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

摘要

To ensure safety and containment during arnmeltdown of a fast breeder reactor core, arnhypothetical case has been considered andrnsimulated via computational fluid dynamics inrnwhich the entire core of a helium cooled fastrnbreeder reactor (875 MWth.) is melted down. Arn"core catcher concept" is suggested whichrnshould encapsulate the core melt. Its thermalrnperformance can only be determined through arnCFD-analysis. The device consists of a graph-iternhemisphere embedded in a two-layer blockrncomposed of concrete and zirconium dioxide,rnwhich serve as thermal insulators. The graph-iternis surrounded with a steel liner on whichrncooling channels are mounted to provide cool-ingrnat the outer shell throughout the meltdownrnprocess. Inside the graphite shell there is arnsolid steel bed. At the beginning of the melt-downrnsimulation, two liquid phases are as-sumedrnto be present in the core catcher: ura-niumrndioxide and contaminated molten steel. Inrnboth phases there is an internal heat sourcerndue to the radioactive decay of the fissionrnproducts present. This heat source power is arnfunction of the reactor thermal power, the timernthat the reactor has been operating underrnnormal conditions, and the time after shut-down.rnThe simulation was performed using thernvolume of fluid (VOF) and phase change mod-elsrnwithin the FLUENT CFD tool. It is foundrnthat once the core melt is in the core catcher,rnthe steel bed begins to melt, and the flow do-mainrnconsists of a colder flow falling downwardrnnear the side walls, an unstable zone in thernbulk with strong turbulence, and a stratifiedrnzone beneath it where heat transfer is domi-natedrnby conduction. The denser uranium diox-idernsinks and eventually solidifies. This is fol-lowedrnby re-melting of the uranium dioxide duernto reduced natural convection of liquid steel atrnthe base of the core catcher. After 26 hours, arnquasi-steady state is achieved in which bothrnphases remain at their respective melting tem-peratures, with a crust of considerable thick-nessrnforming covering the molten steel andrnuranium dioxide beneath it.