Study of the Melt-Cool down in a Suggested Core Catcher Concept for a Helium Cooled Fast Breeder Reactor after a Hypothetical Severe Accident using Computational Fluid Dynamics Simulation

摘要

To ensure safety and containment during a meltdown of a fast breeder reactor core, a hypothetical case has been considered and simulated via computational fluid dynamics in which the entire core of a helium cooled fast breeder reactor (875 MWth.) is melted down. A "core catcher concept" is suggested which should encapsulate the core melt. Its thermal performance can only be determined through a CFD-analysis. The device consists of a graphite hemisphere embedded in a two-layer block composed of concrete and zirconium dioxide, which serve as thermal insulators. The graphite is surrounded with a steel liner on which cooling channels are mounted to provide cooling at the outer shell throughout the meltdown process. Inside the graphite shell there is a solid steel bed. At the beginning of the meltdown simulation, two liquid phases are assumed to be present in the core catcher: uranium dioxide and contaminated molten steel. In both phases there is an internal heat source due to the radioactive decay of the fission products present. This heat source power is a function of the reactor thermal power, the time that the reactor has been operating under normal conditions, and the time after shutdown. The simulation was performed using the volume of fluid (VOF) and phase change models within the FLUENT CFD tool. It is found that once the core melt is in the core catcher, the steel bed begins to melt, and the flow domain consists of a colder flow falling downward near the side walls, an unstable zone in the bulk with strong turbulence, and a stratified zone beneath it where heat transfer is dominated by conduction. The denser uranium dioxide sinks and eventually solidifies. This is followed by re-melting of the uranium dioxide due to reduced natural convection of liquid steel at the base of the core catcher. After 26 hours, a quasi-steady state is achieved in which both phases remain at their respective melting temperatures, with a crust of considerable thickness forming covering the molten steel and uranium dioxide beneath it.