ABSTRACT
This chapter provides an overview of the areas in which thermalhydraulics most strongly influences activity transport during a severe accident in a water-cooled reactor. The influence of thermalhydraulics on activity transport during a severe accident can be divided into six broad areas: fuel temperatures and fission product release from fuel, in-core atmospheric conditions and gas flows, activity transport in the reactor coolant system, fission product chemistry in the reactor coolant system, activity transport in containment, and core-concrete interactions. Gas flow rates are strongly dependent on local pressure gradients, which in turn can be strongly influenced by local condensation/evaporation events, suggesting that there could well be a need for integrated thermalhydraulic/fission product transport analysis, particularly when concentrations of fission products and structural materials are high. In fact, the scatter, covering several orders of magnitude in the fractional release rates of volatile fission product materials at any given temperature, and in determining specific material release rates for specific fuel temperature conditions.
