| Natural circulation capacity is one of the important indicators to evaluate the performance and safety level of marine nuclear reactors.Affected by ocean conditions,marine nuclear power plants will generate complex ship motions.These kinds of ship motions can change the spatial position of the reactor system and cause the thermal parameters fluctuation,which affects the natural circulation operation stability of marine nuclear reactor.In this thesis,the effects of ocean conditions on the flow instability characteristics of natural circulation(NC)in rod bundle channel is studied experimentally.The parallel channels composed of a 3x3 rod bundle channel and a tube channel are used as the testing section.The experiments of NC flow instability in single rod bundle channel and parallel channels under static and ocean conditions are conducted.The experimental system pressure range is 0.2 to 0.6 MPa,and the range of inlet subcooling is 10 to 70?.Two typical ocean conditions,namely inclination and rolling motion,are studied in present experiments.The range of inclination angle is 5 to 20°.The ranges of rolling motion amplitude and period are 10 to 200 and 10 to 30s,respectively.In this thesis,the experimental study on NC flow instability in a single rod bundle channel under static and ocean conditions is firstly conducted.The flow instability existing in the rod bundle channel is Type-I density wave oscillation(DWO1).The parameters oscillation characteristics,instability boundary and parametric effects for DWO1 are obtained.An empirical correlation for the DWO boundary prediction at low pressure NC condition is obtained.The influence law and mechanism of compressible volume position and size on the stability of the closed NC loop is also studied.Two typical two-phase flow instabilities in rod bundle channel under rolling condition are found in experiments:(a)the trough-type oscillation caused by the vapor generation at the minimum point of flow fluctuation and(b)the compound oscillation formed by the superposition of the trough-type oscillation and DWO1.The determination method,classification criterion and evolution law of two-phase NC flow instabilities are obtained.Experimental results show that the rolling motion can reduce the threshold heating power of trough-type oscillation and cause the occurrence of NC flow instability in advance.But the rolling motion cannot affect the dimensionless boundary of DWOi in rod bundle channel.Inclination can cause a decrease in NC flow rate of the current experimental loop.With the inclination angle increases,the threshold heating power of DWO1 decreases.The experimental study on the NC flow instability in parallel channels under static and ocean conditions is further conducted.Under static condition,three typical oscillation modes are found in parallel channels:(a)small amplitude out-of-phase flow instability induced by DWO1,(b)large amplitude out-of-phase flow instability induced by the occurrence of geysering and(c)a kind of compound flow instability superposed by geysering and natural circulation instability.The instability mechanisms,instability boundary and evolution law of above three oscillation modes are obtained.It is found that the NC system stability is weakened with the increase of the asymmetry of the heating power and resistance distribution in parallel channels.The influence of rolling motion on NC flow behavior in parallel channels is related to their spatial arrangement.Similar to the experimental results of single channel,two typical flow oscillation behaviors can also be found in parallel channels:(a)out-of-phase trough-type oscillation induced by the vapor generation at the minimum point of flow fluctuation,and(b)a compound oscillation formed by the superposition of the trough-type oscillation and thermal induced flow instability in parallel channels.The rolling motion can also cause a decrease of the threshold heating power of NC flow instability in parallel channels.The influence of rolling parameters change on the occurrence boundary of thermal induced flow instability in parallel channels is limited.A method for predicting the boundary of trough-type oscillation by using the heating channel exit parameters and the Saha-Zuber correlation is proposed.The influence of inclination on the NC in parallel channels mainly depends on the system spatial arrangement.The threshold heating power of flow instability in parallel channels decreases with the increase of inclination angles.Based on the comparison of experimental results in single rod bundle channel and parallel channels,the NC flow states under rolling condition are divided into three types:forced flow instability,trough-type flow instability,and compound flow instability.The occurrence mechanisms,determination methods,predication methods and effects of rolling motion of these three flow instability are obtained.In order to solve the demand for unsteady flow behavior predication under ocean conditions in the marine reactor design process,this thesis analyzes the effects of static inclination,linear motion and rotation motion on reactor system and establishes an ocean condition mathematical model.A program for the marine reactor thermal hydraulic analysis is developed based on RELAP5 code.The experimental data obtained in zero power loading and NC flow experiments are used to validate the developed code.Results show that the NC flow fluctuation behaviors caused by rolling motion can be well simulated by the developed code. |
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