Study On The Influence Of Large Natural Draft Cooling Towers On The Diffusion Of Gaseous Pollutants In Nuclear Power Plants

With the rapid development of China economy and the nuclear power industry,construction of nuclear power plants(NPPs)has become a very important planning goal in China.The existence of the cooling tower group in the nuclear power plant will change the near-area wind field of the NPPs,resulting in the extreme irregularity of the airborne radionuclide emissions from the nuclear power plant.The traditional Gaussian model will not be fully applicable.In this case,atmospheric dispersion model should be amended to ensure the reliability of the results.In this paper,the combination of calculation and experiment is used to study the influence of atmospheric diffusion on large natural draft cooling towers in the vinicity of nuclear power plants.Impact factors such as cooling tower body,the air inlet at the bottom of the cooling tower and the wet heat plume of the cooling tower are mainly considered.On this basis,through the calculation and analysis of the atmospheric diffusion parameters in the near area of the nuclear power plant,the SMAC model(Segmentation calculation Model for Atmospheric diffusion of Cooling tower)is established,which significantly improves the simulation accuracy of the traditional Gaussian atmospheric diffusion model,and get a better performance in nuclear power plant environmental impact assessment and accident consequence evaluation.The results of the lidar wind test show that the airflow disturbance in the near area of the cooling tower is very strong,and the wind direction is irregularly distributed.The influence of the cooling tower body on the vertical direction of the incoming wind speed will continue until it is higher than the tower body.The results of the on-site tracer tests show that the released tracer is concentrated by the cooling tower and gathers around the cooling tower to form a high concentration area.The axial concentration generally increases first and then decreases with increasing distance.In this paper,the Standard k-?,RNG k-? and MP k-? turbulence models are selected to compare with the results of on-site tracing test and wind tunnel test.The results show that the calculated results of the three turbulence models are consistent with the experimental data.The calculation results of the RNG k-?model are closest to the experimental data,and the other two turbulence models have different degrees of high concentration.The natural ventilation process of the cooling tower is equivalent to increase the effective release height of the gaseous pollutants released from the ground,and the concentration of pollutants in the nuclear power plant area is greatly reduced.The closer the cooling tower is to the release point,the lower the concentration on the down-wind side of the cooling tower.The cooling tower forms a significant blocking effect on the pollutants released below 70 m,so that the maximum ground concentration point does not change with the change of the release height.When the wind speed changes from 0.5m/s to 5.4m/s,the axial concentration of pollutants increases with the increase of wind speed,but when the wind speed increases to 7.9m/s,the ground concentration of pollutants begins to decrease.Atmospheric stability conditions have little effect on the ground release.On the down-wind side of the cooling tower,the axis concentration of pollutants is the largest under the condition of Class A stability,while the stability of Class F is the smallest,which agrees with the conventional atmospheric diffusion principle.The trend of stability is just the opposite,which also illustrates the existence of large natural ventilation cooling towers,which greatly change the process of atmospheric diffusion of pollutants.Based on the engineering design of nuclear power plants,four possible cooling tower layout schemes are given,which are single cooling tower layout,rectangular layout,inline layout and S-shaped layout,which cover most of the nuclear power plant sites.Simulation analysis of atmospheric diffusion of pollutants was carried out for different layout schemes.For a single cooling tower arrangement,the concentration distribution is a standard Gaussian distribution in the distance between the release point and the cooling tower,and its concentration is also significantly higher than the other three arrangements.In addition to the Sshaped arrangement,the other three solutions form a steep drop effect on the leeward side of the cooling tower.In the distribution of pollutants on the leeward side of the cooling tower,the concentration results of the single cooling tower arrangement are smaller than the other three schemes,mainly because a large amount of pollutants enter the cooling tower body and then discharge from the top of the cooling tower,causing a sharp concentration decrease on the leeward side.Combined with the diffusion results of gaseous pollutants under different cooling tower layout schemes,the correction of atmospheric diffusion parameters under different layout schemes is carried out,and the modified atmospheric diffusion parameters are substituted into the Gaussian linear plume model to establish the SMAC model.The calculation results of SMAC model are compared with those of wind tunnel experiment and CFD model,and the validity of SMAC model calculation is verified.In this paper,two typical nuclear power plant sites are selected,namely T site and P site,where T site represents the rectangular cooling tower layout plant,and P site represents the inline cooling tower layout plant.The calculation results of the two sites show that the calculation accuracy of the SMAC model is 1-4times higher than that of the traditional P-G Gaussian model.The application of the SMAC model in the actual site will more scientifically and reasonably reflect the atmospheric diffusion of gaseous pollutants in the near area of the site,and can also guide the division of exclusion area boundary and low population zone of the site,reduce the site construction cost,and enhance the public confidence of nuclear power.The research results of this paper can be directly applied to the atmospheric diffusion analysis under the environmental conditions of large natural ventilation cooling towers at the site,which is of great significance for guiding the environmental impact assessment of nuclear power plants and the evaluation of the consequences of nuclear accidents.