The fact that material with less than 2% fissile content can't go critical on fast neutrons is easily calculated by looking at the well measured fission and absorption cross-sections for U-238, U-235 and Pu-239. There's a *reason* why fast reactors use fuel with fissile content of 15%-20%, despite the higher cost of doing so.
Everything that happened at Fukushima can be explained by loss of AC power -> loss of coolant -> decay heat driven meltdown -> containment failure due to containment pressure & temperature beyond design limits. The decay heat is quite large - tens of megawatts for hours to days after shutdown. There's absolutely no need to invoke criticality to explain what happened, and absolutely no evidence that there was any such criticality. And a detector at the gate certainly couldn't provide any such evidence as the cores were and are inside enough shielding to block the neutrons that result from full power operation. You know, to stop the workers from getting radiation poisoning from normal operations. Any neutron pulse large enough to be detected there would have had to come from a fission reaction large enough to pretty much level the entire site and kill everyone nearby.
And before you mention the spent fuel pools again, the fuel in those has been inspected and found to be intact. So no meltdown there.
The weird thing is that my original post was explaining why a particular nuclear solution *wasn't* a quick and easy answer. I'd have thought that'd be the kind of thing you'd agree with.