Vahid H. Thermal hydraulic features of the corium melt jet penetrating a water pool under reactor vessel during severe accidents

The dissertation is devoted to a study of the thermal hydraulic peculiarities of the high-temperature corium melt interaction with a volatile coolant during severe accidents at the nuclear power plants. The modeling results of the bending perturbations of a melt jet in a coolant pool and disintegration of the jets on the drops are presented. It is shown that among the problems requiring studies for the improvement of the passive protection systems against severe accidents at NPP there are disintegration of the thin jets due to bending instability and the jet penetration length into volatile coolant pool. These problems belong to the initial stage of the severe accident in containment by the corium melt jet spreading in a water pool under the reactor vessel. The successful solution of these problems predetermines an effectiveness of the passive protection systems against severe accidents at the NPPs. Then the mathematical model developed and some results of its application to an element of a passive protection system against severe accidents at NPP with water pool under reactor vessel for corium melt and particles’ cooling are presented. This element of a system is a penetration behavior of a high-temperature corium jet into a volatile coolant pool, with account of the vapor produced on a melt jet penetration. Success of the problem solution determines an effectiveness and durability of the passive protection system against severe accidents. Key words: corium, passive protection systems, severe accident, simulation, multiphase flow, heat transfer, thin jet, thick jet, penetration into a pool.