When it comes to Nuclear reactors?

In a nuclear reactor the fuel will get very hot. This heat will last for years as radioactive decay progresses. In old designs, the fuel pellets would get hotter than the melting point of the ceramic bricks lining the containment vessel. This is not a problem as long as the fuel is covered in water and the steam pressure generated is not too high. If the water runs out, you have the potential of a melt-down. New designs have the uncooled fuel pellet temperature below the brick melting point.