Melted reactor 4
A Fukushima citizen living in 30km area published the book about his experience after 311.
In his book (P43), he refers to a nuclear worker who watched reactor 4 being destroyed.
Though explosion of reactor 4 was not reported, it is significantly destroyed.
Reactor 4 is the most mysterious case. There was no major blasting sound, but the walls of the reactor building were deformed like morphing of SFX in front of our eyes, the walls of lateral side went totally messed up after all.