“Gundersen felt that two were hydrogen gas explosions, but the third seemed to be a steam explosion. This would be consistent with a notably hot critical nuclear pile vaporizing all the cooling water (and vaporizing all the uranium metal and other radioactive gunk too, plus some of the steel and concrete).”
Gundersen’s crackpot theory was that the unit 3 large explosion was due to a prompt critical nuclear detonation originating in the spent fuel pool, triggered by the hydrogen deflagration deforming the spent fuel racks.
That, by the way, was the video in which I discovered what a total fraud Gundersen is. Nobody who knows the first thing about physics or engineering could have proposed such a laughable theory as being even remotely possible. And Gundersen’s specialization in nuclear engineering was supposedly in spent fuel racks, and yet he didn’t know the first thing about their structural strength or about the neutron absorbing borated separators between the fuel assemblies?
“The mushroom cloud looked fairly consistent with this steam explosion theory.”
The far superior steam explosion theory, on the other hand, was advanced by the graphic artist Ian Goddard, and it was well-grounded in evidence and plausible physics. Here’s his detailed presentation in which he lays out the case for his theory:
http://iangoddard.com/fukushima01.html
Goddard had originally accepted Gundersen’s spent fuel detonation theory (Goddard is an anti-nuke, so he can be forgiven for not understanding the nuclear physics involved and deferring to a self-styled expert on the subject), but then Goddard actually looked at the evidence, and here, he demolishes the pool detonation theory:
So far as I’ve seen, he was the first to do so. And then a few months later, underwater images came out showing the spent fuel racks in pool 3:
close-up detail:
The racks were not collapsed or warped, or exploded, or burned, or even dislodged. Absolutely no evidence that a nuclear detonation happened there.
In the mean time, apparently someone had pointed out to Gundersen 1) pool 3 did not go dry, 2) hydrogen would not have sunk and accumulated at the bottom of the pool even if it had gone dry, and 3) you can’t get a prompt critical reaction in adjacent nuclear fuel assemblies without liquid water moderation. (Water vapor, such as would be produced by burning hydrogen, doesn’t have nearly a high enough concentration of hydrogen to moderate the neutrons over such short distances.) So sometime mid to late 2011, he quietly modified his theory by simply leaving the pools full of water, and postulating that the hydrogen deflagration above the pools could have collapsed the racks in the bottom of the pools (!), and caused a moderated prompt-critical detonation.
Here’s Gundersen late in 2013, still peddling his preposterous exploding pool theory (now modified to include the word “moderated”) while also taking a swipe at the steam explosion theory:
“I’ve been saying all along that I think Unit 3 had something called a prompt moderated criticality in the fuel pool and that particles of fuel would be found lying outside Unit 3 is an indication that that happened. If the fuel had come from inside the nuclear reactor, it would have had to go through the containment and through a very circuitous path, so to my mind its very unlikely”
http://enenews.com/gundersen-major-problems-inside-pool-fukushima-unit-3-pieces-nuclear-fuel-rods-blown-during-criticality-explosion-building-could-shatter-big-quake-hits-video
“It wasn’t a real A-bomb explosion in terms of kilotonnage, but it was loaded with 40 years of long-lived radioactive elements such as cesium.”
Anything in the reactor would have been a few years old at most.
“This would be consistent with a reactor going somewhat supercritical, vaporizing the core, and then little bits of core material fusing as the mushroom cloud cooled below 1000 degrees C. … Almost all of Unit 3’s core is gone. If the core went straight up as vapor/plasma and then came back down as microscopic glassy fallout beads, this would explain where the core went.”
The cores melted down. The odds are good that Unit 3’s core melted through though the bottom of the reactor and dropped into the containment vessel. Goddard’s steam explosion theory, elegant as it was, ultimately proved to be incorrect. (That happens with theories sometimes.) In order for an upward explosion to have come from the reactor well, the cap of the pressure vessel would have had to blow off or rupture, and the three tiers of the concrete reactor well shield plug would have been blown high into the air. Here’s the first proof that didn’t happen:
That’s the roof lattice structure lying in crumpled heap after the large explosion. Notice the position of the reactor well. Notice that the lattice structure directly over the well is one of the most intact parts. If tons of steel vessel cap and concrete shield plug had been shot up like a mortar, they would have torn through that lightweight structure like tissue paper. And underneath the lattice structure (most visible towards the top of the well circle) you can see the large gantry crane, which was parked directly over the reactor well at the time of the explosion. That would have been launched as well.
When they cleared off unit 3, this is what they found:
The round thing is the well shield plug. It is three layers of concrete fitted into a layered hole like an upside-down layer cake, and there are three segments to each layer. You can see the middle segment of the top layer has buckled and caved in slighly. That was from the weight of the gantry crane falling on it. But otherwise, nothing dislodged, so clearly no explosion came through there. (You can also see the adjacent spent fuel pool is entirely intact with no surrounding cracks, and if there had been a nuclear detonation in that pool, especially under a few hundred tons of water, that pool would have been obliterated.) But if you look at the region in the red circle, that appears to be the focus of the explosion. And if the explosion originated there, that would also explain why the baffle plate between the reactor well and spent fuel pool was blown out of position towards the pool.
And then in 2015, they managed to photograph a section of the catwalk that runs under the reactor vessel.
They didn’t get a picture of the grate directly below the reactor, but what they did photograph is still informative. If there had been a nuclear detonation or even a steam explosion powerful enough to create that mushroom cloud, the grate should have been massively deformed by the blast. Failing that, there should at least have been shrapnel, rubble, and debris all over the place. But whatever happened there doesn’t even appear to have been violent enough to strip off the peeling paint.