Natural-convection studies for advanced CANDU reactor concepts

AECL is studying advanced reactor designs where natural convection is an important design feature in heat removal processes. The use of a flashing-driven, natural-circulation system to remove moderator heat is being considered. Experiments and code simulations have shown that a flashing-driven system is feasible at normal operating power, but is prone to flow instabilities at low powers. Vapor flashing at superheated conditions and the presence of nucleation sites were found to be important for stable operation over the whole power range. A development concept for CANDU(R) is to increase the primary coolant pressure and temperature to supercritical conditions. With a natural-convection-driven primary flow, the large variations in fluid properties near the critical point introduce the potential for flow instabilities. Analyses have shown that flow instabilities can occur under certain conditions. Experimental and analytical results on the flashing system are described. The experiments and initial analytical results for the supercritical concept are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.