Sputtering of condensed noble gases by keV heavy ions

Bombardment of condensed Kr and Xe by 2–8 keV noble gas ions results in very high sputtering yields. A considerable fraction (10–30%) of the sputtered particles consists of Van der Waals clusters, with Kr₂, Kr₃, Xe₂, XeKr, XeKr₂ and ArKr having been observed. The kinetic energy distributions of the sputtered monomer-species are in agreement with a collision cascade mechanism. However, at the low energy side an excess yield is observed. This is explained by a model which takes into account the large sputtering yields and the damage of the surface during the sputtering process. The energy distributions of the dimers and trimers are satisfactorily explained by a statistical model. It is concluded that the dimer and trimer species are sputtered during an early stage of the cascade.     

Radiation-chemical effect of fast electrons on uranium fluorides

The effect of fast electrons on UF₆, UF₅, and UF₄ is investigated. The radiation-chemical yield of the decomposition reaction of UF₆ molecules is equal to 1.1 ·9 10^–2 molecules per 100 eV. As a result.of long irradiation of UF₆, the dynamic equilibrium UF₆ UF₅ + 1/2F₂ is established, The rate of radiation--fluorination of UF₅ and UF₄ is proportional to the fluorine pressure and to the square root of the radiation intensity.Translated from Atomnaya Énergiya, Vol. 16, No. 6, pp. 510–514, June, 1964     

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.     

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.     

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.     

Neutronic performance of HYLIFE-II fusion reactor using various thorium molten salts

HYLIFE-II is one of the major inertial fusion energy reactor design concepts in which a thick molten salt layer (Flibe = Li₂BeF₄) is injected between the reaction chamber walls and the explosions. Molten salt coolant eliminates the frequent replacement of solid first wall structure during reactors lifetime by decreasing intense neutron flux. This study presents the neutronic analysis of HYLIFE-II fusion reactor using various liquid wall coolants, namely, 75% LiF–25% ThF₄, 75% LiF–24% ThF₄–1% ²³³UF₄ or 75% LiF–23% ThF₄–2% ²³³UF₄. Neutron transport calculations for the evaluation of neutron spectra were conducted with the help of Scale 4.3 by solving the Boltzmann transport equation in S₈–P₃ approximation. The effects of flowing liquid wall thickness and type of coolant on the neutronic performance of the reactor were investigated. Furthermore, radiation damage calculations at the first wall structure with respect to type and thickness of liquid wall were carried out. Numerical results showed that using the flowing liquid wall containing the molten salt, 75% LiF–23% ThF₄–2% UF₄ with a thickness of 70 cm maintained tritium self-sufficiency of the (DT) fusion driver and extended the first wall lifetime to the reactors lifetime (30 full power years). In addition significant amount of high quality fissile fuel was bred through (n, γ) reaction of ²³²Th. Moreover, energy multiplication factor (M) was increased to 12 by high rate fission reactions of ²³³U occurring in the flowing wall. On the other hand, it was concluded that using the other two coolants, 75% LiF–25% ThF₄ or 75% LiF–24% ThF₄–1% ²³³UF₄, as liquid wall did not satisfy the radiation damage and the tritium sufficiency criteria together at any thickness, so that these two coolants were not suitable to improve neutronic performance of HYLIFE-II reactor.     

Experiment and simulation of cluster emission from 5 keV Ar → Cu

The abundance distribution of neutral Cu_n clusters sputtered by 5 keV Ar impact from a polycrystalline Cu surface is measured using single-photon laser post-ionization. Molecular dynamics computer simulation is used to gain insight into the cluster sputtering process. Three different codes and two potentials are used to check the sensitivity of the results on numerics and physical input. Differences in the results obtained by the various codes and the different potentials used are discussed. While the total sputter yield is consistent with experiment, the fraction of atoms bound in clusters, and in particular the dimer fraction, are overestimated by at least a factor of 4. This is also true for a many-body potential which has been fitted to describe both bulk Cu and dimers. In detail, the simulation shows that larger clusters are emitted at later times from the target. Clusters originate mainly from regions of the surface, which are around the melting temperature of bulk Cu. Large clusters are emitted preferably from ion impacts with a high individual sputter yield. Finally, we simulate sputtering from a model Cu material with an artificially decreased cohesive energy. Here, drastic high-yield events (up to Y=78) can be observed, which produce clusters abundantly.     

Investigation of the formation of uranium tetrafluoride in solution

An investigation of the system U(SO₄)₂-HF-H₂O through the solubility, optical density, and pH showed that in this system there is the successive formation of a soluble uranium fluorosulfate and sparingly soluble UF₄ · 1¹/2H₂O in cubic and monoclinic forms. Solubility data are given for the system UF₄-HF-H₂O (t = 25 °C), in which the solid phases are UF₄ · 2¹/2H₂O, UF₄, and UF₄ · 4HF.     

Utilization of Heavy Metal Molten Salts in the ARIES-RS Fusion Reactor

ARIES-RS is one of the major magnetic fusion energy reactor designs that uses a blanket having vanadium alloy structure cooled by lithium [1, 2]. It is a deuterium–tritium (DT) fusion driven reactor, having a fusion power of 2170 MW [1, 2]. This study presents the neutronic analysis of the ARIES-RS fusion reactor using heavy metal molten salts in which Li₂BeF₄ as the main constituent was mixed with increased mole fractions of heavy metal salt (ThF₄ or UF₄) starting by 2 mol.% up to 12 mol.%. Neutron transport calculations were carried out with the help of the SCALE 4.3 system by solving the Boltzmann transport equation with the XSDRNPM code in 238 neutron groups and a S ₈–P ₃ approximation. According to the numerical results, tritium self-sufficiency was attained for the coolants, Flibe with 2% UF₄ or ThF₄ and 4% UF₄. In addition, higher energy multiplication values were found for the salt with UF₄ compared to that with ThF₄. Furthermore, significant amount of high quality nuclear fuel was produced to be used in external reactors.     

Measurements of Total Neutron Cross Sections at 24-keV by means of Iron-Filter Method

Sputtering of the solid hydrogens by light ions has shown isotope effects which are greater than for any other solid targets. These hydrogenic solids are unique because of the extreme volatility and because the first step of the electronic sputtering process is identical for all hydrogenic solids. The sputtering for protons in the energy range from 5 to 10 keV can be qualitatively described by an electronic spike of cylindrical geometry. The sputtering yield of solid tritium has been evaluated on the basis of results for solid H₂, D₂ and HD.     

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.     

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.     

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.     

Internal energy distribution of sputtered sulfur molecules

Bombarding targets of CS₂ and sulfur with keV Ar⁺ ions induces among other species the sputtering of S₂ molecules. We measured the internal energy (vibration and rotation) of these sputtered molecules with a laser induced fluorescence technique. The results show a Boltzmann behaviour for both the vibrational and rotational distributions. A vibrational temperature T_vib = 1500 K and a rotational temperature T_rot = 300 K is obtained for both target materials. The results are compared with different ejection mechanisms, where a molecule on the surface receives one (single collision) or two (double collision) momentum transfers from the solid. Two classical models are able to explain part of the experimentally observed internal energy distribution: a Monte Carlo calculation of the double collision model, and a single collision model assuming a sudden momentum transfer to the molecule as a whole, which yields an analytical expression (via a relation between the internal and kinetic energy). If we assume that some additional rotational cooling, due to time dependent relaxation effects, occurs during the ejection from the surface, the experimental results can be reasonably interpreted within the proposed sputtering models.     

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.     

Synergism in sputtering of copper nanoclusters on graphite substrate at low energy Cu2 bombardment

Molecular dynamics simulation of Cu cluster sputtering by 50-200 eV/atom Cu₂ dimers and Cu single atoms has been performed. The clusters were located on a (0 0 0 1) graphite surface and consisted of 13-195 atoms. Synergy features were identified in the sputtering yield and energy distributions of sputtered particles calculated for the cases of cluster bombardment with Cu dimers and monomers at the same velocity. The reason for the nonlinear effects in surface cluster sputtering is the overlapping of collision cascades generated by each of the dimer atoms.     

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

The rate of the fluorination reaction of UC with fluorine to form UF₆ has been studied in the temperature range between 240° and 300°C by following with a thermobalance the change in weight of the solid phase. The overall reaction takes place in two steps: In the first step, UC is rapidly converted to UF₄, which, in the second step, fluorinates to UF₆. During the first step, polymeric fluorocarbon is also produced and is found dispersed throughout the intermediate UF₄ particles. At reaction temperatures below 270°C, the second step reaction roughly follows the linear law. At temperatures above 270°C, it comes to follow the parabolic law, presumably due to the formation of a compact layer of polymeric fluorocarbon on the surface of the particles. Analysis of the gaseous products using mass and infrared spectrophotometers revealed the formation of several kinds of lower fluorocarbons.     

Sputtering induced by cluster impact on metal targets: Influenceof electronic stopping

The effect of electronic stopping on the sputtering of metals by cluster impact is discussed. We focus on the specific case of Au₁₃ impact on a Au surface. Using molecular-dynamics simulation, we study several strategies to include electronic stopping. Electronic stopping influences both the magnitude of the sputter yield and the duration of the sputter process. In the usual procedure, electronic stopping only affects sufficiently fast atoms with kinetic energies above a threshold energy, which is of the order of the target cohesive energy. When assuming that electronic stopping holds down to thermal energies <1 eV, or even to 0 eV, the collision spike is rapidly quenched and the sputter yields become unrealistically small. Furthermore, we implement a scheme to include electronic stopping based on local (electron) density information readily available in a simulation.     

Neutronic Analysis of the Laser Inertial Confinement Fusion–Fission Energy (LIFE) Engine Using Various Thorium Molten Salts

In this study, a neutronic performance of the Laser Inertial Confinement Fusion Fission Energy (LIFE) molten salt blanket is investigated. Neutronic calculations are performed by using XSDRNPM/SCALE5 codes in S₈–P₃ approximation. The thorium molten salt composition considered in this calculation is 75 % LiF—25 % ThF₄, 75 % LiF—24 % ThF₄—1 % ²³³UF₄, 75 % LiF—23 % ThF₄—2 % ²³³UF₄. Also, effects of the ⁶Li enrichment in molten salt are performed for all heavy metal salt. The radiation damage behaviors of SS-304 structural material with respect to higher fissionable fuel content and ⁶Li enrichment are computed. By higher fissionable fuel content in molten salt and with ⁶Li enrichment (20 and 50 %) in the coolant in form of 75 % LiF—23 % ThF₄—2 % ²³³UF₄, an initial TBR >1.05 can be realized. On the other hand, the 75 % LiF—25 % ThF₄ or 75 % LiF—24 % ThF₄—1 % ²³³UF₄ molten salt fuel as regards maintained tritium self-sufficiency is not suitable as regards improving neutronic performance of LIFE engine. A high quality fissile fuel with a rate of ~2,850 kg/year of ²³³U can be produced with 75 % LiF—23 % ThF₄—2 % ²³³UF₄. The energy multiplication factor is increased with high rate fission reactions of ²³³U occurring in the molten salt zone. Major damage mechanisms in SS-304 first wall stell have been computed as DPA = 48 and He = 132 appm per year with 75 % LiF—23 % ThF₄—2 % ²³³UF₄. This implies a replacement of the SS-304 first wall stell of every between 3 and 4 years.