What's this supposed worst case incident? Chernobyl went prompt critical, that's about as bad as it can get. Chernobyl basically disproved the China Syndrome

What's this supposed worst case incident? Chernobyl went prompt critical, that's about as bad as it can get. Chernobyl basically disproved the China Syndrome fears--everything that was supposed to cause a China Syndrome accident happened--but it only managed to burn it's way into the basement. The core doesn't stay in one blob, it responds to what it's eating through, changes shape and ceases to be critical.