Reply #51: Re-Criticality is likely not continuous

[localroger]

Neutron bursts mean the pile is critical right now. Various isotopes which should have decayed to nothing being found in the environment mean it was critical within the last few days or weeks. There are several signs that the pile has been critical long after it melted in the first couple of days.

One thing that was discovered at the first production reactor in Hanford is that fission products can poison the criticality of the pile by absorbing neutrons. These fission products have to decay according to their own radioactive half-life in order to leave enough neutrons flying around for the pile to go critical again. The original Hanford reactor had to have extra fuel rods added to stop this "heavy breathing."

Since the remains of Fukushima 1-3 are not in a configuration that was designed to be a reactor, if they are going critical it's very likely a close thing that is subject to this heavy breathing. There may also be remelting, which could cause the entire pile of corium to become either more or less reactive as it changes shape.

A few things should be noted. Nuclear fuel is always fissioning, but normally at a low rate. Radioactive decay of fission products will induce extra fissions, which is why spent rods are much more dangerous than new ones. Criticality means that more neutrons are produced by fissioning than are being absorbed to induce fissioning, which means the pile will get hotter and hotter and more and more radioactive until something stops it -- most likely fission product poisoning, since we have not seen the classic China Syndrome steam geyser.

It has been clear since the first few days that containment is toast. The proof of this is the hydrogen explosions. The hydrogen comes from the cooling water inside the core, when the very hot zircalloy cladding oxidizes by taking the oxygen from a water molecule, leaving the two hydrogens standing around like men with no date at the prom. It is important to understand that these hydrogens are released from the interaction of water with the fuel rod cladding -- inside the innermost inside of the reactor. In order to blow the roof off the building that hydrogen, building up inside the reactor, had to get OUTSIDE the reactor. The whole purpose of containment is supposed to make that impossible. This is why so much attention was being paid to pressure -- containment is supposed to contain even gaseous fission products that might build up pressure. But it's been too late for that since the first few days; gases that were in close proximity to the core were reaching the outside world. That is never, ever supposed to happen. Containment is designed from soup to nuts to prevent that very thing.

Also, a little thought about those "totally non-nuclear" hydrogen explosions: The oxidization of Zirconium is an endothermic reaction; it only happens because the reactor core is supplying a lot of energy to get it very, very hot. It takes a lot of energy to separate those hydrogens from their oxygen friend. The hydrogen explosion occurs when the hydrogens find another oxygen and join it exothermically. So in a technical sense, the explosion energy really is nuclear energy; it's just been transferred from inside the reactor to outside the reactor by using the reactor heat to generate hydrogen, which releases the energy used to release them on the outside.

TEPCO is only now admitting that the cores have likely melted out of the steel can into the cement secondary containment. What they still aren't admitting is that the secondary containment is also full of holes. If it isn't, what blew the roofs off those buildings?