[en] The triggerability and explosivity in the molten fuel and coolant interactions(FCIs) were discussed. For the triggerability, small scale experiments using single molten droplets and analytical triggering models were reviewed and large scale experiments and a simple model were discussed for the explosivity. These works revealed that the thermal-hydraulic condition depending upon the physical properties of the molten material is an important role in vapor explosions. However, there exist some uncertainties due to the simulant material or the limited experimental dimension in reasonable explanations for vapor explosions during the reactor severe accidents. Therefore, the real melt simulations under various geometries are necessary for properly answering the reactor safety issues related to vapor explosions during reactor severe accidents