| Nuclear power safety has always been the lifeline of nuclear energy development.When a severe accident occurs in a nuclear power plant,a large amount of radioactive material may be leaked into the ocean environment and diffused rapidly with the movement of seawater,causing significant harm to the public and the environment.In response to severe accidents in nuclear power plants,it is valuable to establish a model for the oceanic diffusion of radionuclides,accurately predict the diffusion range and concentration distribution of radionuclides in the ocean,evaluate the impact of radioactivity caused by the accident,and then verified and studied is investigated by establishing a model verification domain and a credibility calculation method.A hypothetical leakage accident at the Haiyang nuclear power plant of China and a nuclear effluent discharge event at Fukushima were selected as research objects,based on the ROMS model coupled with real-time meteorological data and radionuclide transport model,the radionuclide oceanic diffusion model for China seas and Fukushima nuclear wastewater were established,respectively,and analysis of accident source terms.Selection of a reasonable verification method to establish the verification domain of the nuclide marine dispersion model and validation of the model.Based on the validated model,the diffusion range and concentration distribution of radionuclides in the corresponding sea area after the nuclear wastewater discharge and the accident at the Haiyang nuclear power plant,respectively,were predictive calculated to evaluate the environmental radiation dose caused by the accident.Finally,a credibility calculation method for the radionuclides oceanic diffusion model was proposed and developed,and an example analysis was carried out.Based on the established oceanic diffusion model of radionuclides,the model was verified and analysed by establishing a verification domain.The results indicate that the radionuclide oceanic diffusion model for China seas provides a good description of Yellow Sea warm current invasion path during winter,as well as the temperature range and distribution of the coastal cold water patches and cold water masses during summer.The root mean square errors for the comparison between the calculated values and experimental values of radial velocity and latitudinal velocity are 0.05 m/s and 0.03 m/s,respectively.The correlation coefficients are 0.94 and 0.96,respectively,and the errors are within 10%.By conducting an experiment using offshore buoys as a validation method,it was discovered that the migration path of the buoys aligns well with the simulated migration path of the radionuclides.The difference in direction between the simulation and the experiment is 4.3°,and the distance error is 0.38 km.The temperature and salinity in the waters near Fukushima,calculated by the radionuclide oceanic diffusion model for Fukushima nuclear wastewater,exhibit a striped distribution.The direction of the steepest gradient lines of the water level coincides with the direction of the Kuroshio current.Validation of the Fukushima nuclear accident case has shown that the radioactive concentration reached its peak in early April.with a concentration of 104 Bq/m3 in high-concentration areas.Thereafter.it gradually decreased,and the overall trend of the observed values at the four stations matched the simulated values.Based on the validated radionuclides oceanic diffusion model for Fukushima nuclear wastewater,the diffusion range and concentration distribution of radioactive nuclide 3H in the eastern coastal areas of Japan are calculated.The results indicate that within one day of release,the diffusion of 3H is primarily in the north-south direction from the leakage point,with secondary diffusion towards the east.Specifically,it has spread approximately 55km and 42km in the north and south directions,respectively,while the eastward diffusion is only about 25km.The radionuclides released into the bottom reach the surface of the ocean within 6 hours.In the same vertical position,the concentration of radionuclides in the bottom layer is approximately one order of magnitude average higher than the concentration in the surface layer.The high-concentration region of radionuclides in the bottom layer can reach up to 4 × 104 Bq/m3.During winter,the predominant dispersion of radionuclides is oriented towards the southern direction,while exhibiting a more homogeneous mixing between the bottom and surface layers.This phenomenon exerts the greatest impact on the coastal cities along the eastern coast of Japan,with Mito City experiencing the highest radiation levels-peaking at an alarming 300 Bq/m3.Conversely,during summer,the primary dispersion shifts northwards,accompanied by a notably inferior vertical mixing capacity.Consequently,the diffusion of bottom-layer seawater into the surface layer becomes arduous,resulting in minimal consequences for the coastal cities.Soma City and Sendai City,in particular,encounter radiation levels below 10 Bq/m3.In both spring and summer,the vertical mixing abilities demonstrate a comparable efficacy,as the radionuclides within the surface seawater exert analogous radiation impacts on the eastern coastal cities.Employing a fuzzy comprehensive evaluation method,the credibility assessment of the established radionuclides oceanic diffusion model for Fukushima nuclear wastewater yields a classification of "high."Based on the validated radionuclide oceanic diffusion model for China seas.calculations are conducted to assess the diffusion range and concentration distribution of radionuclides in the China seas following a hypothetical nuclear leakage incident at the Haiyang Nuclear Power Plant.Furthermore,an analysis is performed to evaluate the impact of various factors,including different types of radionuclides,seasons,and duration of dispersion,on the diffusion of radionuclides.Within the span of one month during winter,the diffusion of radionuclides in the China seas primarily manifests as an eastward and southward expansion,covering distances of 160km and 140km,respectively.Conversely,the westward diffusion is limited to a mere 46km.Vertically,there is a commendable mixing of radionuclides in the seawater,resulting in minimal disparity between the concentrations observed in the surface and bottom layers.Due to a substantial release source,during the initial phase of radionuclide diffusion,both 131I and 137Cs exhibit nearly identical ranges and concentrations.However,as the diffusion time extends,one month later,the concentration of 137Cs in Rushan City surpasses that of 131I by an order of magnitude.The results obtained half a year after the winter nuclear leakage incident reveal that the radionuclides in the ocean have formed aggregated clusters and established three distinct migration and diffusion pathways.Following the summer nuclear leakage incident,137Cs exhibits a predominant belt-like distribution.After half a year,it permeates the coastal regions of eastern China,extending southward from the Shandong Peninsula.The southernmost extent has already crossed the mouth of the Yangtze River,reaching the Shanghai area.Concurrently,the northward diffusion of the radioactive plume infiltrates the northern Yellow Sea region of China,albeit with a concentration as low as 10Bq/m3.After two years of diffusion,radionuclides have pervaded the entire Yellow and Bohai Seas.However,the contamination level in the Bohai Sea is lower by 1-2 orders of magnitude compared to the Yellow Sea.Regions with higher radioactive concentrations have shifted from the southern part of Shandong to the coastal areas of Jiangsu.Some radionuclides have entered the Sea of Japan through the Korea Strait and gradually spread within Japanese waters,with radioactive concentrations below 30Bq/m3.Using error analysis,a credibility calculation was conducted for the radionuclide oceanic diffusion model for China seas,yielding a credibility score of 94.44%.The credibility calculation methods for the utilized radionuclide oceanic diffusion model were evaluated,and the advantages and disadvantages of the two methods were analyzed.Subsequently,a fuzzy AHP-ANFIS method suitable for calculating the credibility of radionuclide oceanic diffusion models was established,and this method was applied to calculate the credibility of the models.The research findings demonstrate that the developed fuzzy AHP-ANFIS credibility calculation method effectively overcomes the limitations associated with significant subjective influence from experts.By comparing experimental measurement data with simulated data,the objective credibility of the assessments was enhanced.The application of this method to calculate the credibility of radionuclide oceanic diffusion models resulted in a "high(H)" credibility rating,principles and measures have been proposed for the discharge of Fukushima nuclear wastewater into the ocean. |
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