Where did the worst US nuclear accident happen?

Idaho Falls, Research Reactor SL-1. 1961. Prompt criticality, steam explosion, and partial meltdown when control rod removed too far. Three men killed outright. 80 Ci I-131. 1,100 Ci mixed fission byproducts, recovered and buried. INES level 4.

Even though Three Mile Island was INES level 5, and cost more to recover from, there were no deaths.