What could happen when the uranium pellets in the fuel rods in the core of a nuclear reactor become overheated?

Answer 1

This condition is called a meltdown (or core meltdown or nuclear meltdown). The fuel becomes too hot (quite probably due to a loss of coolant accident - LOCA) and it melts, melting its way in turn through the cladding or "protective layer" of metal around the fuel pellets themselves. This dumps highly radioactive spent fuel (and all those nasty radioactive byproducts of fission - the fission fragments) into the primary coolant to be circulated around the plant. This is a major accident, and emergency shutdown procedures - and emergency cooling - will probably have to be initiated to cool the core enough to keep the problem from getting completely out of control and resulting in the so-called China syndrome. In this accident scenario, the superheated core melts its way through the reactor vessel and through the thick concrete floor of the reactor building and into the ground spreading the radioactive contamination outside the containment building itself. Note that the core continues to generate huge quantities of heat for hours after shutdown (even in a melted condition), so failure of the primary cooling system will result in a disaster if emergency cooling isn't effective.

Answer 2

If uranium pellets in fuel rods become overheated, it can result in a loss of control over the nuclear reaction, potentially leading to a meltdown of the reactor core. This can release harmful radioactive materials into the environment, necessitating evacuation and cleanup efforts to mitigate the health and environmental impact. Cooling systems, like those in a nuclear power plant, are designed to prevent overheating and maintain safe operating temperatures.