Replies

There is one and only one way for Hydrogen to be produced in large enough quantities to produce an explosion. Namely, H is produced as a result of a chemical reaction with the zirconium in the Zircaloy cladding of the fuel rods. And this chemical reaction takes place when the fuel rods reach a temperature of 1200 C (2200 F). Hence the presence of H means the rod temp has reached at least 1200 C. For at least the top parts of the fuel rods.

And this means that the water is boiling away (or leaking) faster than it can be replaced. Because no parts of the fuel rods could reach that temperature if they were completely surrounded by water.

The reaction is similar, but not identical, to these from highschool chemistry.

Na (metal) + H2O -—> NaOH + H
Mg (metal) + HCl -—> MgCl + H [leaving out the 2’s]

With zirconium, it is
Zr + H20 -—> ZrO + H [Zr + 2H20 -—> ZrO2 + 2H2]

That is, Zr metal is converted to Zr-oxide, releasing hydrogen in the process. This does not occur at low temperatures. However, once started the reaction is exothermic, that is, it releases heat. Hence, it tends to get hotter, thus boiling away more water, and speeding up the release of more hydrogen. If nothing happens to get things cool, the process feeds on itself in a runaway fashion.

The problem is made worse by what is known as the “two phase problem.” That is, at the surface of the Zircaloy there is a mixture of liquid water, water vapor (steam), and hydrogen gas. The gases carry heat away poorly, and in effect they provide an insulating layer that hinders the liquid from effective cooling. And zirconium oxide is a much poorer conductor of heat than the metal, so the growing amount of ZrO2 further hinders heat transfer from the fuel rods. Hence, as time goes on, things get hotter.

The rods are about 3 meters long (10 ft), and the water bath normally covers them completely. In this loss of cooling scenario, they are not uncovered all at once. The water level slowly drops, exposing the upper parts of the rods, and the above applies basically to the circa 10-cm region around the water level; above is hot, below is cooler. But at this level, all is turbulent. And as the water level drops, this region keeps revealing new sections of the rods to the H-producing reaction. (The region above the turbulent interface is filled with very hot steam and previously released hydrogen, keeping the ZrO reaction going.)

Worser problems: The Zircaloy tubes (filled with uranium oxide pellets or some mix with plutonium) do not like this situation at all. They are much hotter than they can stand, they experience huge localized hot spots, a thermal gradient, the Zircaloy becomes soft, and the cladding is rapidly being converted to zirconium oxide (there are some other chemical reactions, too). The fuel rods then begin to swell, bend, balloon, buckle, get wart-like bubbles, and eventually develop holes. (The rods always have a fairly high internal pressure owing to the Xenon generated by the radioactive decay). The swelling and buckling may reduce the space between the rods, which then impairs the water circulation, leading to more super-hot spots. At an advanced stage, this becomes a runaway process with a total meltdown unstoppable and inevitable. The whole Pacific Ocean could not stop it.

The holes are what cause isotopes of cesium, xenon, iodine and other unsavory nucleotides to get released.

So far, it seems that only a few upper centimeters of the fuel rods have undergone this process, and only maybe two or three rods have developed holes (out of perhaps 50 rods—not sure of the total number). The concern is, if they have not been able to pump enough water in to prevent this level of Zr-hydrogen reaction, why won’t it get worse? The plant operators know everything—and more—than I have described, but they have not been able to get enough water into the reactor vessels.

The hydrogen problem, again, is that the water level is falling faster than they can pump it in (pump out hot water, replace with cool water). Water, water everywhere but not enough to cool.