Downward heat transfer from heat-generating boiling pools pertaining to PAHR and transition phase

The phenomenon of heat transfer from volume heated boiling pools in the downward direction is of considerable interest in nuclear reactor safety analysis. For example, in the post-accident heat removal studies of LMFBRs the downward heat transfer to a boiling steel layer in molten pool penetration of the MgO substrate is of interest. The bubbles rising from the boiling of stainless steel film, lying at the bottom of the heat generating pool consistaing of MgO and UO2 eutectic solution, cause the upward motion of bulk fluid. This buoyancy effect of two-phase bulk fluid generates a negative pressure gradient along the base, resulting in motion of single-phase fluid in a boundary layer over the stainless steel liquid film. The heat transfer from concrete substrate during molten pool penetration or during sodium-concrete reaction also takes place owing to convection that is driven by gas and water vapor bubbles released from the concrete substrate due to its heatup. The proposed model assumes that owing to the density difference caused by volume boiling or bulk two-phase fluid between the bulk fluid and that near the base of the pool, the lighter two-phase bulk fluid causes upward movement of the fluid and thereby generates a negative pressure gradient along the base of the pool. This in turn causes motion of returning single-phase fluid along the base of the pool in laminar/turbulent boundary layers