Sputtering and ion-induced electron emission of graphite under high-dose nitrogen bombardment

The dependence of the sputtering yield Y and the electron emission coefficient γ of isotropic graphites (POCO-AXF-5Q and Russian MPG-LT) on ion fluence and ion incidence angle θ at near room temperatures and the dependence of γ on target temperature under high dose 30 keV N₂⁺ ion irradiation were measured. It was found that Y and γ are stabilized at fluences F?1×10¹⁹ N/cm². A specific target surface topography develops. At steady-state conditions, the N concentration in MPG-LT is 19 at.% and in POCO16 at.%. In the angular range θ=0-80°, Y and γ increase and the angular dependence of Y is slightly stronger than that of γ. Sputtering yields of POCO are 1.5 times higher than those of MPG-LT. The reasons of the difference between the experimental and calculated sputtering yields using the TRIM.SP code are discussed. The dependence of γ on the target temperature manifests a step-like increase at ?250 °C which may be due to radiation induced structure transformation in the modified surface layer.     

Molecular dynamics simulations of CH3 sticking on carbon surfaces, angular and energy dependence

Sticking cross-sections for CH₃ radicals at different angles of incidence and different energies were calculated using molecular dynamics simulations, employing both quantum-mechanical and empirical force models. The chemisorption of a CH₃ radical at 2100 K onto a dangling bond is found to be highly dependent on the angle of incidence of the incoming radical. The sticking cross-section decreases from (10.4 ± 1.2) to (1.4 ± 0.3) Ų when the angle of incidence of the methyl radical increases from 0° to 67.5°. A simple geometrical model is presented to explain the angular dependence. In the sticking process of CH₃ radicals with higher kinetic energies (1, 5, and 10 eV) both a fully hydrogen-terminated surface and a surface with dangling bond were studied. The sticking probability is enhanced as the radical energy increases. We observed sticking onto the fully hydrogen-terminated surface for all cases except for the case when the methyl radicals had energies corresponding to a temperature of 2100 K.     

Molten salts and nuclear energy production

Molten salts (fluorides or chlorides) were considered near the beginning of research into nuclear energy production. This was initially due to their advantageous physical and chemical properties: good heat transfer capacity, radiation insensitivity, high boiling point, wide range solubility for actinides. In addition it was realised that molten salts could be used in numerous situations: high temperature heat transfer, core coolants with solid fuels, liquid fuel in a molten salt reactor, solvents for spent nuclear solid fuel in the case of pyro-reprocessing and coolant and tritium production in the case of fusion. Molten salt reactors, one of the six innovative concepts chosen by the Generation IV international forum, are particularly interesting for use as either waste incinerators or thorium cycle systems. As the neutron balance in the thorium cycle is very tight, the possibility to perform online extraction of some fission product poisons from the salt is very attractive. In this article the most important questions that must be addressed to demonstrate the feasibility of molten salt reactor will be reviewed.     

Oxidative erosion of graphite in air between 600 and 1000 K

The oxidative erosion of seven types of graphite has been investigated by heating in air at temperatures between 600 and 1000 K. The specimens include pyrolytic graphite, fine-grain graphites, carbon-fibre composites (CFC), and graphites doped with Si and Ti. The weight loss was measured using a microbalance, the surface morphology by scanning electron microscopy, and the composition of the surface layer by MeV ion beam techniques. Pyrolytic graphite is least affected by erosion, while pure and Si-doped CFCs erode particularly fast. Typical erosion rates for specimens with a surface area of ?4 cm² are below 0.2 μg/m² s at 600 K for all graphite types, and at 900 K range from 0.34 mg/m² s for pyrolytic graphite to about 9 mg/m² s for the strongest eroding types. The temperature dependence of the erosion rate of all types of graphite studied is well described by an activation energy of 1.7 eV. The erosion rates of these graphites are by far lower than the removal rates for deposited amorphous hydrocarbon layers. In contrast to all other types, the Ti-doped graphite absorbs a significant amount of oxygen reaching up to ?5% of its original mass. Once the oxygen uptake is saturated, it erodes with rates similar to those of the strongest eroding types.     

Modelling of thermally enhanced erosion of beryllium

This paper presents two concurring models for the thermally enhanced erosion of metals. The modelling particularly deals with the erosion of beryllium by a helium plasma as an example system. Molecular dynamics (MD) simulations are used to reduce the number of free parameters in the models. A model of sublimation of ad-atoms created during ion impact was earlier proposed as an explanation of thermally enhanced erosion. Using MD calculations the parameter space for this model was reduced to a single free parameter, the areal surface defect density δ_Def. Using the reduced parameter space a very low δ_Def has to be assumed in order to reproduce the experimental observations. Therefore a new model is proposed here that is very similar to the ad-atom model but is based on a different mechanism to create weakly bonded surface atoms. The paper shows that inclusion of He atoms during exposure to high flux (10²² m⁻² s⁻¹) of low energy He (50 eV) leads to the formation of weakly bonded atoms in the surface. The comparison of both models with experimental data and their applicability to other projectile/target systems is discussed.     

Modelling of carbon transport in fusion devices: evidence of enhanced re-erosion of in-situ re-deposited carbon

The paper presents new Monte-Carlo transport simulations of methane ¹³CH₄ injected through a hole in a testlimiter and exposed to the edge plasma of TEXTOR. The results show that the spatial distribution of ¹³C re-deposited locally on the testlimiter surface can be modelled if the parameter S for the sticking of returning hydrocarbons ¹³CH_y is set to zero or almost zero. This is interpreted as a negligible effective sticking of the returning hydrocarbon radicals due to the instantaneous re-erosion caused by the hydrogen carried with the CH_y radicals (`self re-erosion'). However, the calculated local deposition efficiency of ¹³C, remains too high compared with the observed value. Therefore, in addition an enhanced yield for chemical erosion caused by the background hydrogen for the fresh re-deposits has to be assumed. Similar assumptions can reproduce also the high amount of carbon deposition found on the inner louvers in the MkIIa divertor configuration of JET and on the plasma-shadowed areas of the MkIIGB divertor.     

Surface mediated isotope exchange reactions between water and gaseous deuterium

Maintaining isotopic purity of hydrogen is one of the major tasks in tritium processing systems. The work with multiple isotopes and isotopomers is accompanied by isotope exchanges which is often accelerated by catalysts e.g. surfaces of various materials. In this work, densities of D₂O, HDO produced via isotope exchange reactions in the mixture of D₂, H₂, D₂O, H₂O, HD and HDO contained in a stainless steel (type SS304) vessel were measured as a function of time (40-36 000 s) and pressures near 3.5 × 10² Pa, using mass spectrometry. The derived rates of change of the isotopomers densities are described accurately by a postulated kinetic model.     

Metallographic preparation techniques for uranium

Existing metallographic preparation techniques for uranium are limited to elucidating specific microstructural characteristics, and some of the techniques are regarded as being environmentally unacceptable. This paper describes a newly developed technique, which is not only more environmentally friendly, but reveals most microstructural features simultaneously. Example microstructures of the various preparation stages are given to highlight the new technique.     

Thermally and plastically induced residual strains in textured Zircaloy-2 plate

The residual intergranular strains in textured Zircaloy-2 plate samples induced by cooling from 823 K to ambient temperatures, by cold-rolling by 1.5% and 25% and by deforming in tension by 1.5% were measured by neutron diffraction. The strong rolling texture, which gives rise to two ideal orientations, permitted the interpretation of much of the data in terms of strain tensors for the two orientations. The experimental results were compared with calculations based on the elasto-plastic self-consistent model with no adjustable parameters. Close agreement was obtained for samples in the as-cooled state and deformation in tension by 1.5% but the agreement is less satisfactory for cold-rolling.     

Pitting corrosion of the laser surface melted Alloy 600

The effect of laser surface melting (LSM) on the resistance to pitting corrosion of Ni-base Alloy 600 was investigated by a potentiodynamic polarization test in 1 M NaCl solutions at pH values of 4 and 10 at temperatures of 30, 60 and 90 °C. The pitting potentials of Alloy 600 were markedly increased by the LSM process, when compared with those of the non-laser treated Alloy 600. From the microscopic examination after the corrosion test, it was found that the pitting was initiated at the junction between a TiN inclusion and the matrix, possibly at the site of a sulfide physically associated with the TiN inclusion. The homogeneous micro-structure associated with the reduction of the inclusion size during the LSM process could be attributed to the improvement of the pitting corrosion properties in the LSM Alloy 600.     

The influence of roughness on tribological properties of nuclear grade graphite

The influence of surface roughness on tribological properties of graphite IG-11 was investigated on a standard SRV tester. The experimental condition was selected as: 30 N normal load, room temperature and a 10 Hz frequency with different strokes. The experiments environments included helium and air. Five types of roughness were studied in the experiments. The experiments revealed that the surface roughness greatly affected the graphite friction behavior. When the friction surface was smooth, the friction coefficient was high because of intensive adhesion accompanied by many pits at the friction surface. When the friction surface was rough, the adhesion was very poor, but the wear was excessive and generated many graphite particles at the friction surface. These particles can separate the friction surfaces, which reduced the friction action between them. For very rough specimens, the friction coefficient decreased with sliding velocity at about 0.004 m/s and then increases gradually.     

A model for the behavior of thorium uranium mixed oxide kernels in the pelletizing process

A behavior model of nuclear fuel kernels in the pelletizing process was developed to predict the microstructure of (Th,5%U)O₂ sintered pellets. Methods, equipments and components were developed in order to measure the density, the specific surface area and the crushing strength of the kernels and produce fuel pellets. It enables a correlation between the kernels properties and the microstructure, density and open porosity that were obtained in the fuel pellet produced with these kernels. It was possible to obtain a mathematical expression that allows one to calculate, from the kernel density and specific surface, the density that will be obtained in the fuel pellet for each compactation pressure value. The investigation showed which kernels properties are desired to obtain fuel pellets that satisfy the quality requirements for a stable performance in a power reactor. This model has been validated by experimental results and fuel pellets were obtained with an optimized microstructure that satisfies the fuel specification for an in-pile stable behavior.     

Tungsten erosion in the baffle and outboard regions of the ITER-like ASDEX Upgrade divertor

Similar to the design of the next-step device ITER, ASDEX Upgrade is equipped with vertical divertor targets with adjacent baffles extending towards the main chamber. In ITER, it is intended to employ tungsten as a plasma-facing material in this baffle area. Tungsten-coated graphite tiles were installed in the divertor baffle and the outboard side regions of ASDEX Upgrade for a full experimental campaign. The erosion behavior of tungsten was investigated by scanning electron microscopy and by measuring the thickness of the tungsten coatings before and after exposure. The coatings had an initial thickness of approximately 450 nm. Two distinct erosion mechanisms were observed: in the outer baffle region a reduction of the coatings’ thickness up to 100 nm was determined after about 6300 s of plasma discharge. On the roof baffle and on the inner baffle modules, no clear reduction of the film thickness was found. In the tracks of arcs, however, the tungsten coatings were completely removed. This represents an erosion of 5-10% of the tungsten-coated surface area in this region.     

Calculation of the threshold displacement energies in UO2 using ionic potentials

A set of ionic potentials matching exactly the crystallographic, elastic and dielectric properties of the uranium dioxide is established. It is further validated upon some basic thermodynamic properties as well as upon the Frenkel pairs formation energies and the activation energies for lattice migration in UO₂. The threshold displacement energies, useful to characterise the radiation resistance of the materials, are calculated for the uranium dioxide along various crystallographic directions applying the optimised force field within the sudden approximation approach.     

Estimation of maximum coated particle fuel compact packing fraction

This work develops an analytic fuel fraction packing model for a high temperature gas cooled reactor fuel compact fabricated from overcoated particles of a single size. The model includes the effects of one dimensional compression and finite matrix grain size. One dimensional compression limits the maximum fuel packing fraction to about 48% for the pressed compact in this single sized particle system. This limit is due to two effects. The first is that the process of die loading limits the pre-compression packing configuration to one that is stable under gravity, which is not the most space efficient one. The second effect is due to the one dimensional compression which reduces only the axial dimension of the particle lattice rather than uniformly compressing the lattice. The die wall can also limit the maximum packing fraction by preventing the nearby particles from moving into a more space efficient configuration.     

Residual stress measurements on a stress relieved Zircaloy-4 weld by neutron diffraction

The macroscopic stress distribution across an annealed Zircaloy-4 gas tungsten arc weld was measured by neutron time-of-flight diffraction at the SMARTS diffractometer at Los Alamos National Laboratory. The stresses after annealing are about 40% lower than those in the same weld prior to heat treatment. The intergranular strains in the reference coupons, which give the macroscopic stress free lattice spacings, are consistent with the difference in cooling the strongly textured plate and the weakly textured weld.     

Desorption of water adsorbed on iron oxide by laser irradiation

Desorption of water adsorbed on iron oxide by laser irradiation was studied by means of a time-of-flight (TOF) technique. The wavelength of the laser for desorption was varied from 355 to 600 nm. The energy threshold of the water desorption ranged around 2.0-2.3 eV. Based on the fact that this energy threshold approximately corresponds to the bandgap of Fe₂O₃, the initial process of water desorption is considered to be the electronic excitation of the iron oxide from the valence band to the conduction band. Analysis of the velocity distribution of the desorbed water suggests that following the electronic excitation of the iron oxide the desorption is caused by both thermal and nonthermal processes. The thermal process is caused by the rise of the surface temperature that occurs after the scattering and de-excitation of the excited electron in the iron oxide. In the case of the laser at λ = 355 nm, the desorption was mainly caused by the thermal process. On the other hand, in the case of the laser at λ = 430 nm, the desorption was mainly caused by the nonthermal process. The desorption caused by the nonthermal process is attributed to the transfer of the electron excited in the iron oxide to the adsorbed hydroxyl.     

Thermal expansion characteristics of a titanium modified austenitic stainless steel: measurement by high-temperature X-ray diffraction and modelling using Grüneisen formalism

The thermal expansion of a titanium modified, swelling resistant austenitic stainless steel designated as D9 is studied by measuring the lattice parameter as a function of temperature in the range 300-1300 K by high-temperature X-ray diffraction technique. The thermal expansion data thus obtained is in reasonable agreement with the typical thermal expansion values reported for similar nuclear grade austenitic stainless steels. However, at temperatures exceeding 900 K, the measured thermal expansivity exhibits a pronounced non-linear increase due partly to the precipitation of complex carbide and intermetallic phases. The high-temperature thermal expansion data obtained in the present study are augmented by modelling the low-temperature thermal expansion behaviour by Grüneisen formalism.