cleavage to UF5 and F. The photolysis products may either recombine to UF6 or the UF5 may dimerize, and ultimately polymerize, to solid UF5 particles. We use four methods to set an upper limit for the rate constant for recombination of kr<2.0×10−12cm3 molecule−1 s−1. We measure the rate constant for UF5 dimerization to be kd=(1.0±0.2)×10−11 cm3 molecule−1 s−1. The principal method employed in these studies is the use of diode lasers to monitor, in real time, the changes in density of the species UF6 and UF5 after laser photolysis of the UF6 gas sample.

Theoretical Analysis of Ternary UF6Gas Isotope Separation by Centrifuge

This paper concerns a theoretical analysis of the isotope separation in a centrifuge for the ternary gas mixture which includes 236U besides 235U and 238U. The expressions for the diffusion velocities are used to derive diffusion equations in terms of the mole concentration and of the mass average velocity. The concentration distributions of the ternary UF6 gas isotopes are calculated by numerically integrating the above diffusion equations with the aid of a flow field for an appropriate model centrifuge. A value function for two of any species in a multicomponent mixture is defined as a natural extension of the value function for a binary mixture. Furthermore, a total value function for the whole multicomponent mixture is defined as a linear combination of the obtained value function. The results of numerical calculations show that the total separative power for the ternary gas isotopes mixture derived from the total value function is an effective expression for the evaluation of centrifuge separation pe...

Theoretical analysis of ternary UF6 gas isotope separation by centrifuge.

Theoretical analysis of ternary UF6 gas isotope separation by centrifuge.

Investigation of the spectral dependences of the excitation of high vibrational levels of the UF6molecule

An experimental investigation was made of the excitation of high vibrational states of the UF6 molecule by infrared laser radiation. The spectral and energy dependences were determined of the absorption of 16 μ radiation by UF6 gas undergoing single-frequency interaction with CF4 laser radiation, and of the visible luminescence of the UF6 molecules in the case of two-frequency interaction with CF4 and CO2 laser radiation in a static gas cell at 300 °K. It was found that the visible luminescence can serve as a quantitative measure of the number of UF6 molecules excited above the first dissociation limit. A possible isotopic dissociation selectivity was estimated and the deexcitation times of electronic and vibrational excitation of the UF6 molecule were measured.

Dissociation of uranium hexafluoride molecules in a two-frequency infrared laser field

Gaseous UF6 was dissociated with radiation from a CF4 laser. A considerable increase in the rate of dissociation of the UF6 occurred under the combined action of CF4 and CO2 lasers. It was found that the CO2 laser radiation acted most efficiently when its frequency was close to that of the ν2+ν3 overtone of the UF6 gas. Visible luminescence, explained by emission from the UF6 dissociation products, was observed in two-frequency dissociation of the UF6. A mass-spectrometric investigation of the residual UF6 gas failed to show any isotopic selectivity of the dissociation.

Evaluation of UF6Vapor Release in Postulated Accident

The rupture of UF6 gas line connected to hot UF6 cylinder, being one of various accidents in UF6 vapor leak-out, is considered as a postulated accident for uranium enrichment plants. For this type of rupture, we will estimate the amount of UF6 vapor release based on a simplified calculation model and then make an evaluation of UF6 vapor release through a ventilation system of feed vaporization facility. Assuming an instantaneous steady state for the change of UF6 states, an unsteady state thermodynamics process is solved. Numerical examples show that about 52% of the initial UF6 quantity are vaporized at 80°C (the temperature of the liquid UF6 in the cylinder). Furthermore, by using the amount of released UF6 vapor and the collection capacity of HEPA filter for IiF gas, the amount of gaseous UO2F2, HF which may be dissipated to the environment are conservatively estimated.

Evaluation of UF6 Vapor Release in Postulated Accident

The rupture of UF6 gas line connected to hot UF6 cylinder, being one of various accidents in UF6 vapor leak-out, is considered as a postulated accident for uranium enrichment plants. For this type of rupture, we will estimate the amount of UF6 vapor release based on a simplified calculation model and then make an evaluation of UF6 vapor release through a ventilation system of feed vaporization facility. Assuming an instantaneous steady state for the change of UF6 states, an unsteady state thermodynamics process is solved. Numerical examples show that about 52% of the initial UF6 quantity are vaporized at 80°C (the temperature of the liquid UF6 in the cylinder). Furthermore, by using the amount of released UF6 vapor and the collection capacity of HEPA filter for IiF gas, the amount of gaseous UO2F2, HF which may be dissipated to the environment are conservatively estimated.

Basic Characteristics of Centrifuges, (IV)

A method of numerically solving the convection-diffusion equation is presented for a binary isotopie gas mixture in the axisymmetric velocity field. A modified Newton's method is employed to perform the numerical integration without the assumptions that the pressure can be estimated from the rigid rotation model and the temperature of gas is uniform. A suitable form of the finite difference equation gives a computationally stable integration with reasonable representation of the molar concentration distribution of isotopie molecules in a rotating cylinder. The method includes a Gaussian elimination procedure which consists of the transformation of the Jacobian matrix to a triangular matrix followed by the backward elimination. Computations are made on UF6 gas in various centrifuges which have the openings for feed, product and waste on the end plates. Discussions are also presented on gas flows and separative efficiencies for the centrifuges which have baffle plates, skirting plates and bellows.

Basic Characteristics of Centrifuges, (III)

A method of numerically integrating the Navier-Stokes equations is presented for axisymmetric compressible flows. A modified Newton's method is employed to determine the steady motion of gas in a rotating cylinder without the use of a time-consuming marching process with respect to time. A suitable form of the finite difference equations gives a computationally-stable integration with reasonable representation of the spatial characteristics of the flow. The method includes a Gaussian elimination procedure which consists of the transformation of the Jacobian matrix to a triangular matrix followed by the backward substitution. By using an auxiliary constant matrix algorithm, the method gives the solution within reasonably acceptable computation time. As an example of the method, some features of solutions are presented for the steady flow of UF6 gas in the centrifuges which have the openings for feed and withdrawal on the end plates.