Let's begin with the ridiculous title:
Nuclear expert??? He's no "expert" (nor even a "scholar" as they occasionally try to pitch him). It doesn't appear that he even has a college degree (and if he does, it's in music). Why would anyone try to sell him as an "expert"???
Oh right... it's because he's saying what they want him to say. I keep forgetting that actual expertise is the antithesis of credibility with this crowd.
And if he's saying that the estimate on the amount of cesium might be off by a factor of five, he's admitting that he doesn't have a clue. There are physical constants involved here. They know how long each fuel assembly was in the core and how long ago it was removed. They know how much cesium would have been in each assembly at that point and how much of that is now barium. They might not know down to the gram, but they know to within a few percent if anyone cares to estimate it (and there's no reason that they should). Anyone who pretends to make an estimate and then says that he might be off by a factor of five... simply doesn't have a clue.
Why on earth would it be relevant to compare the amount of cesium in a pool to the amount released by the worst accident in nuclear history? Even imagining an (almost impossible) collapse of unit 4 followed by the worst possible fire... it isn't as if that's the amount that would be released (or anything close to it). This spin relies on people being ignorant of how Chernobyl progressed. There's no water on that core to this day and yet it isn't burning. The vast majority of the core that wasn't ejected in the explosion is sitting right where it has been for decades. Why would anyone believe the paranoia that a fuel fire in Japan (involving far less active fuel) would spew all of itself into the air?
He makes similar ridiculous comparisons, like comparing the amount of cesium in the pool (without any rational basis for how it would all escape) to the amount released by Hiroshima (and then goes on to remind people how awful Hiroshima was). How stupid does he really expect people to be? (the question can be ignored by those who answer the question by falling for the pitch). Cesium wasn't one tenth of one percent of why Hiroshima was a big deal (and that's likely high by far more than a factor of five)... so why make the comparison?
Oh yeah... because actual relevant facts don't support what you desperately want them to say. So this crowd promotes idiocy to "expertise". Problem solved!