Gas Chromatography of Uranium Hexafluoride at Low Temperatures

Gas chromatography of UF₆ at low temperature was studied with the use of columns of polytrifluoromonochloroethylene oils as liquid phase. The dependence of retention time and HETP of UF₆ on (1) degree of polymerization of oils, (2) liquid phase loading, and (3) kinds of solid support were studied in the temperature range between ?10° and 40°C, and the most favorable conditions for quantitative analysis of UF₆ are discussed.

The relation between the gas chromatographic characteristics of the columns and the B.E.T. surface area of the solid support has been briefly examined.     

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.     

Fluorination of Uranium Metal by Fluorine Gas

A study has been made on the fluorination of uranium metal chips to UF₆ with fluorine gas in the temperature range of 100° to 400°C. The formation of UF₆ was influenced by the concentration and the flow rate of fluorine gas and by the reaction temperature. The reaction occurred apparently above 200°C. Uranium metal was first converted to low fluorine content compound such as UF₃ or UF_4-x, then to UF₄ and finally to UF₆. The intermediate compounds were confirmed by X-ray analysis and by their color.     

Analytical Modelling of Overpack of Enriched Uranium Hexafluoride Package in a Fire Event

Abstract

A fissile package must be subjected to a fire test at 800°C for 30 min. For a package that contains enriched uranium hexafluoride (UF₆), the temperature of the contents has to be kept below 121°C in order to avoid rupture of the 30B cylinder. A previous study on heat transfer mechanisms in an enriched UF₆ package type ‘DOT-21PF-1’ found that decomposed gas with a temperature of around 100°C generated from heated penolic foam thermal insulator was a major heat source heating the cylinder in the initial period of the fire test, in addition to the heat conducted from the outer surface through the thermal insulator into the interior of the package. Mitsubishi Materiass Corporation has developed a new packaging for enriched UF₆ ‘MST-30’ that also uses phenolic foam as the thermal insulator, and has pelformdd a fire test at 800°C for 30 min, which also indicated the temperature history of the package interior affected by heating from the decomposed gas. In this study, a laboratory test was carried out to identify the kind of gas generated from the heated phenoiic foam. The test found that the decomposed gas was water vapour. Since water vapour has a large latent heat, it has a large heat transfer coefficient when it condenses on a cool surface. By taking the water vapour heating mechanssm into account, an analytical model was developed for MST-30 to simulate the measured cylinder temperature history. Analytical models for evaluaiing the cylinder temperature under the 800°C/30 min fire test considering conservativeness are also presented.     

Pyrohydrolysis Reactions of UF4 and UO2F2

This study was made for clarification of the pyrohydrolysis behaviors of UF₄ and UO₂F₂. The progress of the reaction was measured by titrating the amount of produced hydrogen fluoride. When nitrogen was used as carrier gas of water, the pyrohydrolysis of UF₄ proceeded at the temperatures of 350–400°C, and UO_2+x (x?0.3) was formed; the value of x decreased with the decrease of oxygen dissolved in the water. In the case of oxygen carrier gas, the pyrohydrolysis of UF₄ formed U₃O₈-UO₂F₂ mixture as the reaction product in the above temperature region and the reaction was markedly retarded in the course because of lower rate in the pyrohydrolysis of UO₂F₂. The pyrohydrolysis of UO₂F₂ proceeded at a little [higher temperature of 450–500°C in both cases of nitrogen and oxygen carrier gas and α-UO₃ was formed.     

Radical Reaction Mechanisms in Infrared Multiphoton Dissociation of UF6 with Scavenger Gases

Abstract

The radical reaction mechanisms in the presence of F-atom scavenger gases were investigated in the p-H₂ Raman laser-induced infrared multiphoton dissociation (IRMPD) of gaseous ²³⁶UF₆/²³⁸UF₆ cooled to —35°C in a static gas cell. When CH₄ was added as a scavenger of F-atoms produced via IRMPD of UFC, the dissociation rate of UF6 became several tens of times larger than when no scavenger gas was added. Gas-chromatographic analysis revealed that as low as 7% of the nascent CH₃ radicals were involved in the radical reaction with UF₆. On the other hand, H₂ and C₂H₆ were found to increase both the dissociation rate of UF₆ and the contribution of this non-selective reaction. These results agreed with those obtained in the UV photolysis of UF₆ with scavengers.     

On the Rate Controlling Step of UF4-BrF3 and U3O8-BrF3 Reaction

Experiments were made to determine the rate-controlling step in the reactions of gaseous BrF₃ with UF₄ and U₃0₈ under varying diffusivity of BrF₃ brought about by changing the carrier gas.

It was found that, compared with N₂ and Ar, He gas used as carrier brings about a reaction rate twice as high. These relative reaction rates are affected by changes in neither linear gas velocity, BrF₃ partial pressure, nor reaction temperature; the ratio of the rates between the cases of He, N₂ and Ar is roughly equal to the theoretical ratio between the diffusivity of these three gases. The temperature dependence of the reaction rates also corresponds to the theoretical. Hence, the reactions are concluded to be diffusion controlled.     

UF6 Photodissociation through Measurement of Visible and Infrared Luminescence

Infrared multiple-photon dissociation of UF₆ was induced at room temperature by irradiation simultaneously with optically pumped CF₄ laser at 615 cm^?1 and TEA CO₂ laser at 1,073 cm^?1. The dissociation of UF₆ was verified from evidence of fluorine generation, obtained by its reduction with H₂ and observing infrared emission from the resulting excited HF molecules. The rate of UF₆ dissociation was determined from the decrease of infrared absorption shown by UF₆. Quantitative indication of the dissociation yield of UF₆ is known to be provided by the more readily measurable visible luminescence from the irradiated zone. The intensity of the visible luminescence was therefore measured, to seek the dependence of dissociation yield on such factors as the fluences of the CF₄ and CO₂ lasers, and the pressures of UF₆ and of Ar buffer gas. A clear threshold proved to exist around 70 J/cm² for the fluence of CO₂ laser, but not for that of the CF₄ laser, indicating that a very slight excitation by the latter laser serves to induce UF₆ dissociation. This suggests that the dissociation energy is supplied in large part by the COZ laser. In the range of UF₆ dissociation, the visible luminescence intensity was found to rise roughly proportionally with CF₄ and CO₂ laser fluences, as well as with the pressures of UF₆ and of added Ar buffer gas.     

Numerical Analysis of Flow of Binary Gas Mixture with Large Mass Difference in Rotating Cylinder

A numerical analysis is presented of the flow of a binary gas mixture of UF₆ and N₂ in a rotating cylinder. The equations for flow and the diffusion equation are solved simultaneously for the binary mixture in static state, taking account of viscosity and compressibility, using a modified version of the Newton method, commonly applied to rotating fluid flow. An appropriate model is assumed for a centrifuge provided with scoop and baffle plate. Computations are carried out with the N₂ to UF₆ mixing ratio adopted as parameter.

At small values of mixing ratio, the pressure distribution of UF_C in the radial direction is little influenced by the presence of N₂. The N₂ pressure distribution is close to that at equilibrium of N₂ itself in zones inside cylinder of relatively slow gas travel. In the zones of faster gas travel, conversely, the N₂ pressure distribution deviates from that of its equilibrium and approaches that of UF₆.

The pressure distribution of UF₆, on the other hand, is strongly influenced by the presence of N₂ when the mixing ratio is above 0.2. The resulting radial distribution along a section close to the exit scoop presents a peculiar concave configulation with a shallow valley appearing in the intermediate zone, and this significantly lowers the concentration of UF₆ extracted through the scoop. The separation efficiency obtained between the two gases is extremely low, but this is due to the mass flow rate having been chosen to optimize the separation efficiency between UF₆ isotopes.     

Determination of Hydrogen Fluoride and Uranium (VI) in Crude Uranium Hexafluoride by Hydrolysis and Potentiometric Titration

A simple method for determining HF and U(VI) in a hydrolyzed solution of UF₆ by alkalimetry is described.

About 1 g of UF₆ was taken in a polytrifluoromonochloroethylene tube, and hydrolyzed with about 50 ml of water in a closed polyethylene bottle. Using a glass electrode, HF in the hydrolyzed solution was titrated potentiometrically with an alkali solution, after the U(VI) had been masked with ammonium sulfate. When the equivalence point for the free acid was reached, hydrogen peroxide was added to the solution and the liberated acid equivalent to U(VI) was titrated with the same standard alkali solution.

Assuming that the hydrolysis of UF₆ takes place stoichiometrically, the accuracy of this method was roughly evaluated to be ±10% for the ratio of HF/UF₆ and ± 1 % for the value of U(VI) in the range of 0 to 1 mole of HF per mole of UF₆.     

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 with an electron microscope. The depletion of ²³⁵UF₅ concentration by the exchange reaction is very small when averaged over the whole solid UF₅, because the depletion is virtually limited to the solid surface due to the small reaction probability of underlying UF₅ molecules.     

Potential of a fusion–fission hybrid reactor using uranium for various coolants to breed fissile fuel for LWRs

In this study, neutronic performances of the (D,T) driven hybrid blankets, fuelled with UC₂ and UF₄, are investigated under first wall load of 5 MW/m². The fissile fuel zone is considered to be cooled with three coolants: gas (He or CO₂), flibe (Li₂BeF₄), and natural lithium. The behaviour of the UC₂ and UF₄ fuels are observed during 48 months for discrete time intervals of Δt=15 days and by a plant factor of 75%. At the end of the operation time, calculations have shown that Cumulative Fissile Fuel Enrichment (CFFE) values varied between 5 and 8.5% depending on the fuel and coolant type. The best enrichment performance is obtained in UF₄ fuelled blanket with flibe coolant, followed by gas and natural lithium coolant. CFFE reaches maximum value (8.51%) in UF₄ fuelled blanket (in row #1) and flibe coolant mode after 48 months. The lowest CFFE value (4.71%) is in UC₂ fuelled blanket (in row #8) and natural lithium coolant at the end of the operation period. This enrichment would be sufficient for LWR reactor. At the beginning of the operation, tritium breeding ratio (TBR) values were 1.090, 1.3301 and 1.2489 in UC₂ fuelled blanket and 1.0772, 1.2433 and 1.1533 in UF₄ fuelled blanket for flibe, natural lithium and gas coolant, respectively. At the end of the operation, TBR reach 1.1820, 1.3983 and 1.3138 in UC₂ fuelled blanket and 1.2041,1.3266 and 1.2407 in UF₄ fuelled blanket for flibe, natural lithium and gas coolant, respectively. Nuclear quality of the plutonium increases linearly during the operation period. The isotopic percentage of ²⁴⁰Pu is higher than 5% in UF₄ and UC₂ fuel with flibe coolant, so that the plutonium component in these modes can never reach a nuclear weapon grade quality during the operation period. This is very important factor for safeguarding. The isotopic percentage of ²⁴⁰Pu is lower than 5% in UC₂ fuel with gas and natural lithium coolant. In these modes, operation period must be increased to safeguarding.     

Dissociation of the UF6 molecule under the action of fragments from fission of the uranium nucleus

It was shown that when irradiated with neutrons, uranium hexafluoride decomposed to intermediate uranium fluorides (presumably UF₅ and fluorine).The rateof decomposition wasG = 0.5 mole/l00 ev or 0.21mole/hr per kw of power liberated in the gas. It was also shown that in addition to dissociation of UF₆ during irradiation there was also recombination of the decomposition products. As a result, equilibrium concentrations of fluorine and UF₆ are set up, and these depend on the strength of the radiation. When mixed with fluorine, uranium hexafluoride is a radiation-stable compound even at room temperature and may be used as a fuel in a nuclear reactor.In conclusion, the authors consider it their pleasant duty to thank I. K. Kikoin for his interest and valuable advice.     

Computer Calculation of Flame Processes in Uranium Technology

The possibility of using a computer program to calculate three groups of proceses – fluoridation of uranium compounds, reduction of UF₆, pyrohydrolysis, and reductive pyrohydrolysis of UF₆ – is studied.It is established that calculations make it possible to take account of the specific features of the processes in the basic technology and reprocessing of depleted UF₆ and to make an assessment of the desirability and regimes of reactions which are not investigated experimentally. It is shown that the dependences of the temperature generated in the process on the amount of heat removed can be used to estimate the geometry of flame reactors.     

In-line Gas Analyzer for UF6 and F2 through Differential Thermal Conductivity Measurements

For the purpose of analyzing UF₆ and F₂ continuously in the fluorination of U, a system of differential thermal conductivity cells (TCC), equipped with shock absorber, and also NaF and KCl traps, has been trially constructed.

Mixed flow-through/diffusion type katharometers are used, provided with four and two Ni filaments (～30Ω) on the UF₆ and F₂ cells, respectively. Based on data obtained from preliminary operating tests, the standard conditions for operating the system were established i.e., 500 mmHg abs. Cell pressure, 200 ml/min flow rate and 80 mA bridge current.

Under these operating conditions and for UF₆ and F₂ concentrations of 2 and 20 v/o respectively, the overall out-put variations are below 1%. The total response time of the system including the transport lag is below 1 and 2.5 min for UF₆ and F₂ cells, respectively. The dead time due to blow-back noise is observed to have about the same period as the above response time. The sensitivities for UF₆ and F₂ are found respectively to be 11.5 and 0.37 mV/v/o, for which the limits of analytical determination are 0.01 and 0.05 v/o;, respectively.

By practical application to uranium fluorination, the TCC system thus devised, has proven to possess the requisite characteristics for measurements of reaction rate, fluorine utilization and end of reaction in fluid-bed fluorination of uranium oxides.

The system has possibilities of application to the study of reaction steps.     

Industry experience with thermal protectors on 48 in UF6 cylinders

Abstract

The International Atomic Energy Agency (IAEA) Transport Safety Regulations (TS-R-1) require packages filled with non-fissile and fissile excepted uranium hexafluoride (further indicated as natural and depleted UF₆) to pass the accident simulating thermal test. The thermal behaviour of cylinders filled with UF₆ has been studied extensively and also an IAEA Co-ordinated Research Programme was allocated to this subject. The studies show that the standard 48 in UF₆ cylinders have a large thermal mass and some conclude that they would meet the thermal test requirement. A continued unilateral approval, however, was not supported by all parties involved. In order to be able to continue international operations under H(U), i.e. unilateral – approval, an industry consortium developed thermal protection units (BTP's and CTP's) to be added to the standard cylinders. Actual use of the newly developed thermal protectors started in January 2005. The present paper reviews the UF₆ specific regulatory developments, research and analysis activities to assist the regulatory development and to verify compliance with the new requirements and describes the industry experience with these. The present paper describes experience with the use of thermal protectors.     

An Apparatus for Isotopic Ratio Measurement in UF5 Formed by Infrared Multiphoton Dissociation of UF6 in Molecular Beam

A Separation factor was measured in isotopically selective infrared multiphoton dissociation (IRMPD) of supercooled UF₆ in a supersonic expansion by multiphoton ionization (MPI) and time-of-flight mass spectrometry (TOFMS). A pulsed free-jet nozzle was used to generate a UF₆-molecular beam seeded in Ar (–10^?7 Torr in UF₆ partial pressure). Two-frequency ρ-H₂ Raman laser beams around 16μm were used for the dissociation of UF₆ under collisionless conditions in the molecular beam where the flow velocity for UF₆ is about 500m/s. The ²³⁵U/²³⁸U isotopic ratios in nascent UF₅ photoproducts were determined by selective MPI of UF₅ at 532 nm followed by TOFMS with a mass resolution as high as 1200. A separation factor of about 2 was observed under the experimental conditions chosen for the demonstration of this method.     

Measurement of Specific Alpha Radioactivity with Respect to Size for Kerala Monazite Sand Using Aerosol Centrifuge

A numerical analysis is presented on the diffusive separation behavior of a three-component gas mixture of ²³⁵UF₆, ²³⁸UF₆, and light gas in a gas centrifuge. The purpose of analysis was to examine the isotope separation performance of the centrifuge in the presence of light gases such as N2 and HF, which inevitably leak in from the atmosphere or accumulate from impurities contained in uranium hexafluoride.

An approximate basic equation is used, which is almost the same as derived by T. Kai for a ternary system from a generalized form of the Stefan-Maxwell equations, taking account of the pressure diffusion in a rapidly rotating cylinder. The method has previously been proposed for application to counter-current centrifuges, which are characterized by stable axial flow in two concentric streams, assuming that the gas mixture is in thermo-dynamical equilibrium between the light gas and UF6 mixture.

Calculations made using the above basic equation indicated that the separative power of the centrifuge is significantly lowered with increasing penetration of the light gas into the separative zone between the two concentric streams. The results of calculation are in fairly good agreement with previously reported theories over a wide range of light gas concentration.

Measurements made using an experimental centrifuge equipped with scoop substantiated the foregoing trends indicated from calculation up to about 1 mol/o average concentration of light gas in the centrifuge cylinder. At higher concentrations, a more significant lowering of separative power by the presence of light gas was indicated from measurement than from calculation.     

Kinetic Studies of Fluorination of Uranium Carbides by Fluorine, (II)

The fluorination reaction of uranium dicarbide with fluorine to form UF₆ has been studied in the temperature range between 220° and 300°C using a thermobalance. The overall mechanism of reaction is similar to the case of uranium monocarbide, i.e. in the first step, UC₂ is rapidly converted to UF₄, polymeric fluorocarbon and gaseous fluorocarbon, while in the second step, UF₄ and polymeric fluorocarbon are converted rather slowly to UF6 and gaseous fluorocarbon. The amount of polymer is much larger than the case of uranium monocarbide, its weight ratio to UF₄ being 0.2 to 0.25 in the early stages of the reaction. The fluorination of ₄ to UF₆ always follows the linear law derived from the diminishing sphere model. The apparent activation energy was determined to be 19.5 kcal/mole. Differences in the fluorination between UC and UC₂ are discussed, with the effect of polymer taken into consideration.     

Preparation of Uranium Phosphide from Uranium Tetrafluoride

A study was made on UP preparation by two-step reactions starting with UF₄, Si and red phosphorus.

The first step was to produce an intermediate uranium phosphide U₃P₄ from the reaction of UF₄ with Si under phosphorus vapor at temperatures above 900°C.

In the second step, the intermediate phosphide was heat treated under vacuum. To obtain single phase UP from the intermediate phosphide, the treatment required a temperature above 1,050°C, at which temperature the minimum holding time was 60 min.