Fine Research On Dynamic Response Characteristics Of Nuclear Island Subjected To Beyond Design Basis Loads

Nuclear power is an important part of economy and energy sustainable development strategy in China.However,once nuclear leakage accident occurs,its consequences are unimaginable,which will be devastating impact on human life safety and living environment.After 9.11 event and the Fukushima nuclear accident,countries pay more attention to the threat of beyond-design basis event that may suffered during the nuclear power plants(NPPs)operation service period.The policy and rule also have issued in China,which require the nuclear power plants must consider the malevolent impact problem of large commercial aircraft and provide an appropriate margin for the occurrence of beyond-design basis natural disaster event.Therefore,the analysis of dynamic response characteristics and damage mechanism of nuclear power plants under the beyond-design loads,the exploration of corresponding optimization and reinforcement measures are the important content of nuclear power project disaster prevention and reduction.For the extreme loads such as malicious aircraft impact and beyond-design basis earthquake may encountered in the whole life cycle of NPPs,this paper systematically and detailedly carry out the damage evolution analysis of large commercial aircraft impact,and the vibration response analysis and evaluation of internal structure during the impact,and the dynamic response characteristics of NPPs with overburden foundation and pile foundation reinforcement under strong earthquake,and the plastic damage analysis and anti-disaster mitigation measures of pile foundation of NPPs under beyond-design basis earthquake excitation.The research results with scientific research value and practical engineering application significance are obtained.The major contents and innovation of this paper are concluded as follows:(1)On the basis of the method combined with the scaled boundary finite element(SBFEM)and Octree discrete technique,the combination bonding method incorporated structured grids and octree grids is proposed and verified.The combination bonding method overcome the shortcomings of the original method in discrete circular structures and slender rods.Subsequently,a model included the internal structure of the reactor building for the aircraft impact problem is established,meanwhile,a model of nuclear island-pile group-overburden foundation for extreme earthquake problem is built.It is concluded that the combination bonding method has an extremely strong discrete grid and calculation capability,providing a high-quality discrete grid with fewer elements.Furthermore,this method is highly flexible.Compared with the traditional scheme,its modelling efficiency is improved by an order of magnitude.(2)The damage evolution analysis of NPPs in the malicious impact case of large commercial aircraft is developed.The effect of damage results is analyzed by compositive considering the mesh size in impact zone,the shape of the impact zone,impact heights,the foundation effect and SSI effect.The results indicate that the actual projected area of aircraft should be adopted to ensure the accuracy of structural deformation and plastic damage evolution.The junction between the cylinder and the conical dome is an unfavorable impact position.Due to the difference between non-lithologic foundation and lithologic foundation results is about 30%,so the SSI effect in the analysis can not be ignored.(3)The law of vibration response of the internal structure during the impact is studied.The influence of the soil-structure interaction on the vibration response of the internal structure during the impact process and the mechanism are revealed.Finally,safety assessment is carried out according to the corresponding evaluation criteria.The results indicate that the soil-structure interaction effect must be considered when analyzing the response of the internal structure of AP1000.Meanwhile,the acceleration response spectra at the key points of the auxiliary plants and the internal structure of the reactor plants exceed the recommended safety range.(4)The reasonable truncation range under earthquake in consideration of different foundation types and pile foundation effects is suggested.Combined with the nonlinear seismic wave input method and fully considered with the SSI effect,the reasonable truncation range of foundation in lithologic foundation,overburden foundation and pile reinforcement scheme is discussed and determined respectively.The effects of SSI and pile-soil interaction on the seismic dynamic response of nuclear island are also investigated.The results indicate that compared with the existing code,the foundation classification is increased,and the number of degrees of freedom is significantly reduced under the requirements of engineering calculation accuracy,which provides Suggestions for making the code more economical and efficient.(5)The nonlinear dynamic response characteristics of nuclear island-pile foundation-overburden layer system under seismic excitation are furthe analyzed.The influence of dynamic response of nuclear island is analyzed systematically by compositive considering element types of pile,pile-soil contact effects,different foundation depths and the Poisson ratio of foundation soil.The results indicate that the nonlinear effect of pile-soil contact has a significant effect on the distribution of force in pile foundation.In the case of overburden condition,the vertical floor response spectra(FRS)changes significantly with the change of Poisson's ratio.After pile reinforcement,rock-socketed pile bears the main vertical load,and the dynamic response is no longer sensitive to the change of soil Poisson's ratio.(6)The damage mode and mechanism of pile foundation of nuclear island under the beyond-design basis earthquake excitation are studied.With the generalized plastic model adopted to consider the strong nonlinear dynamic characteristics of soil layers,and the plastic damage model used to simulate the damage to pile foundation under the strong earthquake,the damage evolution process of pile foundation in nuclear power plants under the beyond-design basis earthquake excitation is realized.Then,the FRS characteristics change of NPPs after pile damage is illustrated.The targeted seismic design scheme is proposed.It is concluded that the results describe accurately and reasonably the damage process and law under earthquake.With the consideration of pile damage effects,the peak frequency of FRS decreased from the higher frequency(about 3 Hz?4 Hz)to the lower frequency(about 1 Hz?2 Hz),and the peak spectra value decreased at the higher frequency and increased at the lower frequency.