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Journal Articles

Quantitative visualization of a radioactive plume with harmonizing gamma-ray imaging spectrometry and real-time atmospheric dispersion simulation based on 3D wind observation

Nagai, Haruyasu; Furuta, Yoshihiro*; Nakayama, Hiromasa; Satoh, Daiki

Journal of Nuclear Science and Technology, 60(11), p.1345 - 1360, 2023/11

 Times Cited Count:0 Percentile:0.01(Nuclear Science & Technology)

A novel monitoring method for the quantitative visualization of 3D distribution of a radioactive plume and source term estimation of released radionuclides is proposed and its feasibility is demonstrated by preliminary test. The proposed method is the combination of gamma-ray imaging spectroscopy with the Electron Tracking Compton Camera (ETCC) and real-time high-resolution atmospheric dispersion simulation based on 3D wind observation with Doppler lidar. The 3D distribution of a specific radionuclide in a target radioactive plume is inversely reconstructed from line gamma-ray images from each radionuclide by several ETCCs located around the target by harmonizing with the air concentration distribution pattern of the plume predicted by real-time atmospheric dispersion simulation. A prototype of the analysis method was developed, showing a sufficient performance in several test cases using hypothetical data generated by numerical simulations of atmospheric dispersion and radiation transport.

Journal Articles

Front line of countermeasures against radiation terrorism in urban area, 2; Development of LHADDAS and its application to countermeasures for radiological terrorism

Nakayama, Hiromasa; Satoh, Daiki

Nihon Genshiryoku Gakkai-Shi ATOMO$$Sigma$$, 65(10), p.621 - 624, 2023/10

We developed a local-scale high-resolution atmospheric dispersion and dose assessment system (LHADDAS) for safety and consequence assessment of nuclear facilities and emergency response to nuclear accidents or deliberate releases of radioactive materials in built-up urban areas. This system comprises three parts: preprocessing of input files, and main calculation by a local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES) and a real-time urban dispersion simulation model based on a lattice Boltzmann method (CityLBM), and postprocessing of dose calculation by a simulation code powered by lattice dose-response functions (SIBYL). We introduce front line of countermeasures against radiation terrorism in urban area as an example.

Journal Articles

Local-scale high-resolution atmospheric dispersion and dose assessment system; Realization for the first time of dose assessment based on detailed calculation of radioactive material dispersion taking buildings into account

Nakayama, Hiromasa; Onodera, Naoyuki; Satoh, Daiki

Isotope News, (785), p.20 - 23, 2023/02

We developed the local-scale high-resolution atmospheric dispersion and dose assessment system (LHADDAS) for safety and consequence assessment of nuclear facilities and emergency response to nuclear accidents or deliberate releases of radioactive materials in built-up urban areas. This system comprises three parts, namely, preprocessing of input files, main calculation by a local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES) or a real-time urban dispersion simulation model based on a lattice Boltzmann method (CityLBM), and postprocessing of dose calculation by a simulation code powered by lattice dose-response functions (SIBYL). LHADDAS has a broad utility and performs excellently in (1) simulating turbulent flows, plume dispersion, and dry deposition under realistic meteorological conditions; (2) simulating real-time tracer dispersion using a locally mesh-refined lattice Boltzmann method; and (3) estimating the air dose rates of radionuclides from air concentrations and surface deposition in consideration of the influence of individual buildings and structures. This system is promising for use in safety assessment of nuclear facilities (as an alternative to wind tunnel experiments), detailed pre/post-analyses of local-scale radioactive plume dispersion in case of nuclear accidents, and quick response to emergency situations resulting from deliberate release of radioactive materials by terrorist attacks in central urban district areas.

Journal Articles

Parameter optimization for turbulent boundary layer generation using ensemble Kalman filter

Onodera, Naoyuki; Idomura, Yasuhiro; Hasegawa, Yuta; Nakayama, Hiromasa

Dai-36-Kai Suchi Ryutai Rikigaku Shimpojiumu Koen Rombunshu (Internet), 3 Pages, 2022/12

We have developed a wind simulation code named CityLBM to realize wind digital twins. Mesoscale wind conditions are given as boundary conditions in CityLBM by using a nudging data assimilation method. It is found that conventional approaches with constant nudging coefficients fail to reproduce turbulent intensity in long time simulations, where atmospheric stability conditions change significantly. We propose a dynamic parameter optimization method for the nudging coefficient based on an ensemble Kalman filter. CityLBM was validated against plume dispersion experiments in the complex urban environment of Oklahoma City. The nudging coefficient was updated to reduce the error of the turbulent intensity between the simulation and the observation. The mean error of velocity variance is reduced by $$sim$$10% compared to the conventional nudging method with a constant nudging coefficient.

Journal Articles

Development of local-scale high-resolution atmospheric dispersion and dose assessment system

Nakayama, Hiromasa; Onodera, Naoyuki; Satoh, Daiki; Nagai, Haruyasu; Hasegawa, Yuta; Idomura, Yasuhiro

Journal of Nuclear Science and Technology, 59(10), p.1314 - 1329, 2022/10

 Times Cited Count:5 Percentile:86.35(Nuclear Science & Technology)

We developed a local-scale high-resolution atmospheric dispersion and dose assessment system (LHADDAS) for safety and consequence assessment of nuclear facilities and emergency response to nuclear accidents or deliberate releases of radioactive materials in built-up urban areas. This system is composed of pre-processing of input files, main calculation by local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES) and real-time urban dispersion simulation model based on a lattice Boltzmann method (CityLBM), and post-processing of dose-calculation by simulation code powered by lattice dose-response functions (SIBYL). LHADDAS has a broad utility and offers superior performance in (1) simulating turbulent flows, plume dispersion, and dry deposition under realistic meteorological conditions, (2) performing real-time tracer dispersion simulations using a locally mesh-refined lattice Boltzmann method, and (3) estimating air dose rates of radionuclides from air concentrations and surface deposition in consideration of the influence of individual buildings and structures. This system is promising for safety assessment of nuclear facilities as an alternative to wind tunnel experiments, detailed pre/post-analyses of a local-scale radioactive plume dispersion in case of nuclear accidents, and quick response to emergency situations resulting from deliberate release of radioactive materials by a terrorist attack in an urban central district area.

Journal Articles

Development of a surface heat flux model for urban wind simulation using locally mesh-refined lattice Boltzmann method

Onodera, Naoyuki; Idomura, Yasuhiro; Hasegawa, Yuta; Nakayama, Hiromasa

Dai-35-Kai Suchi Ryutai Rikigaku Shimpojiumu Koen Rombunshu (Internet), 3 Pages, 2021/12

A detailed wind simulation is very important for designing smart cities. Since a lot of tall buildings and complex structures make the air flow turbulent in urban cities, large-scale CFD simulations are needed. We develop a GPU-based CFD code based on a Lattice Boltzmann Method (LBM) with a block-based Adaptive Mesh Refinement (AMR) method. In order to reproduce real wind conditions, the wind condition and ground temperature of the mesoscale weather forecasting model are given as boundary conditions. In this research, a surface heat flux model based on the Monin-Obukhov similarity theory was introduced to improve the calculation accuracy. We conducted a detailed wind simulation in Oklahoma City. By executing this computation, wind conditions in the urban area were reproduced with good accuracy.

Journal Articles

Development of local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation, 6; Introduction of detailed dose calculation method

Nakayama, Hiromasa; Satoh, Daiki; Nagai, Haruyasu; Terada, Hiroaki

Journal of Nuclear Science and Technology, 58(9), p.949 - 969, 2021/09

 Times Cited Count:7 Percentile:78.4(Nuclear Science & Technology)

We introduced a detailed dose calculation method considering building shielding effects into LOcal-scale High-resolution atmospheric DIspersion Model using LES (LOHDIM-LES). To estimate quickly and accurately dose distributions considering shielding effects of buildings, we employed the calculation method using dose-response matrices which were evaluated by photon transport simulations with Particle and Heavy-Ion Transport code System (PHITS). Compared to the air dose rate data obtained from monitoring posts in an actual nuclear facility, it was shown that the calculated dose rate is reasonably simulated well. It is concluded that LOHDIM-LES equipped with the calculation method using dose-response matrices can reasonably estimate the air dose rates considering shielding effects of individual buildings and structures.

Journal Articles

Large-eddy simulation of plume dispersion in the central district of Oklahoma City by coupling with a mesoscale meteorological simulation model and observation

Nakayama, Hiromasa; Takemi, Tetsuya*; Yoshida, Toshiya

Atmosphere (Internet), 12(7), p.889_1 - 889_15, 2021/07

 Times Cited Count:2 Percentile:15.85(Environmental Sciences)

Contaminant gas dispersion within urban area resulting from accidental or intentional release is of great concern to public health and social security. When estimating plume dispersion in a built-up urban area under real meteorological conditions by computational fluid dynamics (CFD), a crucial issue is how to prescribe the model input conditions. There are typically two approaches: using the outputs of a meso-scale meteorological simulation (MMS) model and meteorological observations (OBS). However, the influence of the different approaches on the simulation results have not been fully demonstrated. In this study, we conducted large-eddy simulations (LESs) of plume dispersion in the urban central district under real meteorological conditions by coupling with a MMS model and OBS obtained at a single stationary point, and evaluated the two different coupling simulations in comparison with the field experimental data. The LES-MMS coupling showed better performance than the LES-OBS one. However, the latter one also showed a reasonable performance comparable to the acceptance criteria on the model prediction within a factor of two of the experimental data. These facts indicate that the approach of using observations at a single stationary point still has enough potential to drive CFD models for plume dispersion under real meteorological conditions.

Journal Articles

Toward development of a framework for prediction system of local-scale atmospheric dispersion based on a coupling of LES-database and on-site meteorological observation

Nakayama, Hiromasa; Yoshida, Toshiya; Terada, Hiroaki; Kadowaki, Masanao

Atmosphere (Internet), 12(7), p.899_1 - 899_16, 2021/07

 Times Cited Count:1 Percentile:6.95(Environmental Sciences)

In this study, first, we conducted meteorological observations by a Doppler LiDAR and simple plume release experiments by a mist spraying system at the site of Japan Atomic Energy Agency. Then, we developed a framework for prediction system of local-scale atmospheric dispersion based on a coupling of large-eddy simulation (LES) database and on-site meteorological observation. The LES-database was also created by pre-calculating high-resolution turbulent flows in the target site at mean wind directions of class interval. We provided the meteorological observed data with the LES database in consideration of building conditions and calculated three-dimensional distribution of the plume by a Lagrangian dispersion model. Compared to the instantaneous shot of the plume taken by a digital camera, it was shown that the mist plume transport direction was accurately simulated. It was concluded that our proposed framework for prediction system based on a coupling of LES-database and on-site meteorological observation is effective.

Journal Articles

Real-time tracer dispersion simulations in Oklahoma City using the locally mesh-refined lattice Boltzmann method

Onodera, Naoyuki; Idomura, Yasuhiro; Hasegawa, Yuta; Nakayama, Hiromasa; Shimokawabe, Takashi*; Aoki, Takayuki*

Boundary-Layer Meteorology, 179(2), p.187 - 208, 2021/05

 Times Cited Count:9 Percentile:76.92(Meteorology & Atmospheric Sciences)

A plume dispersion simulation code named CityLBM enables a real time simulation for several km by applying adaptive mesh refinement (AMR) method on GPU supercomputers. We assess plume dispersion problems in the complex urban environment of Oklahoma City (JU2003). Realistic mesoscale wind boundary conditions of JU2003 produced by a Weather Research and Forecasting Model (WRF), building structures, and a plant canopy model are introduced to CityLBM. Ensemble calculations are performed to reduce turbulence uncertainties. The statistics of the plume dispersion field, mean and max concentrations show that ensemble calculations improve the accuracy of the estimation, and the ensemble-averaged concentration values in the simulations over 4 km areas with 2-m resolution satisfied factor 2 agreements for 70% of 24 target measurement points and periods in JU2003.

Journal Articles

Simulation code for estimating external gamma-ray doses from a radioactive plume and contaminated ground using a local-scale atmospheric dispersion model

Satoh, Daiki; Nakayama, Hiromasa; Furuta, Takuya; Yoshihiro, Tamotsu*; Sakamoto, Kensaku

PLOS ONE (Internet), 16(1), p.e0245932_1 - e0245932_26, 2021/01

 Times Cited Count:2 Percentile:32.89(Multidisciplinary Sciences)

In this study, we developed a simulation code named SIBYL, which estimates external gamma-ray doses at ground level from radionuclides distributed nonuniformly in atmosphere and on ground. SIBYL can combine with the local-scale atmospheric dispersion model LOHDIM-LES, and calculate the dose distributions according to the map of the activity concentrations simulated by LOHDIM-LES. To apply the SIBYL code to emergency responses of nuclear accidents, the time-consuming three-dimensional radiation transport simulations were performed in advance using the general-purpose Monte Carlo code PHITS, and then the results were compiled to the database for the SIBYL's dose calculations. Moreover, SIBYL can consider the dose attenuation by obstacles and the changes of terrain elevations. To examine the accuracy of SIBYL, typical five cases including $$^{85}$$Kr emission from a ventilation shaft and $$^{137}$$Cs dispersion inside urban area were investigated. The results of SIBYL agreed within 10% with those of PHITS at the most of target locations. Furthermore, the calculation speed was approximately 100 times faster than that of PHITS.

Journal Articles

Development of a data assimilation method using vibration equation for Large-Eddy Simulations of turbulent boundary layer flows

Nakayama, Hiromasa; Takemi, Tetsuya*

Journal of Advances in Modeling Earth Systems (Internet), 12(8), p.e2019MS001872_1 - e2019MS001872_18, 2020/08

 Times Cited Count:1 Percentile:10.2(Meteorology & Atmospheric Sciences)

We proposed a data assimilation method using a vibration equation which can incorporate turbulence winds toward target mean winds while maintaining small-scale turbulent fluctuations. First, we conducted test simulations in which nudging is applied in a basic turbulent boundary layer (TBL) flow toward a target one. It was shown that the basic TBL flow can be reasonably nudged toward the target one while maintaining the turbulent fluctuations well, when prescribing the natural frequency in the vibration equation smaller than the spectral peak frequency in the TBL flow. Then, we applied the proposed nudging method by incorporating data obtained from meteorological observations located in the southern part of the actual city of Kyoto. The mean wind velocity profiles are reasonably nudged toward the target observed profile and the turbulence statistics are also favourably maintained. It is concluded that the LES data assimilation method successfully nudges toward the turbulence winds well.

Journal Articles

Development of a combined LES/RANS model to predict atmospheric dispersion over urban areas

Yoshida, Toshiya; Nakayama, Hiromasa

Nihon Keisan Kogakkai Rombunshu (Internet), 2020, p.20200013_1 - 20200013_9, 2020/07

To quickly and accurately predict the dispersion of hazardous materials released over urban areas, we propose a combined method in which dispersion fields are simulated using a Reynolds-averaged Navier-Stokes model with pre-calculated flow fields from a large-eddy simulation (LES) model. First, the combined model is conducted for dispersion in a simple street canyon. The results of the combined model are compared with those of a wind-tunnel experiment to adjust empirical parameters in the turbulent scalar flux. The horizontal dispersion fields predicted in the combined model with the best parameters are well consistent with those calculated from our LES model. We then apply the combined model to predict the scalar dispersion over a real urban area. The combined model well predicts the results obtained from the LES model with less calculation time. Therefore, we find that the combined model is potentially effective for emergency response to hazardous-material release over urban areas.

Journal Articles

Development of the data assimilation method suitable for large-eddy simulation model using the vibration equation

Nakayama, Hiromasa; Takemi, Tetsuya*

Proceedings of 19th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes (HARMO-19) (USB Flash Drive), 5 Pages, 2019/06

We proposed the data assimilation method using the vibration equation suitable for LES models. First, we generated the basic turbulent boundary layer (TBL) flow with a power law exponent of 0.14 at the upstream area and then attempted to nudge toward the TBL flow with a power law exponent of 0.25 by our proposed data assimilation. For a case that the natural frequency set in the vibration equation was larger than the peak frequency of the basic TBL flow, the turbulent fluctuations were rapidly damped although the mean velocity profile matched with the target one. On the other hand, for a case that the natural frequency was smaller than the peak frequency, the mean wind velocity profile was reasonably nudged toward the target profile while maintaining the turbulent fluctuations. It is concluded that our proposed data assimilation method using the vibration equation successfully nudges toward the target mean winds with small-scale turbulent fluctuations.

Journal Articles

Large-eddy simulation studies for predicting plume concentrations around nuclear facilities using an overlapping technique

Nakayama, Hiromasa; Takemi, Tetsuya*

International Journal of Environment and Pollution, 64(1/3), p.125 - 144, 2018/00

We have attempted to develop a practical and quick local-scale atmospheric dispersion calculation method using an overlapping technique for plume concentration distributions in an emergency response to nuclear accidents. In order to evaluate the overlapping approach, we performed LESs of turbulent flows and plume dispersion under varying mean wind directions using the meteorological data as the model input. When compared with the simulated results under the real meteorological condition, it is shown that the concentration distribution patterns are reasonably simulated by the overlapping method. It can be concluded that the atmospheric dispersion calculation method using the overlapping technique has potential performance for emergency responses to nuclear accidents.

Journal Articles

LES studies for predicting plume concentrations around nuclear facilities using an overlapping technique

Nakayama, Hiromasa; Takemi, Tetsuya*

Proceedings of 18th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes (HARMO-18) (USB Flash Drive), p.843 - 847, 2017/10

We have attempted to develop a practical and quick local-scale atmospheric dispersion calculation method using an overlapping technique for plume concentration distributions in an emergency response to nuclear accidents. In order to evaluate the overlapping approach, we performed LESs of turbulent flows and plume dispersion under varying mean wind directions using the meteorological data as the model input. When compared with the simulated results under the real meteorological condition, it is shown that the concentration distribution patterns are reasonably simulated by the overlapping method. It can be concluded that the atmospheric dispersion calculation method using the overlapping technique has potential performance for emergency responses to nuclear accidents.

Journal Articles

Development of local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation. 5; Detailed simulation of turbulent flows and plume dispersion in an actual urban area under real meteorological conditions

Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu

Journal of Nuclear Science and Technology, 53(6), p.887 - 908, 2016/06

 Times Cited Count:16 Percentile:83(Nuclear Science & Technology)

We extend the local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation (LES) to turbulent flows and plume dispersion in an actual urban area under real meteorological conditions by coupling with a meso-scale meteorological simulation model. The LES results of wind speed, wind direction, and concentration values are generally reproduced well. It is concluded that our coupling approach between LES and meso-scale meteorological models is effective to simulate turbulent flows and plume dispersion in urban areas under real meteorological conditions.

JAEA Reports

Local-scale high-resolution atmospheric dispersion model using large-eddy simulation; LOHDIM-LES

Nakayama, Hiromasa; Nagai, Haruyasu

JAEA-Data/Code 2015-026, 37 Pages, 2016/03

JAEA-Data-Code-2015-026.pdf:2.48MB

We developed LOcal-scale High-resolution atmospheric DIspersion Model using Large-Eddy Simulation (LOHDIM-LES). This dispersion model is designed based on LES which is effective to reproduce unsteady behaviors of turbulent flows and plume dispersion. The basic equations are the continuity equation, the Navier-Stokes equation, and the scalar conservation equation. Buildings and local terrain variability are resolved by high-resolution grids with of a few meters and these turbulent effects are represented by immersed boundary method. In simulating atmospheric turbulence, boundary layer flows are generated by a recycling turbulent inflow technique in a driver region set up at the upstream of the main analysis region. This turbulent inflow data are imposed at the inlet of the main analysis region. By this approach, the LOHDIM-LES can provide detailed information on wind velocities and plume concentration in the investigated area.

Journal Articles

The Numerical analysis of the capping inversion effect in a convective boundary layer flow on the contaminant gas dispersion

Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu

Procedia Earth and Planetary Science, 15, p.560 - 565, 2015/09

 Times Cited Count:0 Percentile:0.02

The local-scale high-resolution atmospheric dispersion model can simulate turbulent flows considering the effects of not only local terrain variability and buildings but also atmospheric stability conditions, and has been validated for representative meteorological condition cases. In this study, first, we conducted numerical simulations of plume dispersion under various thermally-stratified boundary layer flows by changing the stability conditions and compared with the wind tunnel experimental data. Then, we categorized the patterns of vertical profiles of mean concentrations based on the ratio of the convective velocity scale to the friction velocity scale. It is found that a plume quickly begins to touch the ground for a strongly unstable case with the ratio value less than 0.34 while the plume touchdown is observed at the downstream positions located away from the point source for a weak condition case with the ratio value greater than 0.34.

Journal Articles

Large-Eddy Simulation of turbulent winds during the Fukushima Daiichi Nuclear Power Plant accident by coupling with a meso-scale meteorological simulation model

Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu

Advances in Science & Research (Internet), 12(1), p.127 - 133, 2015/00

 Times Cited Count:8 Percentile:92.73

One of the important issues to be solved to couple LES and MM models is to impose time-dependent turbulent inflow data for LESs from the MM model outputs, because the MM models are not able to reproduce high-frequency turbulent fluctuations appropriate to drive LES models. In this study, we apply the recycling technique to couple the CFD and MM models. We conduct an LES of turbulent winds during the Fukushima Daiichi Nuclear Power Plant accident and report the usefulness of our approach by comparing the simulated results with the observations.

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