Development of a Numerical Simulation Method for Air Cooling of Fuel Debris by JUPITER

Abstract It is known that fuel debris is widely spread inside the primary containment vessel (PCV) at the Fukushima Daiichi Nuclear Power Plant (1F) by on-site observations. To retrieve fuel debris reliably and safely, it will be required that an accurate understanding of where and how much fuel debris locates in the PCV and how high the temperature of fuel debris rises. However, the details of the circumstances are not yet fully understood, and due to the high radiation dose, on-site investigation of it still has a lot of difficulties. Instead of on-site investigations, numerical simulations may provide essential information for the accurate understanding of thermal behaviors, including melt relocation behaviors, effects of unsteady natural convection around fuel debris, and the detailed temperature distribution inside fuel debris. We focus on the development of a detailed numerical simulation method for unsteady natural convection around fuel debris. JAEA has developed the detailed three-dimensional thermal-hydraulics code, JUPITER, which was originally developed for the melt relocation behavior of core internals in severe accidents. JUPITER has governing equations for incompressible fluids, i.e., equations of the Navier-Stokes, heat conduction, the advection of free interfaces, and solute transport. Because fuel debris may exist as a porous medium, in this study, we newly implemented the equation of porous medium model in JUPITER to calculate natural convection induced by a decay heat of fuel debris. We confirmed that JUPITER gives a reasonable result for the temperature of fuel debris as a porous medium.