| The passive containment cooling system(PCS)is widely used in the field of advanced nuclear reactor design.It can long timely remove the containment residual heat and maintain the containment integrity without using external energy.Take the case of Chinese Generation III nuclear reactor HPR 1000,the PCS of which utilizes the open natural circulation structure.It consists of inner-containment heat exchangers,inner-containment cooling water tanks and connecting pipelines.Under the accidental condition,the working fluid in the PCS establishes the natural circulation flow gradually driven by the density difference between the hot leg and cold leg,to remove the heat in the containment.The cooling water tanks in direct contact with atmosphere when the PCS operating,which makes the operation pressure of PCS relatively low.The vaporization processes,such as the flashing and the boiling,have greater potentials to occur,which makes the two-phase flow instabilities are more easy to happen.Morover,the flow characteristics of PCS are relatively complex due to the multi-mechanism impacts of the flashing and the boiling.Therefore in this paper,experimental investigations,theoretical analysises and RELAP5 code simulations on the flow characteristics of open natural circulation system were carried out,the purposes of which are to reveal the transient flow characteristics and heat transmission ability of open natural circulation system;and to figure out the characteristics and mechanisms of flow modes and flow mode evolution process of open natural circulation system.Moreover,the influences of thermal-hydraulic parameters and geometric parameters on the flow characteristics were also analyzed,which are aimed to provide the technical supports for the design and the improvement of PCS.The experimental investigations on the start-up behaviors and flow characteristics of open natural circulation were carried out.The experimental results show that,along with the increase of heating tube inlet temperature,the flow modes of single-phase natural circulation,geysering,pure flashing instability,flow instability induced by flashing and boiling,boiling instability and steady boiling flow occur sequentially.The steady flashing natural circulation flow is also triggered by the gas-injection method in the experiment.The flow characteristics and mechanisms of those flow modes were analyzed meticulously.Moreover,a water hammer phenomenon induced by flashing instability(FIIWH)was observed in theexperiment.The mechanism and influence factors of FIIWH phenomenon were analyzed by the experiment.At last,the flow mode distribution map of open natural circulation was drew according to the experimental data,and hysteresis phenomenon during the open natural circulation power changes process was discovered.A linear disturbance model for describing the stability characteristics of open natural circulation system was bulit up,based on the homogeneous flow model and method of statespace equations.The stability characteristics of flashinng and boiling in the open natural circulation system were studied by the linear model.By comparing the analysis results with experimental data,the interactions between flashing and boiling,and the mechanism of open natural circulation evolutionary process were obtained,which provides us a novel understanding to the open natural circulation flow behaviors.Moreover,the influences of geometric parameters on the open natural circulation evolutionary process were also investigated respectively by experiment and by analysis.The analysis results showed good agreements with the experimental data.The relation formula between mass flowrate and heating rate of open natural circulation system was obtained by theoretical derivation.The theoretical calculation results show good agreements with the experimental data.The discriminant formula for open natural circulation flow excursion region was also obtained by theoretical derivation.Moreover,the influences of thermal-hydraulic parameters and geometric parameters on the flow ability and heat transfer ability of open natural circulation system were investigated by the experiment and by the theoretical analysis.The theoretical analysis results fitted well with the experimental results.At last,the open natural circulation experiment loop was modeled by RELAP5/MOD3.4code.The simulation results showed good agreements with the experimental results,which indicates that RELAP5 code can be used for simulating the transient flow behaviors and the flow ability and heat transfer ability of open natural circulation.However,there are still some differences between the simulation results and the experimental results in the analysis of the transient flow behavior of geysering and boiling instability. |
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