Define "nuclear explosion" - one could argue that regards of the nature of the explosion it was nuclear-sourced, given that the heat generated was done through fission, but the typical definition is a fully uncontrolled thermonuclear detonation, which did not take place. -------- It's not. What you simply have is a reactor design that has a potentially unstable thermal output: reactor cores cannot undergo any kind of thermonuclear detonation.