| The inside of nuclear island containment(reactor building)is a complex and closed special space,which has both high and narrow workplace.Due to the wide variety of construction operations,occupational health protection of the working environment in the process of nuclear power plant construction has always been one of the difficult problems that plague China’s green nuclear power construction.At present,no systematic and targeted research has been carried out in China,and there are few complete plans for foreign nuclear power construction for reference.In this paper,the problem of welding aerosol pollution control during the construction of nuclear island containment is selected.Firstly,the welding aerosol pollution and the individual aerosol exposure are investigated and measured in the CPR1000 containment of Liaoning Hongyanhe Nuclear Power Station.The physical and chemical characteristics of particulate matter are analyzed.Then,mathematical models of momentum and energy coupling between plasma,hot air and particles are established by combining the turbulent Brownian motion of particles with the flow characteristics of hot air and plasma.Next,the particle size distribution of welding aerosol is obtained by experimental measurement,theoretical calculation and numerical simulation,and the main stress and diffusion characteristics of particles are analyzed.The effects of spatial position,electrode inclination angle,trapping wind velocity and field angle of the dust hood on collection efficiency are studied.On the basis of the above,based on Elman neural network,a neural network prediction model of aerosol particle collection efficiency under multiple factors is established Finally,CFD numerical simulation is used to study the movement law of welding aerosol particles in the high space of nuclear island.And the relationship between particle size and aerosol main distribution height is obtained.According to the actual situation,a comprehensive exhaust combined with local purification method were adopted and applied in practice,which effectively solved the welding aerosol pollution in nuclear island.The main contents and conclusions of this paper are as follows:1.According to the investigation of current situation of welding aerosol pollution in nuclear island.The results show that:(1)Without using any pollution control measures,the weighted average concentration of individual aerosol contact time in the construction site of nuclear island containment exceeds the national standard value of 4mg/m~3,and the individual contact concentration of some welders exceeds the limited value by more than 5times.When the effective local control of aerosol concentration is not adopted in the welding spot area,the aerosol concentration in the breathing area around the welding spot can exceed the limited value by more than 10times in a short time.(2)According to TEM morphology observation of welding aerosol particles,the welding aerosol secondary particles are complex in shape,which are formed by coalescence of spheroid-like primary particles with smooth surface in tens of nanometers.The primary particles combine with other particles around by coagulation,resulting in the increase of particle volume and mass and the decrease of particle quantity.According to the EDS analysis of welding aerosol,the single spherical particles with regular shape are mainly composed of Fe and O,which mainly come from the metal part of the welding core.The irregular polymeric particles are mainly composed of Fe and Cu,in addition,there are large mass fractions of Ca and F.It can be inferred that this type of particles comes from the welding core and coating of the electrode.2.Through the dust emission test of the welding rods used in the nuclear island construction site,it is concluded that the dust emission rate per unit mass of the welding rods is between 6.2g/kg and 9.3g/kg,and the average dust emission of the welding rods is 7.6g/kg.Through further particle size analysis,it can be concluded that sub-micron fine particles with a particle size below 0.5μm account for more than 80%of the total aerosol particles.With the increase of air relative humidity,the average particle size of aerosol particles tends to increase,but the increment is not significant.3.From the stress analysis of welding aerosol particles,it can be seen that for submicron welding aerosol particles,the main forces are drag force,Brownian force,saffman lift force,thermophoresis force and gravity.Especially for fine aerosol particles with particle size below 0.3μm,Brownian force can not be ignored,and the particles make irregular diffusion motion in all directions of space under its action.When the particle size is larger than 0.3μm,the influence of Brownian force decreases rapidly,and the drag force becomes the control particle.In the free diffusion process of welding submicron aerosol particles,the particles with smaller particle size are more diffusive,and their trajectory is more likely to deviate from the airflow direction.4.When the electrode angle changes,the shape of plasma arc and hot air flow field will change accordingly,which will affect the initial position distribution and motion state of aerosol particles.The initial position of the particles will shift in the opposite direction of the electrode inclination angle,and this shift will increase with the increase of the electrode inclination angle.When the electrode angles are 90°(inclined 0°),60°(inclined 30°)and 45°(inclined 45°),the percentage of aerosol particles causing the shift is49.9%,62.5%and 94.9%,respectively.5.By studying the local collection efficiency of sub-micron aerosol,it is found that the appropriate position and field angle of the dust hood are of great significance to improve the collection efficiency,and increasing the horizontal angle of the dust hood or reducing the trapping distance can improve the trapping efficiency.When the vertical dip angle of the dust hood changes from 0°to 90°,the collection efficiency increases with the increase of the hood dip angle.When the vertical dip angle of the dust hood changes from 90°to 180°,the fixed electrode dip angle will correspond to an optimal vertical dip angle,so that the closer to this angle,the higher the trapping efficiency.In addition,according to the research results,it is a more suitable angle of the trap cover.6.The prediction results of local trapping efficiency based on neural network established in this paper have been proved to be in good agreement with CFD numerical simulation results.Using this prediction model,the collection efficiency can be calculated under any dust hood position,electrode inclination angle,trapping wind speed and other factors.At the same time,it can also predict the effective action area of the dust hood under fixed working conditions,which can provide reference for the design and practical application of welding aerosol dust hood.7.In the high space above 20 meters of containment,the distribution of aerosol particles is stratified at vertical height,and the average distribution height increases with the increase of particle size.More than 77%of welding aerosol particles are distributed in the height range of 30 meter to 46 meter of nuclear island containment,with an average distribution height of 40.78meter.By combining the methods of comprehensive exhaust and local purification,and after actual field measurement and evaluation,the control scheme of welding aerosol in nuclear island designed in this paper can effectively control the welding aerosol concentration in the working environment,and has achieved good practical application results. |
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