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Session Overview
6.09: Dispersion of radionulides
Wednesday, 18/Mar/2020:
11:30am - 1:00pm

Session Chair: NATHAN BIXLER, Sandia National Laboratories, United States of America
Location: B-1049

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Atmospheric Transport and Consequence Analysis of the Fukushima Daiichi Accident

Nathan Bixler, Daniel Clayton

Sandia National Laboratories

The accident at Fukushima Daiichi Units 1, 2, and 3 began on March 11, 2011, and continued for approximately three weeks. The pattern of deposition resulting from the accident has been well characterized. Thus, this accident, while highly unfortunate, is valuable for validation of atmospheric transport modeling and for evaluation of consequence analysis methods and tools. A significant portion of OECD’s Benchmark of the Accident at the Fukushima Daiichi Nuclear Power Station (BSAF) Projects, Phases I and II, and the follow-on ARC-F Project, are focused on understanding the accident progressions, source terms, and consequences resulting from the three-unit accident.

As a participant in these projects, Sandia performed accident progression and source term analyses for each of the three units using MELCOR. These analyses account for the full three-week period of the accident. Furthermore, the results of the source terms were used to perform atmospheric transport and consequence analyses using HYSPLIT and MACCS. The agree remarkably well with the observed deposition patterns in the region surrounding the Fukushima Daiichi site.

Several sources of uncertainty contribute to overall uncertainties in the deposition pattern, including the source term signatures for the three plants and the weather data used to perform the atmospheric transport analysis. These uncertainties are explored in the paper. A third source of uncertainty, wet deposition modeling, is not assessed. A comparison of results using Lagrangian particle tracking and Gaussian plume segment modeling is also provided to assess the errors introduced by the atmospheric transport model. Finally, several consequence metrics are assessed to validate the modeling tools against the actual data from Fukushima.

Dispersion of radionuclides released by severe accidents and consequences analysis in Guangzhou city

Junjie Cai1, Jiyun Zhao1, Haimei Cheng2, Shuangfei Zi2, Jinchao Xiao2

1City University of Hong Kong; 2Chinese Academy of Sciences

The severe accidents in nuclear power plants will probably release radioactive pollutants, which will be transported to human beings by air, water, soil and food. Among all these approaches, the atmospheric dispersion is most difficult to control, has a fastest transportation and can cover the widest area. Numerical simulation is a powerful tool to study the underlying physics of atmospheric transportation and reasonably quantify and analyze the impact of radionuclides dispersion on the environment. In this study, we postulate an accident at Daya Bay nuclear power plant (DBNPP) to evaluate the possible consequences to the city of Guangzhou. The CALPUFF code is applied to solve the non-steady-state dispersion of radionuclides, due to its outstanding performance in long-distance transportation simulation and low computational demand. First the Fukushima case is numerically studied in order to validate the model and related parameters. The airborne concentration and deposition rate of both Iodine-131 and Cesium-137 agree well with the measured data. Then, an assumed source is imposed on DBNPP and the subsequent dispersions are resolved. The deposition of Cs-137 at landmarks of each administrative district are analyzed. Primarily based on the concentration of I-131, a dose assessment is established to evaluate the short-term consequences. Finally, the recommendations of emergency preparedness are proposed according to the generic criteria.

Numerical Simulation of Aerosol Scavenging by Spray Droplets with OpenFOAM

Hui Liang1, Nejdet Erkan1, Virginie Solans2, Shunichi Suzuki1

1The University of Tokyo; 2École polytechnique fédérale de Lausanne (EPFL)

When decommissioning the damaged Fukushima D1 reactors, the fuel debris in the bottom of primary containment vessel need to be cut into small pieces prior to removing them from reactor buildings. In the cutting process, submicron aerosol particles will be generated and dispersed in the upper part of the primary containment vessel. The spray system is an effective and applicable method to remove these radioactive aerosol particles before they escape to the environment. In this paper, a numerical model of aerosol scavenging by water spray droplets with considering inertial impaction, interception and Brownian diffusion was implemented into an open source computational fluid dynamic code OpenFOAM. The dispersed spray droplets were described using Lagrangian particle tracking method while the continuous particle-laden gas was described using Eularian method. The aerosol particles were treated as species of continuous phase. The size distributions of spray droplets as well as aerosol particles measured from experiment were used in the simulations. Firstly, the aerosol removal model was validated by comparing the simulation results with experiment data and it was found they were in good agreement. Then, the effects of different factors, including spray droplet size distribution, spray nozzle height and spray injection flow rate, on the aerosol removal performance were numerically investigated using the validated aerosol removal model. The details of aerosol removal process, including time evolution of aerosol mass and flow field of gas phase, were provided by numerical simulations. The simulation results will be used to optimize the design of water spray system. The numerical aerosol removal model is expected to be capable of predicting the aerosol removal performance by optimized water spray system in real Fukushima decommissioning situation once the initial aerosol size distribution was provided.

Formation of Type A Cs Particles from HEPA Filter Materials in Unit 3 during Fukushima Dai-ichi Nuclear Power Station Accident - From Viewpoint of Similarity in Silicate Glass Composition -

Akihide Hidaka1,2, Hiroki Hagiwara2

1Khalifa University; 2Japan Atomic Energy Agency

During Fukushima Dai-ichi NPS accident, part of Cesium (Cs) was released into the environment as insoluble Cs particles covered with silicate glass. At least two kinds of Type A (Unit 2 or 3 origin) and Type B (Unit 1 origin) particles were observed so far. Although the formation mechanisms of the Type B are gradually being elucidated, for the Type A, there are still some discussions. Author recently proposed that the Type A may have been formed by melting and atomization of glass fibers of High Efficiency Particulate Air (HEPA) filter in Stand-by Gas Treatment System (SGTS) line in Unit 3 during the hydrogen detonation. In the present study, the constituent elements of silicate glass covering the Type A and glass fibers of HEPA filter were examined using Electron Probe Micro Analyzer (EPMA). The results showed that the constituent elements of glass fiber were all contained in the Type A, and the elemental composition ratios of glass fiber origin were preserved also in the Type A. When the glass fiber was irradiated with the electron beam of EPMA under vacuum condition, spherical particles of several μm size were easily formed. These strongly suggest that the HEPA filter is Si source of the Type A. In addition, the activated carbon or carbon contained in the charcoal filter or separator of HEPA filter could have ignited and combusted by the flame of hydrogen detonation, and this could have enhanced the melting of HEPA filter. Immediately after the detonation, the gravity damper closed. The pressure became vacuum before the damper while the atmospheric pressure behind. This could cause a difference in the particle temperature and surface tension of liquid SiO2, indicating the possibility of spherical and non-spherical particles formation. Based on these new findings, formation mechanisms of the Type A were extensively refined.

Review and Application of Emergency Phase Resuspension Models to Radiological Consequence Analysis

Sung-yeop Kim1, Nathan E. Bixler2, Sukhoon Kim3

1Korea Atomic Energy Research Institute; 2Sandia National Laboratories; 3FNC Technology Co.

Atmospheric dispersion and deposition is one of the major migration pathways of radioactive materials which can be released from a nuclear power plant accident. Radionuclides released to the atmosphere are deposited to the ground by dry and wet depositions which act as a depletion process to air concentration. Deposited materials on the ground can be suspended again to the air by man-made or natural stress and this physical process is called resuspension. Resuspension inhalation can be an important internal exposure pathway and the contribution of resuspension inhalation to dose or health effects is influenced by the application of resuspension models when performing radiological consequence analysis. Previous study investigated long-term resuspension models and it was found that the choice of long-term resuspension models is not highly influential on total dose and cancer risk. As the shorter the deposition time is, the more likely the deposited materials are resuspended, man-made stress such as vehicular traffic and human activity can be necessary for the resuspension process in the emergency phase. In this study, various resuspension models which can be applied for emergency phase resuspension are reviewed. In addition, the impact of changing emergency phase resuspension models is evaluated from the results of consequence analyses applying reviewed resuspension models by using MACCS (MELCOR Accident Consequence Code System). Further study considering various influential parameters, such as more various source terms and duration of each phase, can be performed as a further work.

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