(An) application of nano-fluids to enhance CHF in in-vessel retention system

Abstract

In-vessel retention (IVR) by external vessel flooding to maintain the molten core in reactor vessel by cooling the outer of it is one of the accident management strategies currently being employed in some advanced water reactors. However, the ability of the reactor vessel to transfer heat into coolant beneath the vessel is limited by the occurrence of critical heat flux (CHF) at its outer surface. Enhancing CHF at the outer surface of the reactor vessel allows a higher thermal margin and a significant enhancement of IVR capability. Higher CHF can be achieved by using either an advance coolant and/or an engineered fuel cladding surface. Nano-fluids, colloidal dispersions of nano-size particles in base fluids, have been of interest for the last decade as advanced coolant for heat transfer systems, including nuclear reactors. One of the most interesting properties of nano-fluids is to enhance CHF over that of the base fluids, which has been obtained in many previous works. This opens a possibility for using nano-fluids to enhance the IVR capabilities of Advanced Light Water Reactor (ALWR) systems. In this study, pool boiling CHF behavior on a small plate at varying inclination angle of the heated surface under atmospheric pressure was examined. Three nano-fluids with high suspension stability were selected by the Zeta Potential method to investigate the effects of each nano-fluid on pool boiling CHF, which were 0.05% $Al_2O_3$, 0.05% CNT + 10% $H_3BO_3$, 0.05% $Al_2O_3$ + 0.05% CNT. CHF increased as the heated surface was inclined from downward facing to vertical position. Nano-fluids enhanced CHF significantly in comparison with pure water. Among the three selected fluids, combination of $Al_2O_3$ + CNT nano-particles showed the highest CHF enhancement with the maximum of 122% at downward facing position, while $Al_2O_3$ nano-fluid exhibited the lowest CHF enhancement. The underlying mechanism of CHF enhancement in nano-fluids was investigated. Surface morpho...