What is a "meltdown"?

Question

fusion of control rods
melting of fuel rods
a huge explosion of the power plant
when gases are vented from the bottom of the containment vessel

Answer 1

A "meltdown" refers to the melting of fuel rods in a nuclear reactor, typically as a result of insufficient cooling or other safety failures. It can lead to the release of radioactive material and pose a serious threat to health and the environment.