External cooling of reactor vessels during severe accident

Chapter 16 is devoted to the so called external cooling of reactor vessels during severe accident. It is a technology allowing arresting the melt inside the vessel of some initial conditions are fulfilled. First the state of the art is presented. Then a brief description of the phenomenology leading to melt in the lower head is discussed: dry core melting scenario, melt relocation, wall attack, focusing effect. Brief mathematical model description is given appropriate for a set of model assumptions. The model describes: the melt pool behavior, the two-dimensional heat conduction through the vessel wall, the total heat flow from the pools into the vessel wall, the vessel wall ablation, the heat fluxes, the crust formation and the buoyancy driven convection. Solution algorithm is provided for a set of boundary conditions adequate for real situations. A summary of the state of the art regarding the critical heat flux for externally flowed lower head geometry is provided. On a several practical applications different effects are demonstrated: the effect of vessel diameter, the effect of the lower head radius, the effect of the relocation time, the effect of the mass of the internal structures. Varying some important parameters characterizing the process the difference between high powered pressurized- and boiling water reactor vessel behavior is demonstrated.