Simulation Research On The Migration And Diffusion Of Radionuclide In The Ocean Under Accident Conditions

With the rapid development of nuclear technology, nuclear, as a kind of clean energy, has become an important part of national energy strategy of electric power. But the nuclear safety has always been the focus of attention. Simulation of radionuclide migration in the ocean is an important problem to reveal the variation of the concentration along with time and distance, and to predict the influence of the radionuclide on the marine water environment after the severe accident.In this paper Fukushima nuclear accident was selected as the research subject, which is near the Fukushima nuclear power station, as the study area, puts forward improved instantaneous point source model considering the radionuclide decay characteristics, which can be applied to the large scale migration and diffusion in the shallow sea area. According to the leakage and efflux data of radioactive waste water discovered and published by Japanese Tokyo Electric Power Company, as well as the monitoring data of radionuclide concentration in seawater, this paper also estimates the accident source term, and determines the model parameters. And on this basis, the Joseph model and improved model are used respectively to simulate the concentration of 131I and 137Cs of four monitoring sites. The laws of concentration varying with time and distance is analyzed respectively, and the simulation accuracy of the model is also discussed. The main conclusions are as follows:1. At the same research point, the concentration of radionuclide decreased through its decay and dilution of seawater with time increasing. The impact which 131I has on water quality reduces sharply within one month, and eliminates after two or three months. The impact which 137Cs has on water quality slashes within two or three months to the acceptable level. And the impact is persisitant,low-concentration and cumulative which can be last several decades.2. The nuclear accident in Fukushima has caused severe radioactive contamination for the surrounding sea areas. At the same time,.the farther the distance is from leakage point, the smaller concentrations of radionuclide is. According to the concentrations of leakage and diffusion conditions which the article identifies, if the radionuclides concentrations released from Fukushima Nuclear Plant are diluted to the standard value, the distances of diffusion will be more than 30km. The main affected areas are the waters around the nucler power plant within 30km range. And the water within 10km is most severely polluted.3. Instantaneous point source Joseph model can be used to simulate the large-scale radionuclide diffusion in sea areas.For the longer half-life radioactive nuclides such as 137Cs, the simulation accuracy is within an acceptable range. But for the shorter such as 131I, it is not enough. Considering the characteristics of radionuclide decay, this paper puts forward improved instantaneous point source model. The simulation results show that By improving the model, the simulation accuracy of 131I increases from 175.18% to 35.89%, and the error of simulation results obviously reduced. But modification effect of 137Cs rsimulation results is very small.