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At least 955 records · Page 53
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

The Virtual Test Bed (VTB) repository: a library of multiphysics reference reactor models using NEAMS tools

With the next generation of nuclear reactors under development, modeling and simulation (MS) tools are being developed by the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program in order to support their design, licensing, and future operation. Mirroring the physical test beds currently under construction (i.e., EBR-II and ZPPR), the Virtual Test Bed (VTB) was launched by the National Reactor Innovation Center (NRIC) in collaboration with NEAMS to support the advanced reactor community. This collaborative effort, which involves multiple teams at both Idaho National Laboratory and Argonne National Laboratory aims to use NEAMS tools to model a wide range of reactor designs. Those models are automatically tested to ensure their continued functionality as the tools are further developed. Examples are extensively documented, each acting as a tutorial for applying the relevant NEAMS tools to that reactor design. Currently, five advanced reactor types (with a total of eight specific design variants) are simulated by a variety of different models. These models range from steady-state, core multiphysics simulations to integrated plant analysis during loss-of flow transients. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2016

Magnetic field line random walk in isotropic turbulence with varying mean field

In astrophysical plasmas, the magnetic field line random walk (FLRW) plays an important role in guiding particle transport. The FLRW behavior is scaled by the Kubo number R=(b/B{sub 0})(ℓ{sub ∥}/ℓ{sub ⊥}) for rms magnetic fluctuation b, large-scale mean field B{sub 0}, and coherence scales parallel (ℓ{sub ∥}) and perpendicular (ℓ{sub ⊥}) to B{sub 0}. Here we use a nonperturbative analytic framework based on Corrsin’s hypothesis, together with direct computer simulations, to examine the R-scaling of the FLRW for varying B {sub 0} with finite b and isotropic fluctuations with ℓ{sub ∥}/ℓ{sub ⊥}=1, instead of the well-studied route of varying ℓ{sub ∥}/ℓ{sub ⊥} for b ≪B{sub 0}. The FLRW for isotropic magnetic fluctuations is also of astrophysical interest regarding transport processes in the interstellar medium. With a mean field, fluctuations may have variance anisotropy, so we consider limiting cases of isotropic variance and transverse variance (with b {sub z} = 0). We obtain analytic theories, and closed-form solutions for extreme cases. Padé approximants are provided to interpolate all versions of theory and simulations to any B {sub 0}. We demonstrate that, for isotropic turbulence, Corrsin-based theories generally work well, and with increasing R there is a transition from quasilinear to Bohm diffusion. This holds even with b {sub z} = 0, when different routes to R→∞ are mathematically equivalent; in contrast with previous studies, we find that a Corrsin-based theory with random ballistic decorrelation works well even up to R = 400, where the effects of trapping are barely perceptible in simulation results.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Revisiting the Bulge–Halo Conspiracy. I. Dependence on Galaxy Properties and Halo Mass

We carry out a systematic investigation of the total mass density profile of massive (log M{sub star}/M{sub ⊙}≳11.3) early-type galaxies and its dependence on galactic properties and host halo mass with the aid of a variety of lensing/dynamical data and large mock galaxy catalogs. The latter are produced via semi-empirical models that, by design, are based on just a few basic input assumptions. Galaxies with measured stellar masses, effective radii, and Sérsic indices, are assigned, via abundance matching relations, host dark matter halos characterized by a typical ΛCDM profile. Our main results are as follows. (1) In line with observational evidence, our semi-empirical models naturally predict that the total, mass-weighted density slope at the effective radius γ′ is not universal, steepening for more compact and/or massive galaxies, but flattening with increasing host halo mass. (2) Models characterized by a Salpeter or variable initial mass function (IMF) and uncontracted dark matter profiles are in good agreement with the data, while a Chabrier IMF and/or adiabatic contractions/expansions of the dark matter halos are highly disfavored. (3) Currently available data on the mass density profiles of very massive galaxies (log M{sub star}/M{sub ⊙}≳12), with M{sub halo}≳3×10{sup 14} M{sub ⊙}, favor instead models with a stellar profile flatter than a Sérsic one in the very inner regions (r ≲ 3–5 kpc), and a cored NFW or Einasto dark matter profile with median halo concentration a factor of ∼2 or ≲1.3, respectively, higher than those typically predicted by N-body numerical simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Observational Signatures of a Kink-unstable Coronal Flux Rope Using Hinode /EIS

The signatures of energy release and energy transport for a kink-unstable coronal flux rope are investigated via forward modeling. Synthetic intensity and Doppler maps are generated from a 3D numerical simulation. The CHIANTI database is used to compute intensities for three Hinode /EIS emission lines that cover the thermal range of the loop. The intensities and Doppler velocities at simulation-resolution are spatially degraded to the Hinode /EIS pixel size (1″), convolved using a Gaussian point-spread function (3″), and exposed for a characteristic time of 50 s. The synthetic images generated for rasters (moving slit) and sit-and-stare (stationary slit) are analyzed to find the signatures of the twisted flux and the associated instability. We find that there are several qualities of a kink-unstable coronal flux rope that can be detected observationally using Hinode /EIS, namely the growth of the loop radius, the increase in intensity toward the radial edge of the loop, and the Doppler velocity following an internal twisted magnetic field line. However, EIS cannot resolve the small, transient features present in the simulation, such as sites of small-scale reconnection (e.g., nanoflares).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

A Dissipation-Based Method for Improving the Accuracy of Computational Fluid Dynamics Simulations of High Level Non-Newtonian Waste - 17351

Analysis of shear rate in dissipative scales of turbulence was performed using a quasi-direct numerical simulation (Q-DNS) of flow through a pipe involving Bingham fluid. Dependency of deformation rate on dissipative scales of turbulence was studied and a modification algorithm was developed and implemented in Reynolds-averaged Navier-Stokes (RANS) modeling of the same flow using the standard k-e model. It was shown that the modification was effective in increasing accuracy of the RANS simulations when the velocity profile was modified to match experimental data. In both the Q-DNS and RANS simulations, the Herschel-Bulkley (HB) rheology equation was used to relate shear stress of the non-Newtonian fluid to shear deformation. Additional analysis of dependency of shear on the turbulent dissipation rate (TDR) was performed using various flow conditions in RANS simulations with a modified viscosity model (alpha model). In the past, the alpha model has been shown to produce accurate simulations for the same flow conditions. Results of the analysis shows that a qualitative agreement exists between shear dependency in the RANS-alpha and Q-DNS-HB simulations when the ratio of velocity to length scale is used. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2017

Advancement of Functional Expansion Tallies Capabilities in Serpent 2

The need for accident tolerant fuels (ATFs) became self-evident after the series of unfortunate nuclear events at Fukushima Daiichi in 2011. Restarting the Transient Reactor Test Facility (TREAT) at INL is one component in this concerted effort to establish the ATF development and testing program under the United States Department of Energy. Prior to the closure of TREAT in 1994, each experiment required multiple low-power calibration tests before and after each run. Since then, computing advancements have set computer-based modeling and simulation (M and S) as the preferred calibration method. The anticipated savings are both temporal and monetary. MAMMOTH, a collection of M and S tools built from the finite element-based MOOSE Framework, has been established to support the designed experiments intended for TREAT. This collection of tools includes: 1) Rattlesnake, a striking diffusion-based neutronics code, 2) BISON, an advanced materials and fuels performance code, and 3) RELAP-7, a powerful thermal-hydraulics solver. Each is capable of independently performing its function accurately and effectively; unified, they create a powerful multiphysics reactor core M and S application. One unfortunate weakness in this configuration lies in the unique geometry and design of TREAT's core. Although simple to model, this poses a challenge to the diffusion solver in Rattlesnake. One alternative is to replace Rattlesnake with Serpent, a fully-fledged Monte Carlo (MC) neutronics code. Coincidentally, Rattlesnake calculations are frequently validated against Serpent simulations.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Development and Test of a New Verification Scheme for Transient Core Simulators

Transient calculations in commercial nuclear reactors are performed while typically relying on a time-dependent neutron transport solver or a low-order solver (i.e. diffusion). In order to be licensed, the codes used by the industry need to go through a process of verification and validation, with the verification carried out by comparing the results of simulations to analytical or semi-analytical solutions. Such analytical or semi-analytical solutions can only be obtained if the system to be modelled during the verification process is either fully homogeneous or piecewise homogeneous. This paper reports on the development of a different verification approach that can be applied to fully heterogeneous systems. It relies on the extraction of the point-kinetic response of the reactor (which can be estimated from the results of core simulations) and on its subsequent comparison with its expected analytical form.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2017

Molten Salt Reactor Simulations using MPACT-CTF

The US Department of Energy and industry have shown significant interest in advanced reactors, as evidenced by over 1.3 billion dollars of private investments in companies to develop advanced reactor concepts. More than seven of these advanced reactor concepts use molten salt reactor (MSR) technology. Based on this interest in MSR technology, ORNL has invested in the development of the MSR simulation capability, extending some capabilities in VERA to model this high-profile reactor concept. In this paper, the MPACT deterministic neutron transport capability and the COBRA Two-Fluid (CTF) subchannel capability are adapted to handle liquid fuel MSRs. Specifically, this work focuses on modifications to the depletion algorithms to account for isotopic mixing, continuous feed, and continuous removal. Models will be added to account for drift of precursors and decay heat emitters as the liquid fuel flows through the reactor coolant system. Modifications to CTF, including the addition of molten salt properties, are also addressed. Finally, the coupled MPACT-CTF capability is used to simulate the 30-year operation of an MSR.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2014

A Grid Intensity Map-Based Source Model for Monte Carlo Modeling of IMRT Field Irradiation - Paper 5

An accurate source model is important for TPS (Treatment Planning System) dose calculation and verification, especially for IMRT (Intensity Modulated Radiotherapy) irregular and inhomogeneous fields. Monte Carlo simulation is the most accurate way to calculate dose distributions in the heterogeneous media like human bodies. A full radiotherapy modeling generally includes Linac modeling and phantom dose calculation. Full Linac modeling is very time-consuming and requires highly detailed knowledge of the accelerator composition and structure, however, which is very difficult to obtain for commercial interests. Often all relevant parameters are not known with sufficient accuracy, and thus a time-consuming tuning of e.g. the electron beam spot size and energy must be performed. A multiple source model (MSM) regards particles coming from different parts of an accelerator as if they are coming from different virtual sources. MSM uses a set of virtual sources representing the treatment beam (e.g. primary photons, head scatter, electron contamination) by a set of analytic functions or histograms with parameters derived from dosimetric measurements or a full MC simulation. Tillikainen et al built a three sub-source model by matching with measurement data using an automatic process to optimize the free model parameters for a Varian 2100 C/D accelerator. Lin et al have built a convenient five-photon-sub-source model for Varian 2300C 6 MV-X beam based on Monte Carlo simulated Linac for ARTS (Accurate Radiotherapy System). The particle generation of MSM is very fast compared to full Linac simulation, and its sampling efficiency is not limited by the size of the phase space file (PSF). However, generally MSM is generated based on regular field PSF and needs an irregular component to form the beam shape meanwhile apt to bringing error Linac modeling and MSM producing from PSF analysis actually both are multi-freedom optimization process by adjusting several parameters (e.g. electron energy and angular divergence). Linac modeling adjusts its parameters from the beginning of electron hitting target through all components to fit the simulation to the measurement. MSM tunes its PSF-derived parameters by optimizing to minimize the deviations between the simulation and the measurement. Since the full Linac modeling and the MSM model both need to fine tune their parameters to match the simulation to the measurement, and sometimes maybe offsets the real accelerator for its multi-parameter optimization process. So, why not fine tune directly all source model parameters just behind all Linac components to fit the simulation to the measurement? A grid intensity-based surface source model was suggested here, which assumed the particle emitting from the bottom surface of Linac last beam trimmer MLC (Multi Leaf Collimator). The particle emitting position is sampled on the grid intensity map optimized by TPS, and the particle weight is adjusted by its original grid intensity. This source model not only takes advantage of the Monte Carlo method in the heterogeneous media, but also avoids effectively the transportation speed shortcoming of Monte Carlo simulation through Linac components. (authors)

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2022

Simulation of Peach Bottom turbine trip transients TT1, TT2 and TT3 with SIMULATE-3K

The Peach Bottom 2 Turbine Trip transients tests TT1, TT2 and TT3 are simulated with SIMULATE-3K. This work provides the initial basis for evaluation of the capabilities and accuracy of coupled 3D neutronics/thermal hydraulics best estimate calculations for support of safety analysis. Selected results, obtained with SIMULATE-3K, are compared to available measurements and benchmark results. The comparisons are analyzed and the application of SIMULATE-3K for best estimate analysis approach within the safety analysis methodology is discussed. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2018

Adaptive Sampling of Large Deviations

We introduce and test an algorithm that adaptively estimates large deviation functions characterizing the fluctuations of additive functionals of Markov processes in the long-time limit. These functions play an important role for predicting the probability and pathways of rare events in stochastic processes, as well as for understanding the physics of nonequilibrium systems driven in steady states by external forces and reservoirs. The algorithm uses methods from risk-sensitive and feedback control to estimate from a single trajectory a new process, called the driven process, known to be efficient for importance sampling. Its advantages compared to other simulation techniques, such as splitting or cloning, are discussed and illustrated with simple equilibrium and nonequilibrium diffusion models.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Numerical simulations of chaotic and complex spatiotemporal patterns in fractional reaction–diffusion systems

The generalized fractional reaction–diffusion equations which exist in the form of noninteger order partial differential equations have now found wide application for illustrating important and useful physical phenomena, such as subdiffusive and superdiffusive scenarios. The space fractional derivatives are defined in the Riesz sense on the intervals 0 < α < 1 and 1 < α ≤ 2. We propose robust numerical techniques, such as a spectral representation of the fractional Laplacian operator in conjunction with the exponential time differencing method, and present the equivalent relationship between the Riesz fractional derivative and fractional Laplacian operator. We apply these techniques to numerically solve a range of chaotic processes, such as the Chua’s equations, Rössler system, Lorenz and Lorenz-type systems. Simulation results revealed various complex and spatiotemporal chaos, spiral chaos, intermittent chaos and spots patterns in two-dimensional space fractional reaction–diffusion problems.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2018

New approach for stability and bifurcation analysis on predator–prey harvesting model for interval biological parameters with time delays

This paper presents a prey–predator harvesting model with time delay for bifurcation analysis. We consider the parameters of the proposed model with imprecise data as form of interval in nature, due to the lack of precise numerical information of the biological parameters such as prey population growth rate and predator population decay rate. The interaction between prey and predator is assumed to be governed by a Holling type II functional response and discrete type gestation delay of the predator for consumption of the prey under impreciseness of the biological parameters. Parametric functional form of interval number with two parameters is introduced. This study reveals that not only delay and harvesting effort play a significant role on the stability on the system but also interval parameters play a crucial role on the stability of the system. Computer simulations of numerical examples are given to explain our proposed imprecise model. We also address critically the biological implications of our analytical findings with proper numerical example.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2021

A dissolution model of alite coupling surface topography and ions transport under different hydrodynamics conditions at microscale

Portland cement is the most produced material in the world. The hydration process of cement consists of a group of complex chemical reactions. In order to investigate the mechanism of cement hydration, it is vital to study the hydration of each phase separately. An integrated model is proposed in this paper to simulate the dissolution of alite under different hydrodynamic conditions at microscale, coupling Kinetic Monte Carlo model (KMC), Lattice Boltzmann method (LBM) and diffusion boundary layer (DBL). The dissolution of alite is initialised with KMC. Two Multiple-relaxation-time (MRT) LB models are used to simulate the fluid flow and transport of ions, respectively. For solid-liquid interface, DBL is adapted to calculate the concentration gradient and dissolution flux. The model is validated with experiment from literature. The simulation results show good agreements with the results published in the literature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Lattice Monte Carlo simulation of Galilei variant anomalous diffusion

The observation of an increasing number of anomalous diffusion phenomena motivates the study to reveal the actual reason for such stochastic processes. When it is difficult to get analytical solutions or necessary to track the trajectory of particles, lattice Monte Carlo (LMC) simulation has been shown to be particularly useful. To develop such an LMC simulation algorithm for the Galilei variant anomalous diffusion, we derive explicit solutions for the conditional and unconditional first passage time (FPT) distributions with double absorbing barriers. According to the theory of random walks on lattices and the FPT distributions, we propose an LMC simulation algorithm and prove that such LMC simulation can reproduce both the mean and the mean square displacement exactly in the long-time limit. However, the error introduced in the second moment of the displacement diverges according to a power law as the simulation time progresses. We give an explicit criterion for choosing a small enough lattice step to limit the error within the specified tolerance. We further validate the LMC simulation algorithm and confirm the theoretical error analysis through numerical simulations. The numerical results agree with our theoretical predictions very well.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Comparison of three-dimensional morphologies of abnormally growing grains between Monte Carlo simulations and experiments of Fe-3% Si steel

Highlights: • Grains with sub-boundaries grew abnormally in the MC simulation with realistic grain boundary energy. • The experimental 3-D morphology of Goss grain was reconstructed from serially sectioned images. • 2-D sectioned microstructures of the MC simulation and experiment have highly irregular shape. • Both simulated and experimental 3-D morphologies of AGG have similar complicated shapes. - Abstract: Three-dimensional (3-D) Monte Carlo simulations of grain growth with sub-boundaries in the presence of precipitates were performed based on the distribution of realistic grain boundary energy data. The simulated grains with sub-boundaries grew abnormally. The simulated and experimental morphologies of the abnormally growing grains in Fe-3%Si steel were compared. The experimental 3-D morphology was reconstructed from serially sectioned images. Both the simulated and experimental 3-D morphologies of abnormally growing grains have highly irregular and complicated shapes whereas both the simulated and experimental 3-D morphologies of normally growing grains have relatively regular and simple shapes. Growth by wetting is responsible for the highly irregular shape of the experimental abnormally growing grains of Fe-3%Si steel as well as for the highly irregular shape of the simulated abnormally growing grains.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Monte Carlo study for optimal conditions in single-shot imaging with femtosecond x-ray laser pulses

Intense x-ray pulses from x-ray free electron lasers (XFELs) enable the unveiling of atomic structure in material and biological specimens via ultrafast single-shot exposures. As the radiation is intense enough to destroy the sample, a new sample must be provided for each x-ray pulse. These single-particle delivery schemes require careful optimization, though systematic study to find such optimal conditions is still lacking. We have investigated two major single-particle delivery methods: particle injection as flying objects and membrane-mount as fixed targets. The optimal experimental parameters were searched for via Monte Carlo simulations to discover that the maximum single-particle hit rate achievable is close to 40%.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗