What is the worst case scenario when dealing with a nuclear meltdown?

Okay, I'm writing a script and since I'm just a schmuck who does research and not a physicist I need some more information to properly tell a story. Essentially, I want to figure out exactly how catastrophic a potential meltdown can be. I'm talking scorched earth here. I have to ume it gets worse than Chernobyl, right? Take a look at the Fermi plants in southeast Michigan and tell me exactly how bad it can get. Thanks in advance.