Three-dimensional Monte Carlo Code Research Articles

Studies on the impacts of the pore size and shape on deuterium retention in tungsten fuzzy nanostructures

Tritium retention in plasma-facing materials is a critical issue that can significantly impact the long-term and steady-state operation of fusion devices. The experiments conducted in the laboratory device MIES have confirmed that the presence of the tungsten (W) nanostructure (called ‘fuzz’) leads to a substantial retention of hydrogen isotopes within W fuzz layer. This observation motivates us to conduct dedicated modeling to investigate the influence of W nanostructures on deuterium (D) retention using the three-dimensional kinetic Monte Carlo code SURO-FUZZ. The SURO-FUZZ code offers a great flexibility in generating diverse microscopic structures of the W fuzzy surface through the quartet structure generation set (QSGS) approach, which allows us to explore the effects of the pore size and shape on D retention. In this study, several different W nanostructures generated by QSGS approach are utilized to conduct a comprehensive comparison between MIES experiments and SURO-FUZZ simulations. It is demonstrated that the simulated D retention can be brought into a reasonable agreement with the experimental data. On this basis, predictive estimations of D retention on EAST and ITER have been performed with SURO-FUZZ modeling. The simulation results indicate that the total D retention induced by W fuzz remains well below the administrative limit of 700 g.

Optimization analysis of nuclear thermal coupling for a small nuclear thermal propulsion (SNTP) reactor

As a main structure, tie-tube elements are used both in the traditional and modern LEU nuclear thermal propulsion reactors. This complex structure and its corresponding complex convective heat transfer process have been studied, including CFD calculations. But these studies did not effectively evaluate the applicability of the models and typical problems. In this study, a general small nuclear thermal propulsion (SNTP) reactor model is established by using three-dimensional Monte Carlo code and commercial CFD program Fluent. The applicabilities of material properties, turbulence models and grids are clarified by analyzing various influencing factors. The calculation results show that due to the small enthalpy difference and heat transfer, the convection effect of inner flow based on hydrogen can be ignored. There is no factor that can significantly reduce the maximum fuel temperature, but the porosity of the insulator can significantly improve the heat transfer ratio. In terms of thermal safety and overall performance, 2:1 support ratio is still the best choice. Based on the simulation results, the phenomenon of steep temperature rise of outer tie-tubes is found, and the SST k-omega model with Y + <3 is recommended for simulation of similar problems.

Modelling of physical sputtering properties during tungsten fuzz growth under various impact energies on NAGDIS-II

Fibre-like nanostructure named fuzz can form on tungsten (W) surface under the irradiation of helium (He) plasma. While the W fuzz growth can be suppressed by the bombardment of energetic He particles. The experiments of tungsten fuzz growth and erosion exposed by He plasma with impact energies in the range of 200 ∼ 500 eV have been conducted on the linear divertor simulator NAGDIS-II. To study W physical sputtering properties during fuzz growth on NAGDIS-II, three-dimensional kinetic Monte Carlo code SURO-FUZZ has been upgraded by including the migration of W adatoms and the deposition of sputtered W particles. By means of the renewed SURO-FUZZ, dedicated simulations of W fuzz growth and erosion under various He impact energies have been performed to reproduce the NAGDIS-II experiments. The simulated fuzz layer thicknesses under erosive regimes show a good agreement with the measurements on NAGDIS-II. The W net physical sputtering yield on W fuzzy surfaces increases with He impact energy according to SURO-FUZZ modelling. Further, the relative W net physical sputtering yields of the fuzzy surface to the smooth surface (Yfuzz/Ysmooth) have been studied under different He impact energies, which reveals that the decay trend of Yfuzz/Ysmooth is mainly dependant on the fuzz layer thickness.

Multi-physics coupling analysis on neutronics, thermal hydraulic and mechanics characteristics of a nuclear thermal propulsion reactor

As a highly developmental potential propulsion technology, nuclear thermal propulsion (NTP) technology has characteristics of high power and high specific impulse. A large number of relevant studies for NTP reactor have been carried out, but most studies only focused on the overall reactor performance parameters, ignoring the core reactivity safety and local assembly behavior safety. In this study, a cross-scale neutronics, thermal hydraulic and mechanics coupling model of the core-assembly is established by using three-dimensional Monte Carlo code and commercial CFD program to analyze three aspects: reactor control response security characteristics, heat release homogeneity of reactor and thermal hydraulic parameters. By means of value comparison of control system, three-dimensional power flattening and sensitivity analysis of single variable thermal–hydraulic parameter, the thermal hydraulic temperature safety and mechanical thermal stress safety of assemblies are significantly improved under control safety and inherent safety, which proves the importance of the neutronics, thermal hydraulic and mechanics characteristic balance.This paper can provide a theoretical basis for study on the assembly multi-physics behavior and safety characteristics of NTP reactors.

Nuclear thermal coupling optimization of baffled NTP reactor

Nuclear thermal propulsion (NTP) technology has characteristics of high power and high specific impulse. In particular, CERMET fuel reactors have received a lot of research due to simple form and structure. A general CERMET fuel reactor model is established by using three-dimensional Monte Carlo code and commercial CFD program. After nuclear thermal coupling optimization, three partition schemes of baffled reactor were proposed to reduce the temperature difference between the wall and the coolant and lower the relative spike factor in the inner region. A feasible design of redundant control system was then proposed, with a cold clean excess reactivity greater than $4.59 and a minimum dry shutdown reactivity lower than -$7.94, where there was a high control accuracy of 6.7 pcm/mm. The reactor finally realized safety of the core temperature lower than 3000 K under the conditions of total exit temperature of 2650 K and thrust of 25000 lb.

Modelling of tungsten sputtering by argon particle bombardment on a fuzzy surface

• The physical sputtering of tungsten fuzz by the Ar ions have been investigated by SURO-FUZZ. • The detailed studies of the impacts of Ar incident energy and fuzzy layer thickness on tungsten physical sputtering have been conducted in this work. • The physical sputtering yield is in reasonable agreement with measurements. The physical sputtering yield of a tungsten (W) fuzzy surface by argon (Ar) plasma bombardment was measured in the linear plasma device PISCES-B, which showed an evident reduction in the physical sputtering yield on the fuzzy surfaces in comparison with a smooth surface (Nishijima D . et al 2011 J. Nucl. Mater. 415 S96). In order to reproduce and explain this phenomenon, dedicated modelling of W physical sputtering on smooth and fuzzy surfaces by Ar bombardment has been performed with the three-dimensional kinetic Monte Carlo code SURO-FUZZ. According to a measured porosity distribution, W fuzzy surface morphology is constructed in our simulation, on which physical sputtering, trapping and escaping of W atoms under Ar bombardment are simulated with SURO-FUZZ. Detailed comparison between simulation and experiment reveals that microscopic structures of W fuzzy nanofibers play a critical role in the trapping of W atoms and hence in the resulting physical sputtering yield. For the same porosity distribution, the simulated physical sputtering yields of W fuzzy surface morphology with shallow valleys are higher than the measured values, while W fuzzy surface structure with deep and narrow slots results in a lower physical sputtering yield compared to the experimental data. The good agreement between simulation and experiment can be attained for W fuzzy surface morphology with deep and relatively open recessions.

Aerodynamic Comparison of Two Concept Capsules in Earth Reentry

Aerodynamics of two capsules of different conception both equipped with aerodynamic control surfaces has been compared at high-altitude Earth reentry path. The capsules are comparable in size but completely different in shape and, hence, in the aerodynamic behavior. The capsules are: 1) a circular cylinder provided with four flapped fins (finned cylinder), 2) a capsule, here tagged as Reentry Space Vehicle (RSV), very similar in shape and dimensions to the Intermediate eXperimental Vehicle. The study was carried out computationally in hypersonic, rarefied flow in terms of aerodynamic global coefficients (lift, drag, moments, etc.) computed by a three-dimensional direct simulation Monte Carlo code at the altitudes of 100 and 120 km, in the interval of angles of attack 0–50 deg and with flap angles of 0 and 20 deg. Computations verified that while RSV exerts a stronger aerodynamic force for the reduction of the capsule reentry velocity, the finned cylinder is better in terms of the attitude control capability. In fact, equilibrium of the finned cylinder is stable at zero angle of attack at the two altitudes, whereas the angle of attack of stable equilibrium of RSV changes with altitude. Hence, RSV needs to be provided with an active attitude control system.

Link between plasma properties with morphological, structural and mechanical properties of thin Ti films deposited by high power impulse magnetron sputtering

The main focus of this work is to correlate the basic plasma properties with morphological, structural and mechanical properties of thin films to bridge the gap between the energy source, the plasma and materials. For this purpose, the deposition and growth of thin titanium films deposited by high power impulse magnetron sputtering (HiPIMS) at various discharge power densities, from 0.17 kW/cm 2 to 3.5 kW/cm 2 were studied, both experimentally and by kinetic Monte Carlo simulation. Simulations of film growth were performed with a three-dimensional kinetic Monte Carlo code (NASCAM) with ion fraction and species energy determined experimentally by mass spectroscopy . Our approach, which is not purely empirically driven, intends to reveal some insights of the mechanisms underlying the synthesis process, which determine the intrinsic material properties. In order to link HiPIMS plasma parameters and Ti film properties, we used different techniques to analyse Ti films. TEM, X-ray diffraction and AFM were used to evaluate the structural and morphological properties of the films, and nano indention was used to evaluate their mechanical properties. We observed that the orientation of micro-crystals, which constitute the films, changes when the discharge power density increases. At the same time, we show that the films nano hardness changes non-monotonically with the increase of the discharge power density; it decreases first, then increases. The surface roughness behaviour is also non-monotonic; first increasing, then decreasing with the further increase of the discharge power density. 3D modelling helped to reveal that these non-monotonic evolutions are due to a transition between thermally-driven to ballistically-driven Ti atom mobility. • Evolution of the film properties with increase of discharge power density in HiPIMS deposition • Combined analysis of the film morphology by a range of experimental methods and simulations • A correlation between ion energy distribution functions and properties of the deposited Ti film is demonstrated. • The average energy per deposited atom is the driving force for the changes in morphology and structure of deposited films.

Modelling of hydrogen atoms reflection from an annealed tungsten fuzzy surfaces

Abstract Modelling of the hydrogen reflection on tungsten smooth and fuzz surfaces under the low-energy hydrogen plasma bombardment has been performed with the three-dimensional kinetic Monte Carlo code SURO-FUZZ. The relative reflection coefficient of hydrogen atoms, i.e. the ratio of reflected hydrogen atoms on the tungsten fuzz surface to that on the tungsten smooth one, is employed to illustrate the change in the hydrogen reflection on the tungsten fuzz and smooth surfaces. The simulated relative reflection coefficient of hydrogen atoms shows a large discrepancy with the measured values obtained in the hydrogen plasma bombardment experiment. The impinging hydrogen particles with a low incident energy result in a small sputtering and large residual nanostructure existing on the tungsten fuzz surface, which lead to a small change in the relative reflection coefficient of hydrogen atoms. The discrepancy between simulations and experiments motivates us to take the impacts of the annealing effect into account in SURO-FUZZ code. Implementation of annealing effect into SURO-FUZZ has been benchmarked against the experimental data. The simulated temporal evolution of the hydrogen reflection coefficient is in reasonable agreement with the experimental data.

ERO2.0 modelling of the effects of surface roughness on molybdenum erosion and redeposition in the PSI-2 linear plasma device

The surface morphology of plasma-facing components (PFCs) and its evolution during plasma irradiation has been shown to have a significant effect on the erosion and subsequent transport of sputtered particles in plasma. This in turn can influence the resulting lifetime of PFCs. A model for treatment of the effect of surface roughness on the erosion of PFCs has recently been incorporated into the three-dimensional Monte Carlo code ERO2.0. First simulations have confirmed a significant influence of the assumed surface roughness (for both regular and stochastic numerically constructed samples) on both the effective sputtering yields Yeff and the effective angular distributions of sputtered particles. In this study, a series of experiments at the linear plasma device PSI-2 are conducted to test the effect of surface roughness on the sputtering parameters. Graphite samples prepared with a 100 nm molybdenum layer with various surface roughness characteristic sizes (Ra = 110 nm, 280 nm, 600 nm and Ra < 20 nm) were exposed to a helium plasma in the PSI-2 linear plasma device at a magnetic field B = 0.1 T. These PSI-2 experiments were simulated using ERO2.0 with a surface morphology model. Simulations are able to reproduce the experimentally observed significant suppression of erosion for higher Ra values.

Possis: predicting spectra, light curves, and polarization for multidimensional models of supernovae and kilonovae

ABSTRACT We present possis, a time-dependent three-dimensional Monte Carlo code for modelling radiation transport in supernovae and kilonovae. The code incorporates wavelength- and time-dependent opacities, and predicts viewing-angle dependent spectra, light curves, and polarization for both idealized and hydrodynamical explosion models. We apply the code to a kilonova model with two distinct ejecta components, one including lanthanide elements with relatively high opacities and the other devoid of lanthanides and characterized by lower opacities. We find that a model with total ejecta mass $M_\mathrm{ej}=0.04\, \mathrm{M}_\odot$ and half-opening angle of the lanthanide-rich component Φ = 30° provides a good match to GW 170817/AT 2017gfo for orientations near the polar axis (i.e. for a system viewed close to face-on). We then show how crucial is the use of self-consistent multidimensional models in place of combining one-dimensional models to infer important parameters, such as the ejecta masses. We finally explore the impact of Mej and Φ on the synthetic observables and highlight how the relatively fast computation times of possis make it well-suited to perform parameter-space studies and extract key properties of supernovae and kilonovae. Spectra calculated with possis in this and future studies will be made publicly available.

Criticality Calculations and Basic Sensitivity/Uncertainty Investigation of LR-0 Benchmark Core

Three-dimensional Monte Carlo code KENO-VI of SCALE-6.2.2 code system was applied for criticality calculation of the LR-0 reactor core. A central module placed in the center of the core was filled by graphite, lithium fluoride-beryllium fluoride (FLIBE), and lithium fluoride-sodium fluoride (FLINA) compounds. The multiplication factor was obtained for all cases using both ENDF/B-VII.0 and ENDF/B-VII.1 nuclear data libraries. Obtained results were compared with benchmark calculations in the MCNP6 using ENDF/B-VII.0 library. The results of KENO-VI calculations are found to be in good agreement with results obtained by the MCNP6. The discrepancies are typically within tens of pcm excluding the case with the FLINA filling. Sensitivities and uncertainties of the reference case with no filling were determined by a continuos-energy version of the TSUNAMI sequence of SCALE-6.2.2. The obtained uncertainty in multiplication factor due to the uncertainties in nuclear data is about 650 pcm with ENDF/B-VII.1.

First three-dimensional edge plasma transport simulations with magnetic perturbations induced by lower hybrid waves on EAST

Recent experiments from the Experimental Advanced Superconducting Tokamak (EAST) show that lower hybrid waves (LHWs) can profoundly change the magnetic topology by inducing helical current filaments flowing along magnetic field lines in the scrape-off layer. Here, it is investigated for the first time how these magnetic perturbations caused by LHWs affect the edge plasma transport utilizing the three-dimensional Monte Carlo code EMC3-EIRENE, both in double-null and single-null configurations. The 3D magnetic topology structure is reflected in the plasma properties, due to much stronger parallel field transport compared with cross field diffusion. Good qualitative agreements between simulation results and experimental data from various edge diagnostics demonstrate that the EMC3-EIRENE code now is capable of taking into account the LHW-induced magnetic perturbation fields with both physical and geometrical effects being considered. Combined with experimental observations, the simulation results strongly support that total current amplitude of LHW-induced filaments increases with an increase in LHW input power. It can further deepen the penetration depth of the additional transport channel by extending the stochastic edge layer, and influence the ratio of heat (or particle) flux between split striated and original strike line on divertor targets. The 3D simulation results also indicate that the additional plasma transport channel induced by LHWs can significantly cause the redistribution of heat load between inner and outer divertor targets, which could not be found by the field line tracing method in previous works.

Modeling dark signal of CMOS image sensors irradiated by reactor neutron using Monte Carlo method

The dark signal degradation of the CMOS image sensor (CIS) was induced by neutron radiation, and it was modeled by Geant4, which is a three-dimensional Monte Carlo code. The simplified model of the CIS array was established according to the actual pixel geometry, material, and doping concentration. Nuclear elastic interaction and capture interaction were included in the physical processes, and the displacement damage dose in the space charge region of the pixel was calculated. The mean dark signal and dark signal distribution were modeled using Geant4, and the physical mechanisms were analyzed. The modeling results were in good agreement with the experimental and theoretical results.

3D modelling of tungsten fuzz growth under the bombardment of helium plasma

The fiber form nanostructure of tungsten surface called “fuzz”, which degrades the performance of tungsten material, has been observed on linear devices, tokamak and stellarator. We developed a three-dimensional (3D) kinetic Monte Carlo (KMC) code SURO-FUZZ to investigate the formation of fuzzy nanostructure in the micro- and second-scales. The time evolution of fuzz morphology under helium bombardment is studied by SURO-FUZZ. The growth rate of tungsten fuzz shows a good agreement with the experimental results.

Monte Carlo Simulation of the X-Ray Halos of GX 5−1 and GX 13+1 to Test Models of Insterstellar Dust Grains

Abstract Simulated dust-scattered X-ray halos, generated by a three-dimensional ray-tracing Monte Carlo code, are fitted to the observed halos of GX 5−1 and GX 13+1 by automatic adjustment of parameters that specify the distributions of the dust grains in size and location. Three distributions in size a are tested for the quality of fit they yield: the WD01 distribution, the BARE-GR-B distribution, and power laws of the form . The best fits are obtained with multicloud models of the spatial distributions and power-law size distributions. Correlations between the fitted fractional distances of the dust clouds and the distance estimates of molecular clouds toward GX 5−1 and GX 13+1, derived from the velocity spectra of CO and the galactic rotation curve, favor distance estimates of 4.2 and 5.6 kpc for the two sources, respectively.

DSMC analysis of bipropellant thruster plume impingement on a geostationary spacecraft

To quantify the potential contamination induced by exhaust plume from the MBB 10 N bipropellant thruster firing, we performed plume impingement analyses on the sensitive surfaces. The heat and mass fluxes of chemical species on the spacecraft surfaces and on the specific area of sensor units (optical port and radiator) were evaluated. A fully unstructured three-dimensional Direct simulation Monte Carlo (DSMC) code was developed and validated with measurements. A geostationary satellite with an optical port and radiator was selected and the actual configuration of the satellite was simplified by considering a bipropellant thruster, which is a major plume source to the sensitive surfaces, two communication antennas, and satellite panels. The DSMC computations show that the mass distribution or plume gas composition is unevenly distributed due to the mass separation effect, which occurs from the rapid expansion into vacuum especially around the nozzle lip at the backflow region. H2O is known as the most critical contaminant species; however, its plume impingement effect on the sensitive surfaces is negligible compared to the evaporation rate calculated by Langmuir’s evaporation equation. Not only have the results contributed to the geostationary satellite development, but also these findings are expected to be significant in designing spacecraft configurations and useful in the trade-off studies.

A comparison of SOLPS5.0 and 3D code EMC3-EIRENE for EAST double null configuration

The three-dimensional (3D) edge Monte Carlo code coupled with EIRENE (EMC3-EIRENE) was recently successfully implemented to the double null of EAST. The SOLPS5.0 code package has been used to investigate the validation and consistency of the 3D edge EMC3-EIRENE code on EAST. These two codes show a good agreement with each other with the average error less than 20%. However, there exist discrepancies for ne and Te profiles along target between calculations and measurements. The evaluation of kinetic corrections by heat flux limiters which is not included in EMC3-EIRENE has been presented in a low density discharge. With considering the correction of heat flux limit, the upstream ion density is strongly affected and the target parameters slightly increase and get closer to the experimental measurements. Our previous analysis of parameters sensitivity showed the most possible reason is uncertainty of the separatrix position [J. Huang et al., Plasma Phys. Control. Fusion 56(2014) 075023]. Agreement is achieved in both Te and ne at targets when the innermost separatrix shift ∼7mm inward at outer midplane for both codes.

Simulation of gross and net erosion of high-Z materials in the DIII-D divertor

The three-dimensional Monte Carlo code ERO has been used to simulate dedicated DIII-D experiments in which Mo and W samples with different sizes were exposed to controlled and well-diagnosed divertor plasma conditions to measure the gross and net erosion rates. Experimentally, the net erosion rate is significantly reduced due to the high local redeposition probability of eroded high-Z materials, which according to the modelling is mainly controlled by the electric field and plasma density within the Chodura sheath. Similar redeposition ratios were obtained from ERO modelling with three different sheath models for small angles between the magnetic field and the material surface, mainly because of their similar mean ionization lengths. The modelled redeposition ratios are close to the measured value. Decreasing the potential drop across the sheath can suppress both gross and net erosion because sputtering yield is decreased due to lower incident energy while the redeposition ratio is not reduced owing to the higher electron density in the Chodura sheath. Taking into account material mixing in the ERO surface model, the net erosion rate of high-Z materials is shown to be strongly dependent on the carbon impurity concentration in the background plasma; higher carbon concentration can suppress net erosion. The principal experimental results such as net erosion rate and profile and redeposition ratio are well reproduced by the ERO simulations.

Hydrocarbon deposition in gaps of tungsten and graphite tiles in Experimental Advanced Superconducting Tokamak edge plasma parameters

The three-dimensional (3D) Monte Carlo code PIC-EDDY has been utilized to investigate the mechanism of hydrocarbon deposition in gaps of tungsten tiles in the Experimental Advanced Superconducting Tokamak (EAST), where the sheath potential is calculated by the 2D in space and 3D in velocity particle-in-cell method. The calculated results for graphite tiles using the same method are also presented for comparison. Calculation results show that the amount of carbon deposited in the gaps of carbon tiles is three times larger than that in the gaps of tungsten tiles when the carbon particles from re-erosion on the top surface of monoblocks are taken into account. However, the deposition amount is found to be larger in the gaps of tungsten tiles at the same CH4 flux. When chemical sputtering becomes significant as carbon coverage on tungsten increases with exposure time, the deposition inside the gaps of tungsten tiles would be considerable.