Turbulent Dispersion Simulation of Radioactive Aerosol Applied to the Infrastructure of the NPP-2006 Industrial Site

The goal of this work is to simulate the processes of transport and deposition of aerosol particles in a turbulent flow, taking into account the infrastructure of the industrial site of the NPP. The developed model for calculating the dynamics of the spread of the pollutant emissions in emergency situations is presented, the limits of applicability of turbulence models are determined and the main mechanical and thermal sources of turbulence in the NPP infrastructure are analyzed. The mechanisms of radioactive substances deposition for emergency situations have been assessed taking into account turbulent effects. According to the results of the numerical modeling, the zones of predominant deposition of radioactive aerosols on the characteristic surfaces of the NPP infrastructure have been determined, which is the basis for emergency actions planning and assessment of the personnel doses.

Ensemble Dispersion Simulation of a Point-Source Radioactive Aerosol Using Perturbed Meteorological Fields over Eastern Japan

We conducted single-model initial-perturbed ensemble simulations to quantify uncertainty in aerosol dispersion modeling, focusing on a point-source radioactive aerosol emitted from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011. The ensembles of the meteorological variables were prepared using a data assimilation system that consisted of a non-hydrostatic weather-forecast model with a 3-km horizontal resolution and a four-dimensional local ensemble transform Kalman filter (4D-LETKF) with 20 ensemble members. The emission of radioactive aerosol was not perturbed. The weather and aerosol simulations were validated with in-situ measurements at Hitachi and Tokai, respectively, approximately 100 km south of the FDNPP. The ensemble simulations provided probabilistic information and multiple case scenarios for the radioactive aerosol plumes. Some of the ensemble members successfully reproduced the arrival time and intensity of the radioactive aerosol plumes, even when the deterministic simulation failed to reproduce them. We found that a small ensemble spread of wind speed produced large uncertainties in aerosol concentrations.

Nature of Relationships Between Atmospheric Electricity Parameters at Ground Surface and Air Ionization on the Basis of Nuclear Accidents in Power Plants and Weapons Tests

In this work we present an analysis of selected atmospheric electricity parameters, measured at the Geophysical Observatory in Świder (near Warsaw, Poland), in a review of the major events that resulted in the release of a significant amount of artificial radioactive substances in the Earth’s atmosphere: the radioactive accident in Fukushima, Japan, beginning 12 March 2011, followed by the 9.0 earthquake and tsunami; Chernobyl disaster (27 April 1986); and nuclear weapons testing (1958–1965). The physical mechanisms of the impact of radioactive sources on the electrical parameters of the atmosphere are analyzed. The formation of free charge (small ions, represented by electric air conductivity) and bound-induced charges (measured vertical electric field and current) by radioactive aerosol and cloud nuclei were taken into account. The values of electric field Ez, atmospheric air conductivity λ, and aerosol concentrations measured at a certain site depend on the time and space location of the released radioactive materials in relation to the measurement site and the meteorological situation. A frontal inflow of air masses containing radioactive substances may be noticeable at a large distance from the atmospheric electricity measuring site in fair weather conditions (Chernobyl disaster). Atmospheric precipitation plays a very important role in the transport of radioactive substances to the ground level (nuclear weapons testing). The relationship between the ionospheric potential Vi and the electric field near ground level Ez resulting from the Global Electric Circuit (GEC) concept for the presence of a strongly ionized air layer in the lower stratosphere and the ground level was disturbed in nuclear weapons testing time. The aim of this work is a qualitative characterization of discussed events. Future modeling works are needed to investigate the dependence of quantitative GEC parameters in situations of global or regional high air ionization. For this purpose, available measurements of recorded atmospheric electricity parameters will be used.

Modeling study of the atmospheric transport of radioactivity after wildland fires and a dust storm in the Chernobyl Exclusion Zone in April 2020

<p>In April 2020, the largest forest fire occurred in the Chernobyl Exclusion Zone (ChEZ) in its history. The results of modeling the atmospheric transport of radioactive aerosols released into the atmosphere as a result of wildland fires in the ChEZ and around it are presented. The atmospheric transport model LEDI, developed at the Institute for Safety Problems of NPPs, and the Atmospheric Dispersion Module of the real -time online decision support system for offsite nuclear emergency RODOS, which development was funded by the EU, were used. The <sup>137</sup>Cs activity concentration in the surface air is calculated on a regional scale (in Ukraine) and a local scale (within the ChEZ). The <sup>137</sup>Cs activity in the surface air of Kyiv (115 km from the ChEZ borders) is found to have reached 2–4 mBq m<sup>−3</sup> during the period April 3–20. The modeling results are generally consistent with measured data pertaining to radioactive contamination in Kyiv, within the ChEZ, and areas around four operating nuclear power plants in Ukraine.</p><p>A method for estimating the radionuclide activity emissions during wildland fires in radioactively contaminated areas is proposed. This method is based on satellite data of the fire radiative power (FRP), the radionuclide inventory in the fire area, and an emission factor for radioactive particles. A method was applied for forest fires in the ChEZ in April 2020. Preliminary estimations of an emission factor are made using FRP values obtained from NASA's MODIS and VIIRS active fire products.</p><p>On April 16, 2020, a strong dust storm was observed in the ChEZ, which coincided with the period of intense wildland fires. The additional <sup>137</sup>Cs activity raised by the dust storm from burned areas in the meadow biocenoses was estimated to be about 162 GBq, i.e. up to 20% of the total activity emitted into the air during the entire period of forest fires on April 3-20, 2020. According to the modeling results, during April 16-17, the input of resuspension of radioactive particles due to a dust storm was up to 80-95% of the total <sup>137</sup>Cs activity in the surface air near the Chernobyl NPP. In Kyiv, this value decreased to only about 4%.</p><p>The total effective dose to the population of Kyiv during the fire period is estimated to be 5.7 nSv from external exposure and the inhalation of <sup>137</sup>Cs and <sup>90</sup>Sr, rising to 30 nSv by the end of 2020. This is about 0.003% of the annual permissible level of exposure of the population. A committed effective dose up to 200-500 nSv is estimated for the personnel of the Chernobyl NPP from the radioactive aerosol inhalation during the 2020 forest fires, which is not more than 0.05% of the established control levels of internal exposure for them.</p>

Design of nuclear radiation detector for stack exhaust of plasma melting system

The Plasma melting technology can reduce the low and medium level radioactive waste to a minimum. The emission of high-temperature radioactive aerosol generated in the process of treatment needs to be monitored. After treatment with a high efficiency filter and meeting national standards for gaseous effluents, it will be allowed to be discharged into the atmosphere. In order to achieve stable and reliable radiation monitoring, through the research on the radionuclide analysis of stack exhaust and the design of detector selection, the combined design of plastic scintillator & NaI scintillator is adopted. At the same time, the real-time and reliable monitoring of radioactive aerosols in stack gases under high temperature environment is also solved.

Characterization of a non-thermal plasma source for use as a mass specrometric calibration tool and non-radioactive aerosol charger

In this study the charging efficiency of a radioactive and a non-radioactive plasma bipolar diffusion charger (Gilbert Mark I plasma charger) for sub-12 nm particles has been investigated at various aerosol flow rates. The results were compared to classic theoretical approaches. In addition, the chemical composition and electrical mobilities of the charger ions have been examined using an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF MS). A comparison of the different neutralization methods revealed an increased charging efficiency for negatively charged particles using the non-radioactive plasma charger with nitrogen as the working gas compared to a radioactive americium bipolar diffusion charger. The mobility and mass spectrometric measurements show that the generated bipolar diffusion charger ions are of the same mobilities and composition independent of the examined bipolar diffusion charger. It was the first time that the Gilbert Mark I plasma charger was characterized in comparison to a commercial TSI X-Ray (TSI Inc, Model 3088) and a radioactive americium bipolar diffusion charger. We observed that the plasma charger with nitrogen as the working gas can enhance the charging probability for sub-10 nm particles compared to a radioactive americium bipolar diffusion charger. As a result, the widely used classical charging theory disagrees for the plasma charger and for the radioactive chargers with increased aerosol flow rates. Consequently, in-depth measurements of the charging distribution are necessary for accurate measurements with differential or scanning particle sizers for laboratory and field applications.

Rain-induced bioecological resuspension of radiocaesium in a polluted forest in Japan

It is the conventional understanding that rain removes aerosols from the atmosphere. However, the question of whether rain plays a role in releasing aerosols to the atmosphere has recently been posed by several researchers. In the present study, we show additional evidence for rain-induced aerosol emissions in a forest environment: the occurrence of radiocaesium-bearing aerosols in a Japanese forest due to rain. We carried out general radioactive aerosol observations in a typical mountainous village area within the exclusion zone in Fukushima Prefecture to determine the impacts and major drivers of the resuspension of radiocaesium originating from the nuclear accident in March 2011. We also conducted sampling according to the weather (with and without rain conditions) in a forest to clarify the sources of atmospheric radiocaesium in the polluted forest. We found that rain induces an increase in radiocaesium in the air in forests. With further investigations, we confirmed that the fungal spore sources of resuspended radiocaesium seemed to differ between rainy weather and nonrainy weather. Larger fungal particles (possibly macroconidia) are emitted during rainy conditions than during nonrainy weather, suggesting that splash generation by rain droplets is the major mechanism of the suspension of radiocaesium-bearing mould-like fungi. The present findings indicate that radiocaesium could be used as a tracer in such research fields as forest ecology, meteorology, climatology, public health and agriculture, in which fungal spores have significance.

Effect of Containment Vessel’s Size Scale on the Aerosol Spray Scavenging Efficiency With Water Mist

In the foreseen decommissioning and debris removal plans of the damaged Fukushima Daiichi reactors, the fuel debris will be broken into small pieces using laser or mechanical cutting techniques prior to removing them from the reactor buildings. Regardless of the technique to be employed, submicron radioactive aerosol particles will be generated and dispersed in the gas space of primary containment vessel during cutting operations. The water spray system has been proven to be an applicable method in removing aerosol particles. However, it cannot remove Greenfield-gap aerosol particles (with diameters between 0.1–1 μm) so effectively. To solve this problem, a new agglomeration method by addition of water mist before spray injection was developed. With preexisting water mist, aerosol particles were expected to aggregate with water mist and form larger-sized agglomerated aerosol-mist particles, which increased the effect of inertial impaction mechanism leading to higher scavenging efficiency. The new method has been verified to be capable of improving the spray scavenging efficiency for the Greenfield gap particles by conducting aerosol scavenging experiments without and with mist in the newly built UTARTS facility in the University of Tokyo. The experiment results showed that the aerosol removal rate increased along with the increasing of mist concentration level. To verify the new agglomeration method in different experiment facilities and to investigate the effects of vessel’s size scale on aerosol collection efficiency, similar experiments were repeated in the TOSQAN facility of IRSN, France. Though the cylindrical vessel in two facilities have same internal diameter, the vessel’s height of TOSQAN facility is 4.8 m, which is larger than the one in the UTARTS facility (2.5 m). The experiment results in TOSQAN facility also showed that water mist has potential to improve aerosol spray scavenging efficiency. The corresponding numerical simulations about aerosol removal by spray droplets without mist in both UTARTS and TOSQAN facilities were conducted to better understand the aerosol removal process, including time evolution of aerosol mass fraction and flow field of the gas phase.