Hysteresis Impacts in Steady Flow Transitions of the Rayleigh Bénard Problem

Policymakers are searching for conscious solutions to the global threat of climate change and nuclear power is an unavoidable part of any combatting strategy. Public perception of nuclear safety was eroded by the Fukushima Daiichi accident in the spring of 2011. Research programs after the accident have focused on safety improvements of modern reactor designs. Important to these efforts is the refinement of Severe Accident Management strategies, such as External Reactor Vessel Cooling. This strategy relies on a low enough outward facing heat flux to prevent failure of the submerged reactor vessel lower head when under thermal assault from the naturally circulating molten reactor core. Correlations used to predict the subject heat transfer problem have low confidence in applicability, being derived from simplified, steady-flow experiments. We will present transient direct numerical simulations of the Rayleigh Bénard problem with internally heated convection using a scalable, in-house Navier-Stokes solver, with optimal concervation properties. We will showcase the presense of a hysteresis effect to the subject heat transfer problem. The observed transition Nusselt numbers are under-predicted by the steady value by more than the thermal margin available in published safety analyses in some cases. This transition is brief and may likely abated by the heat capacity of the reactor vessel.