The Pyrolysis of the Spent Fuel Rods at Fukushima
As the detailed story of the Fukushima accident slowly emerges, it is increasingly apparent that much of the radiation that was emitted came from overheating spent fuel rods. The inflection point in the crisis came when the Fukushima 50 managed to get cooling water into the spent fuel ponds to cool the rods back down. Much of the public's attention was drawn to the risk that the pressure vessels would fail as the fuel rods inside them melted down. But post-accident inspections of the pressure vessels reveals that they survived even though the fuel rods inside went through a partial meltdown. The "China Syndrome" did not occur.
So it is necessary to examine what was going on in the spent fuel ponds that were inside the reactor buildings, but outside the containment structures. There were three primary mechanisms of interest to the public. There was a release of gaseous hydrogen that resulted from a chemical reaction between the water in the pressure vessels and spent fuel ponds and the zirconium cladding on the fuel rods. In essence, the hydrogen atoms separated from the oxygen atoms in the water molecules as the oxygen "rusted" the zirconium. When the big shots in Tokyo finally gave approval to vent that hydrogen, it was too late. There was so much hydrogen released in one big burp that it formed a chemically explosive mixture with the oxygen in the air and exploded. The sequential chemical explosions blew off the roofs of the reactor buildings, leaving the pressure vessels inside intact. With very limited cooling of the cores, the fuel rods partially melted down. But this was not the primary source of the airborne radiation.
Much of the airborne radiation came from the overheating of the fuel rods in the spent fuel ponds. Not having experienced such a violent earthquake before the event, no one had considered what would happen to the water in the ponds as the "Earth moved"! If you have a relatively full cup of water and violently shake it, the water inside sloshes out. It seems that exactly that happened to the spent fuel ponds, but everyone assumed they were still full of water, with disastrous results. With a greatly reduced water level, the top ends of the fuel rods became exposed and started overheating.
This heating is the result of the radioactive decay of the isotopes remaining in the fuel rods, not an atomic chain reaction. As a particle is emitted from a given atom, it travels until it hits another atom, and the kinetic energy of the particle is converted to heat. The fuel rod thus begins a process of getting hotter and hotter. Normally the water bath around the rods absorbs the heat and keeps the rods cool. Without water, the rods get hot.
A simple analogy of what occurs would be to compare it to the pyrolysis of a wood log in a fire. As the log gets hotter, the various constituent molecules start to turn to gas from the solid state. The more volatile the molecule, the faster it starts to vaporize. As the log burns, it gets hotter, and more of the molecules start to vaporize. Let it burn long and hot enough, and all that will be left is the ash. The progression is related to the boiling point of the molecule in question.
As the fuel rods got hotter, the various radioactive isotopes began to vaporize according to their boiling points. Iodine came first, then cesium, and finally plutonium. So the initial burst of radiation was largely iodine. This blew out to sea, and what did come ashore had an eight-day half-life and quickly abated.
The crisis passed shortly after the abortive attempts to refill the cooling pond in Unit 3 by Chinook helicopter were supplanted by water injected by water cannon from ground level. As the pond filled up, the rods were submerged in the water and gave up their heat as steam. Once the water level covered the rods, the worst of the crisis was over.
In Japan, the immediate need is to restart the existing fleet of reactors. So the lessons learned from Fukushima are:
1) Allow the onsite operators to vent hydrogen early enough so that it does not accumulate to a concentration rich enough to explode.
2) Be sure to keep the spent fuel ponds full of water.
3) Have backup generators and pumps available (typically via military units) through people with sophisticated logistic capabilities (e.g., helicopters) and communications to supply supplemental cooling water to both the pressure vessels and the spent fuel ponds.
4) In case of a nuclear plant problem, consult with the world's most experienced operators of nuclear power plants: the United States Navy.
5) Do not try to run the operation under the control of government or corporate weenies in an office in the nation's capital!
So the radioactivity part of the problem is largely over. But as long as Japan fails to restart its fleet of nuclear power plants, it will rush toward the precipice of financial ruin! The prototype of such a disaster was the largest municipal bankruptcy of the age before "Too Big to Fail" economics, the Washington Public Power Supply System (WPPSS), more commonly referred to as WHOOPS! Meanwhile, Japan is driving the cost of gasoline in the United States higher due to its higher demand for oil. So saving Japan from financial ruin and lowering American gas prices are both readily achievable if Japan would only restart its fleet of nuclear power plants!