Prediction of thermal loads on nuclear reactor vessel lower plenum after core melting and relocation during a severe accident requires knowledge about the core melt behavior, especially the circulation pattern. To analyze the heat transfer dynamics on the lower plenum walls, two-dimensional numerical simulations of a fluid flow with internal heat generation were performed for Rayleigh numbers 106, 107, 108, 109, 1011 and 1013 at Prandtl number 0.8. For subgrid motion modeling, a Large-Eddy Simulation Smagorinsky model was implemented.

The minimum, time-average and maximum Nusselt numbers on the boundaries were calculated. The dynamics of fluid structures were analyzed to reveal the instability mechanisms and transition to turbulence. Results disclose Rayleigh-Taylor instabilities as a dominant mechanism for turbulence appearance, which occurs when the Rayleigh number is increased over 108.

The structure dependence of fluid motion at high Rayleigh numbers makes the time-average of heat transfer hard to assess. The time-average values should be supplemented with probability distributions of related variables.

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