Fukushima Disaster 11th Anniversary Report
There is lots of good info in the report. Short and straight to the point.
Our early estimation soon after the initial disaster as that unit 1's failure was rapid and dire. TEPCO dismissed such ideas, claiming the unit might have some fuel damage but surely hadn't melted down. Nuclear wonks from the US also insisted the damage to unit 1 was trivial. We now know with confidence that it was indeed a rapid and major meltdown.
By 2012 there was clear evidence that molten fuel had escaped containment into the torus room
What TEPCO found on inspection indicates a major series of destructive events took place beyond merely melting the reactor fuel. Layers of sediment and fuel debris were apparent on the containment floor and partway up the torus downcomer tubes, documenting the migration of the molten fuel out of the pedestal and out of the containment structure.
Important note: I shamelessly copied their report from their site ( here ) as their embedded reader is a nightmare to use, and I often just get grey pages. I don't think they will mind as it is in the spirit of getting info out. If anyone has an issue with this then please let me know.