Models! A great deal of work goes into the prevention of a serious core event. If such an event were to occur, three different physical processes are expected to increase the time between the start of the accident and the time when a large release of radioactivity could occur: - The time required for the water to boil away (coolant, moderator). - The time required for the fuel to melt. - The time required for the molten fuel to breach the primary pressure boundary. Models, do your job!