Turbulent Conformation and Flow Fluctuation Analysis in Reactor Lower Chambers

The uniformity and stability of the flow field in the reactor lower plenum are crucial factors for ensuring the core cooling efficiency and operational safety. This study is based on the shear stress transport (SST) turbulence model combined with the scale-adaptive simulation (SAS) method. Computational fluid dynamics (CFD) technology was adopted to conduct transient numerical simulations of the flow fields in the lower plenums of three typical reactor structures. The turbulence characteristics and fluid pulsation properties in reactors with different structures were systematically investigated, and the pulsation power spectral density (PSD) was quantitatively analyzed through the energy cascade theory. The numerical results show that in the lower plenum of Structure 1 reactor, there are “columnar structures” such as support columns and energy absorbers, which cause the coolant to generate a large number of small-scale vortices inside the lower plenum. From the power spectral density analysis results, its power spectral density is at a medium level, and the high-energy cutoff frequency is relatively high, about 5-10 Hz. For Structure 3 with a full-coverage hemispherical flow distribution device, the coolant entering the lower plenum is “squeezed” into the narrow gap close to the lower plenum wall, effectively suppressing the formation of large-scale vortices, and the turbulence energy is low. According to the power spectral density analysis results, its maximum PSD value is only 1/100 of that of Structure 2. The high-energy cutoff frequency is at a medium-high level, about 5 Hz. It can be seen that the “hemispherical” structure plays a key role in reducing the fluid pulsation energy and stabilizing the flow field. In the lower plenum of Structure 2 reactor, the flow distribution components only cover the central area of the core, and a “cavity” is formed at the edge position. The coolant generates a significant large-scale vortex below the radial support key, making the fluid pulsation in the edge area obvious. The power spectral density analysis shows that its fluid pulsation PSD value is the highest, about 10 times and 100 times that of Structure 1 and Structure 3 respectively, and the high-energy cutoff frequency is relatively low, about 1 Hz. Such high-energy, low-frequency large-scale vortices are extremely likely to generate large hydraulic excitations on the core fuel assemblies, which may trigger low-frequency oscillations of the fuel assemblies, and then lead to problems such as large fluctuations in nuclear power and grid wear. Therefore, during the power operation period, Structure 2 needs to be closely monitored, and structural modifications should be carried out when necessary to improve the turbulence pattern and pulsation conditions inside the lower plenum and ensure the safe operation of the reactor.