Research On The3-D Coolant Temperature Fluctuation In Upper Plenum Of The Core In Nuclear Reactor

In nuclear reactor, the complicated temperature fluctuation phenomenon caused by the mixed coolant flows out of the core with different temperatures in upper plenum. It may not only cause thermal fatigue of the solid structure in upper plenum, but also interfere with the temperature measurement in the outlet of the core. Further, this phenomenon may affect the control and protection actions of nuclear power plant, which may interfere with its normal operation. Therefore, it is very important to study the phenomenon of three dimensional temperature fluctuation in upper plenum.In this paper, the characteristics of3-D temperature fluctuation are analyzed based on the variations of the inlet temperatures, inlet velocities, the cold and hot velocity ratios, the geometric structures and the physical properties of the fluids etc, as follows:Firstly, the axially symmetric test section was designed with the coaxial-jet nozzles and circumferential overflow outlet, based on the complicated geometric structure of upper plenum. The experimental system also was completed in these studies. In experiment, the3-D transient temperatures were measured by a mobile thermocouple bridge, while the integrated2-D velocity vector fields were measured by the particle image velocimetry (PIV) with interference-free flow field characteristics.Secondly, based on experimental data, the spatial distributions of3-D temperature fluctuations in the radial, circumferential and axial directions were obtained. For grasping the mechanism of temperature fluctuation, this characteristics was more accurate than the international2-D experiment results. In addition, the characteristics of3-D temperature fluctuation also were analyzed with the variations of the inlet temperatures, inlet velocities, the inlet cold and hot velocity ratios in the experiment.Thirdly, the temperature fluctuations caused by parallel triple-jet flows were simulated by the software of FLUENT. Its results were compared with Japan experimental data, which verfied the correctness of experimental and numerical methods. The commonly used turbulence models were applied to prediction of3-D temperature fluctuations caused by coaxial-jet flows, i.e. Reynolds averaged Navier-Stokes method (RANS)(Reynolds stress model and standard k-ε model) and large eddy simulation model (LES). By the comparison of the computed3-D data and experimental ones (temperature and velocity fields), it was shown that LES was capable of predicting the mixing process. The temperature fluctuations caused by coaxial-jet flows and parallel triple-jet flows also were simulated by Large Eddy Simulation. By comparison of the results in different geometric structures, it was shown that the temperature fluctuations were three dimensional in coaxial-jet flows, while those were two dimensional in parallel triple-jet flows. Finally, the temperature fluctuations caused by coaxial-jet flows were simulated by LES method, the liquid metal sodium on the outlet of the core in China Experimental Fast Reactor as the boundary conditions. The distributions of the amplitudes and frequencies of temperature fluctuations were obtained in the cases with the different inlet temperature differences and velocities. These will provide a theoretical guidance for the engineering design.In this study, the experimental and numerical methods are valuable for the engineering application.