Abstract: A gap forming structure for use with water cooled nuclear reactors is invented to prevent overheating and ultimately structurally failing of the lower head of a reactor vessel in a nuclear reactor core meltdown accident by virtue of a cooling effect in the gap structure for facilitating the retention of accumulated molten core debris. Single layer or multilayer gap structures can be installed either inside or outside the vessel lower head by joining, or fastening structures or secured to the instruments/control guide tubes within the vessel. The water cooling capacity inside the gap can prevent the vessel lower head from overheating and subsequently failing and thus defend against severe accidents by preventing the lower head of the reactor vessel from rupturing.