I don't necessarily agree with his hypothesis, but that's not what I was surprised when I saw the video. He says that the highly radioactive debris have been "bulldozed under", and when the debris are unearthed in the future we will see more high radiation spots.
I don't remember reading the news that the radioactive debris at the plant were ever actually buried. I remember one person who did say "Dig a hole, bury the debris so that the workers can work", Michio Ishikawa of the Japan Nuclear Technology Institute, a strong proponent of nuclear power who expressed a very pessimistic view of the TEPCO/government response to the Fukushima nuclear accident in late April.
Ishikawa also said the core had melted down and probably out of the RPV. That was officially admitted 2 weeks later. He also said a bridgehead must be built do the reactor, and forget about the turbine building. That also happened. Workers entered the reactor buildings, and they installed the heat exchangers for the Spent Fuel Pools (Reactors 2, 3, 4, and Reactor 1's heat exchanger has just been installed).
Is it possible that Ishikawa's suggestion on radioactive debris was also carried out? Or Is Gundersen simply mistaken? Or does he have some insider source?
Anyway, here's Mr. Gundersen's video:
Lethal Levels of Radiation at Fukushima: What Are the Implications? from Fairewinds Associates on Vimeo.