Research On Operation Characteristics Of The Flashing-Driven Natural Circulation Integrated Reactor

Flashing-driven natural circulation technique is an effective method to improve the reactor's natural circulation capability,which has been widely adopted in advanced SMR designs and passive safety systems.In a flashing-driven natural circulation integrated reactor,the coolant approaches to saturation temperature and triggers subcooled boiling at the core outlet.When it enters the adiabatic chimney,the coolant will continuously evaporate after a certain position as a result of the decrease of hydrostatic pressure.This effect greatly promotes the natural circulation driving force and therefore allows the reactor operate by means of natural circulation.Flashing-driven natural circulation technique can significantly simplify the reactor's structure and enhance the reactor's inherent safety features.However,it is still littleunderstood on the operation characteristics of the flashing-driven natural circulation integrated reactor,and it is not yet possible to master its thermal-hydraulic characteristics and safety performance comprehensively.In this thesis,the flashing-driven natural circulation integrated reactor IP100 design is proposed.The key techniques involved in the IP100 reactor,including flashing-driven natural circulation,self-pressurization,neutronic/thermal-hydraulic reactivity feedback mechanism,coolant flow load tracking mechanism and In-vessel control rod drive mechanism are described.The phenomenology involved in the IP100 reactor,including core enthalpy variation characteristic,self-pressurization characteristic,the relationship between coolant mass flux and CHF,and the onset of flashing,are investigated and revealed.According to the temperaturepressure joint control mechanism and the matching characteristics between primary loop and secondary loop,the coordinate operation scheme is proposed,which maintains both the coolant pressure and the steam pressure constant.Based on this operation scheme,the reactor control system of the IP100 reactor that using feed-forward control and PI controller are designed.The thermal-hydraulic model of the IP100 reactor are established by RELAP5/MOD4.0code,including the RCS,OTSG,and passive safety systems.The steady-state operation characteristics of the IP100 reactor are investigated.The variation of the thermal hydraulic parameters in reactor core,OTSG and chimney along the coolant flow direction are demonstrated.The variation of these parameters at different load conditions are also revealed.The transient performance of the IP100 reactor is tested by using load rejection transient and step load reduction transient.The contribution of reactor control system and self-regulation mechanisms on IP100 reactor's flexibility and safety are illustrated.In order to deal with the flow instability of the OTSG at low load conditions,the improve proposals including OTSG grouping operation and pressure sliding operation are applied to the original operation scheme.The testing results show that these two improvement measures on the reactor operation scheme can effectively extend the stable operation region of the IP100 reactor.In order to investigate flashing-induced instabilities under low pressure conditions,a natural circulation simulation model is established according to the Purdue NMR natural circulation experimental facility by RELAP5/MOD4.0 code.Three oscillation modes,including intermittent oscillation,compound oscillation and sinusoidal oscillation,were found in the study.The features and characteristic parameters of these oscillations were analyzed.By comparing the flashing-induced instability with other similar flow instabilities,the flashinginduced instability can be classified as the DWO-I.The parametric sensitivity analysis of the flow instability thresholds is carried out,according to which suggestions on the stable start-up procedure of the IP100 reactor are put forward.This thesis also carried out safety characteristic analysis of IP100 reactor based on three typical accidents,including Small break LOCA,LOFW and MSLB.The research focuses on the performance of passive safety systems and reactors' self-regulation mechanisms in mitigating the consequences of accidents and ensuring the IP100's safety.Different aspects of reactor thermal safety criterion under conservative assumptions are emphatically verified during different accidents,which proves the excellent safety characteristics of the IP100 reactor under the synergistic effect of various passive technologiesThis thesis systemically illustrates the thermal-hydraulic characteristics of the flashingdriven natural circulation integrated reactor.The contributions of the self-regulation mechanisms in the IP100 reactor on the transient performance and safety is proved.The feasibility,safety and flexibility of the IP100 reactor is proved in this research,which provides significant guidance to the development and application of the flashing-driven natural circulation integrated reactor.