Zircaloy seems to be one of the major reasons why the Fukushima crisis is scary. When the earthquake happened, the reactors automatically inserted neutron-absorbing control rods to stop the nuclear chain reaction. This process is called a SCRAM. Despite this, the fuel rods are still producing heat because of other ongoing nuclear reactions (decay heat). This is what makes Zircaloy so worrisome, because when it gets heated up it can oxidize or burn exothermically, producing even more heat. That adds to the risk that fissile material will end up in the wider environment. It also adds to the risk that the nuclear fuel will melt, dribble down to the bottom of the reactor containment vessel, and re-form a critical mass.
That’s the really scary possibility, as far as reactors one, two, and three at Fukushima are concerned. If the fissile material in the fuel rods forms a critical mass again, it could melt through the bottom of the containment vessel. It could also trigger a large steam and/or hydrogen explosion that could spread radiation further.
There are other risks with Zircaloy. When hot, it oxidizes in the presence of water, stripping oxygen from water molecules and producing explosive hydrogen. Zircaloy is also what makes spent storage pools so scary. If they lose cooling, the rods can heat up, burn, and release large amounts of radiation into the environment. Cooling ponds are not placed inside containment vessels in the same way reactor cores are.
So, what seem to be possible lessons learned here?
1) If it is possible to find something better than Zircaloy, we should. It needs to have a low neutron cross-section, so that neutrons from different fuel rods can induce fission in one another. At the same time, it would be really nice if it would not oxidize exothermically, generate hydrogen in the presence of water, or burn.
2) Perhaps spent fuel cooling ponds should be inside containment structures.
3) Perhaps containment pools should be embedded in solid rock, not perched atop buildings.
It’s possible there is no material that satisfies (1) and it is possible that (2) would make nuclear reactors impractically expensive. If so, perhaps the appropriate option is to pull back from nuclear power as an energy source and concentrate on reducing total energy demand, while deploying renewable forms of energy.