Research On Operation And Safety Characteristics Of The Supercritical Carbon Dioxide Reactor

The application of supercritical carbon dioxide（SCO₂）in nuclear reactor system has become a research hotspot because of its high efficiency and compact layout.The low compressibility of SCO₂ near the pseudocritical region can greatly reduce the power consumption of the compressor and improve the cycle efficiency.The high density of supercritical fluid can realize the miniaturization of equipment and system,which meets the energy supply demand in the special application environment.Meanwhile,the inherent safety of the reactor system is guaranteed by the characteristics of moderate critical point parameters,no phase change during operation,and easy to establish natural circulation in passive residual heat removal system.At present,the research on the application technology of SCO₂ in nuclear power system and the operation characteristics of this reactor system is insufficient.The research of SCO₂ reactor system mainly focuses on the preliminary design of the system and equipment.However,the dramatic change of SCO₂ physical properties,the strong coupling characteristics of the closed Brayton cycle,the coaxial design of the turbomachinery and the recuperator pinch-point problem make the operation mechanisms of this reactor system more complex.Therefore,to deepen the understanding of SCO₂ reactor system operation mechanism,the further research from the perspectives of multi-scale matching evaluation of equipment and system,multi-dimensional comparison of specific objects,system operation control and accident safety analysis based on the complete systematic research framework should be urgently carried out.In this thesis,the 15MWth SCO₂ miniature reactor（SCMR-15）aiming at energy supply for underwater equipment is taken as the research object.Firstly,the core layout scheme,operation mechanism and thermal hydraulic parameter configuration are described respectively.The effects of innovative structures,such as closed channel fuel element,multi-stage control drum and its driving mechanism,and dual flow distribution of coolant,on the operation safety of the reactor core are discussed.And the application characteristics of SCO₂ in nuclear reactor are revealed.Based on the reactor single channel calculation method,the core thermal hydraulic characteristics of SCMR-15 are analyzed preliminarily.The effects of coolant fraction and fuel element size on the temperature distribution of reactor internals and the coolant flow loss are discussed.The rationality and safety of the core layout scheme and thermal hydraulic parameter configuration are demonstrated.Meanwhile,1-D analysis codes for SCMR-15 dual loop Brayton cycle energy conversion system and its key equipment are developed to complete the research on equipment structure calculation,performance evaluation and system matching demonstration.In addition,the calculation results provide input boundary for subsequent simulation analysis in modified RELAP5.Meanwhile,the feasibility and superiority of SCO₂applied in nuclear power system can be demonstrated by the preliminary introduction and analysis of SCMR-15.The flow and heat transfer characteristics of SCO₂ in semicircular microtube of PCHE are studied by computational fluid dynamics.The effects of the physical properties,operating conditions and the channel geometry on local heat transfer performance are discussed.And the influence of buoyancy effect on localized heat transfer mechanism is revealed from the aspects of dimensionless parameter quantitative analysis,the hysteresis of physical properties inhomogeneous distribution and the secondary flow intensity.Based on the requirements of SCO₂ reactor system simulation calculation,the function of RELAP5 was expanded by adding SCO₂ thermophysical properties,modifying the thermal-hydraulic correlations and developing the key equipment models.The modified RELAP5provides a reliable method for the design verification,theoretical analysis and simulation research of SCO₂ reactor system.Meanwhile,according to the system layout and equipment characteristics,the SCMR-15 simulation model is established.The effects of different operation strategies and control schemes on reactor power step reduction conditions are compared.The optimization scheme is proposed to solve the problem of equipment and system under low load conditions,which provides technical support for research on operation and control of this reactor system.From the perspective of system reliability,safety and environmental adaptability,the loss of flowrate accident（LOFA）,loss of external load accident（LOEL）and loss of coolant accident（LOCA）are selected to carry out the simulation research on SCMR-15 thermal-hydraulic response characteristics in the deterministic reactor accident safety analysis.The positive effects of auxiliary safety strategies and facilities such as the single loop operation,turbomachinery speed protection system and core passive residual heat removal system on the flexibility,stability and safety of SCMR-15 are demonstrated,which provides reliable theoretical basis and technical support for accident response and safety redundancy design of this reactor system.