Modeling Molten Fuel-Moderator Interactions for the CANDU Flow Blockage Accident

CANDU reactors are pressurized heavy water-moderated and heavy water-cooled reactor designs. During commissioning of nuclear power plants, a range of possible accidents must be considered to assure a plant's robust design. Consider a complete channel blockage in the CANDU reactor. Such an extreme flow blockage event would result in fuel overheating, pressure tube failure, partial melting of fuel rods, and possible molten fuel-moderator interactions (MFMIs). The MFMI phenomenon would occur immediately after the pressure tube rupture and would involve a mixture of steam, hydrogen, and molten fuel being ejected into the surrounding moderator water in the form of a high-pressure vapor bubble mixture. This bubble mixture would accelerate the surrounding denser water, causing interfacial mixing due to hydrodynamic instabilities at the interface. As a result of these interfacial instabilities, water is entrained into the growing two-phase bubble mixture with attendant mass and heat transfer, e.g., water vaporization and fuel oxidation. A comprehensive model has been developed to investigate these complex phenomena resulting from a postulated complete flow blockage and complete pressure tube failure. This dynamic model serves as a baseline to characterize the pressure response due to a pressure tube rupture and the associated MFMI phenomena. Theoretical modeling of these interrelated complex phenomena is not known a priori, and therefore, a semiempirical approach is adopted.