Thermal shock in a nuclear containment wall due to loss-of-coolant accident

The safety analysis of reinforced concrete containments for nuclear reactors requires evaluation of thermal effects due to the loss-of-coolant accident. It is assumed that the inner surface of the containment is suddenly heated by the coolant getting out of the primary circuit. The wall is assumed to behave as a beam-column. Hoop forces and moments created by shell action are ignored. The plane wall section of the containment, normal to the reinforcing rods, is studied for evaluation of the stress increment due to the thermal shock. It is assumed that the section remains plane during the mechanical and thermal loading. The elastic-plastic model of material is chosen both for concrete and reinforcing steel. The section is considered cracked whereever the concrete is subjected to tensile stress. Thermal and mechanical material data are included in the program.The input-data for the computer program consist of the temperature of coolant inside the containment, the coefficient of heat transfer from the coolant to the wall, the axial force and the bending moment imposed by loading conditions before the thermal shock and the conditions of restraint for the considered wall section during the thermal shock. The computer program, based on the finite element method, consists of two sets of subroutines. The first set calculates the temperature increment after the prescribed time step. The second set calculates the elastic and plastic strain increments resulting from the increment of combined mechanical and thermal loading. The wall section of an actual containment is used, as an illustrative example, for the determination of thermal effect under various loading conditions. Results are presented in a diagram of axial force versus bending moment. A point on the diagram represents the load combination to which the wall section might be subjected. The thermal effect for various thermal loads, in form of the equivalent bending moment, it also plotted in the diagram.