Don't know about"bye bye world", but here is part of a definition that supports what you are saying.
"In the worst case scenario, the molten reactor core could penetrate the containment vessel and hit ground water. The combination of molten radioactive material and water could cause an enormous steam explosion which would spread radioactive contamination over a large area. In addition, the ground water itself would likely be severely contaminated, and its flow could carry the contamination far afield."
