What really is the worst case scenario in nuclear meltdown? I've understood that actual fission explosion is nearly impossible, given the delicate structure in

What really is the worst case scenario in nuclear meltdown? I've understood that actual fission explosion is nearly impossible, given the delicate structure inside a nuke to make this happen. Chernobyl scene will not be seen again (except maybe in Russia) because graphite is somewhat rare slowing material. So it's something about bunch of radioactive materials that have buried themselves below ground. Then they get in contact with water, and steam explosion can occur. How big is this? How much radiation will spread? How probable is this?