Thermal stress analysis of containment building in case of a Main Steam Line Break (MSLB)

In Indian PHWR, containment building is one of the primary barriers for mitigating the consequences of a Loss of Coolant Accident (LOCA), and Main Steam Line Break (MSLB) accident. It is desired to know the temperature transients as well as the resulting thermal stresses in the containment structures of 220MW(e) PHWR, Kaiga Nuclear Power plant under postulated MSLB event. The high enthalpy steam discharged into the containment space comes in contact with the Structural Wall (SW) of containment, Inner Containment Wall (ICW) and Raft. The containment wall temperature rises due to heat transfer from steam–air mixture. To calculate the transient temperature distribution across the containment walls, it is necessary to determine containment ambient temperature and heat transfer coefficient for the condensing steam on the internal structures. Hence, at the outset, a thermal hydraulic code was developed to predict the pressure–temperature transients and condensation heat transfer coefficient transients (using various condensation models) based on the mass and energy of high enthalpy steam released into containment. The effect of various condensation models on containment pressure–temperature was evaluated. The thermal boundary conditions such as containment temperature and heat transfer coefficient, evaluated from the thermal hydraulic code using Uchida condensation model, were subsequently applied as boundary conditions to a two-dimensional axi-symmetric containment model developed using a FEM code for estimating two-dimensional temperature profiles and the resulting thermal stresses.