[en] During core melt accident significant amount of core may relocate in the reactor pressure vessel lower head. During its cooling it may form cracks inside the corium and gap between corium and reactor vessel. Gap also may appear due to deformation of the lower head if its temperature exceed creep limit. Slotted channels ensure ingress of the cooling water into the corium, and exit of the generated steam. Study of the cool-down mechanism of the solid core debris in the lower head of the reactor vessel through gap and cracks is the objective of experimental work on the CTF facility. Thermal hydraulics in the heated channels closed from the bottom and flooded with the saturated water from the top of the channel, is characterized by the counterflow of the steam and water, attended by such specific phenomena as the dry out when boiling, flooding and overturning of the coming down flow of water at the certain flow rates of the steam going up, partial dry out of the channel, and reflooding from the top of the heated channel with the saturated water. The above phenomena may reveal independently or in different combinations depending on geometric parameters of the channel, heat release, and coolant parameters. Interchange of these processes with a certain cyclic sequence is possible. Experimental study was performed at the CTF (Coolability Test Facility) facility, which is a part of the thermohydraulic KC test facility in the RRC 'Kurchatov Institute'. Presented results are obtained at the CTF-1 test section which represents a vertical flat channel modeling a single crack in the solidified corium or the gap between the corium and reactor vessel