If you search on YouTube for "fuel air explosion" you will find rather a lot of yellow flashes associated with much bigger bangs than Fukushima Dai-ichi 3. Gundersen is just talking nonsense here.

And clouds of black smoke can mean almost anything - certainly doesn't have to be nuclear fuel. Burning thermal foam insulation would look the same. So would diesel fuel, and it would make sense for them to have that on-site to power the pumps.

When I was a kid in New Zealand there was a dockside accident where an acetylene cylinder fell out of a crane net, hit concrete and went bang. The top end of the cylinder was found a few weeks later on the other side of the harbour, two miles away. The same distance that Gundersen is saying a conventional vapour explosion couldn't throw a fuel rod.