[en] Under failure conditions atomic power plant reactors in which a cold heat carrier is supplied into the reactor to remove rapidly the heat from the active zone, nonuniform thermal stresses form in the wall of the reactor body which may greatly exceed the level of permissible stresses. Operation of the system of emergency cooling of the zone is accompanied by the formation of a thermal shock with subsequent long-term cooling of the walls of the body by the continuously arriving cold heat carrier. As regards vessels made of domestic materials, this work is the first study of this problem. The main objective of the study was to obtain information on the behavior of a vessel containing a crack and subjected to the operating pressure in heating to 300 degrees C and then rapid cooling with cold water. It was required to design special standards, develop a test procedure, and ensure thermometric measurements in the process of cooling throughout the entire thickness of the wall of the model and also evaluate the stress-strain state (mainly in the zone of the notch or crack). To carry out the thermal shock experiments, a model of 15Kh2MFA steel was produced in the form of a shell with an external diameter of 980 mm, a wall thickness of 150 mm, and 1850 mm long. The unfavorable conditions during service of the reactor body were simulated by embrittlement of the model by heat treatment under the following conditions: quenching 1000 degrees C, holding 4 h, cooling in oil, tempering 620 degrees C, holding 10 h, cooling in air. These treatments made it possible to simulate the effect of neutron irradiation on the level of the mechanical properties and critical brittleness temperature. 14 refs., 4 figs., 3 tabs