Research On Flow-induced Vibration Of Fuel Rods In Supercritical Water-Cooled Reactor

The supercritical water reactor is a Generation IV reactor concept which uses supercritical water as the working fluid. The high thermodynamic efficiency and plant simplification make SCWR attractive for consideration as a promising advanced nuclear system. However, unlike the other Generation IV reactor, there is no SCWR has been built in the world before. Considering the SCWR in the commercial road must pass through an important phase of experimental reactor building, so carrying out the technical studies associated with the experimental reactor is very important. Thus, European Union made a research plan of establishing a supercritical water loop to estimate the heat transfer coefficient and test the materials for the cladding. However, the fuel rods would vibrate in the case of coolant flowing through them, leading to the fatigue of cladding. In order to make sure the safety of fuel rod and get the vibration characteristics, it is necessary to carry out flow-induced vibration experiment and related numerical work. In the first part of this thesis, the flow field around the fuel rods was calculated by employing three-dimensional computational fluid dynamics (CFD) approach. According to the results, it is found that the wrapped wire would play a role of increasing the cross flow mixing effect in the flow channel, which would have a significant influence on the stability of fuel rod. Then an experiment was made to investigate the vibration behavior of fuel rods. According to the experimental result, the significant vibration induced by the fluid-elastic instability has not been observed in the experiment, and there is only a small random vibration caused by turbulent-boundary-layer pressure fluctuations. In addition, the vibration energy is concentrated near the first order natural frequency of the fuel rods in frequency domain, indicating the fuel rod mainly vibrate in in its first order natural frequency. Despite the vibration behavior of fuel rod is random, the maximal amplitudes is almost the same within several times experiments. By taking the maximal amplitude as input for ANSYS software to calculate the stress distribution on the fuel under the condition of maximal deformation, it can be found that the maximal stress occurs on the constrained end, and the maximal stress is small enough to make sure there is no chance of cladding occurring fatigue under the set flow rate, meaning the structure of fuel rod is safe enough in the supercritical water loop.Since it is necessary to conduct refueling in the loop, there must be a tiny clearance between the fuel rod and the spacer to make unloading easy, meanwhile, the wear between the fuel rod and spacer will make the clearance becomes larger over the time. When water flow through a narrow clearance, it may cause the fuel rod vibration. Another experiment is made to investigate the mechanism of leakage-flow-induced vibration. According to the experiment result, it is found that when flow rate is low, the vibration of rods is mainly caused by turbulence, but the effects of leakage flow would become obvious with the increase of flow rate. The influence of leakage flow contains two aspects: one is the change in the distribution of vibrational energy in spectrum, and the other is the increase in spectrum magnitude in a certain frequency band. Thus, for increasing the structure safety of fuel assembly, it is more reasonable to use the spacers with blind hole to fix the both ends of fuel rods.