From a retired nuclear engineer US based consultant (if anyone knows his/her name, please give me a tinker).
“First of all, yes, this is very serious, for one reason because there are multiple reactors involved. There are 10 reactors total in that locality; 6 older ones on one site (Fukushima I, units 1 through 6) and 4 larger, newer ones (Fukushima II, Units 1 through 4) sited on property 7 miles to the south. All are located on the oceanfront in order to use sea water for cooling the (non-nuclear) turbine-generators. It looks to me as if the Fukushima I plants are located on a peninsula that sticks out into the ocean. All of the reactors are a type known as BWRs (Boiling Water Reactors). The number of reactors and the similar names are currently confusing the hell out of US television and press reporters.
“At the time the earthquake struck, Fukushima I Units 1, 2, and 3 were operating (the others on that site were shut down for maintenance) as well as all 4 units at Fukushima II. All would have been equipped with seismic triggers which would have scrammed (rapidly automatically shut down) the reactors as soon as the earthquake registered. All shut down normally and their multiple, redundant emergency diesel generators were started as a precautionary measure. Then, within the hour, the tsunami struck. I don’t know for sure, but I doubt any of the plants were designed for an 8.9 earthquake followed by a 33 foot tsunami.
“Nuclear power plants are big machines and require a lot of power to run their pumps and equipment. Under normal operation, power to run the plants is taken off their own turbine-generators. If the plants shut down, the power is taken off the national grid. If the grid fails, large installed backup, redundant emergency generators start up. If the emergency generators fail, there are big emergency batteries, but they only last for about 8 hours and cannot operate big pumps. As in all nuclear plants, there are multiple routings for the power, to guard against single failure problems and make sure the power gets to where it’s needed.
“When a reactor is shut down, the fission process stops, but for several days afterwards significant “residual heat” is given off by the decaying fission products trapped in the fuel. This is normal, and the plants have (multiple, redundant) shutdown cooling systems to deal with removing the unneeded heat, which could damage the fuel if left un-cooled. In a perfect storm of seismic-related events, the shutdown cooling system(s) were damaged (or maybe plugged by debris) and the emergency diesel generators flooded and rendered inoperable for multiple plants at Fukushima by the tsunami. Because of the ruined roads, portable generators and replacement batteries could not be brought in.
“Perhaps due to the lack of power, the operators could not get sufficient cooling water into the reactor immediately after shutdown, which is when the residual heat is at its greatest. The reactor(s) continued to heat up, boiling the water already in the reactors into steam. With no operable main turbine generator to send the steam to, the steam was deliberately vented, per design, into the “containment buildings” which house the individual reactors. This steam, coming as it did from inside the reactors is generally mildly radioactive. These containment buildings are not the massive concrete pressure domes most common for US reactors, but weaker metal-framed concrete-sided designed to shield the reactors inside and prevent any contamination of the outside areas. If only mildly radioactive steam was in the containments, the steam could be filtered and released to the atmosphere and diluted to insignificance. Very slight offsite radiation would be noticed, but nothing dangerous.
“At some point, the water level probably dropped to below the top of the cores, over heating the zirconium alloy tubes that contain the fuel in the core. At 2200 deg F, the alloy begins to break down and one of the products is a lot of hydrogen gas. If the temperature in the core reaches 4000 deg F, the fuel in the tubes will also melt. This is the so-called “meltdown” scenario. Fukushima I, Unit 1, was the first to reach the limit of 2200 degrees. In my opinion, it was probably hot enough to breach the tubes and damage the fuel inside. Now the steam contains explosive hydrogen, and potentially highly radioactive debris, so it cannot be filtered and vented to the atmosphere and the pressure begins to climb in the containment building. At 2.1 times the design pressure, a spark or spontaneous combustion ignites the hydrogen in Unit 1 containment, resulting in the massive explosion seen on television and completely destroying the outside shell of the building. Real bad news, although the reactor is not exposed or damaged by this explosion. The reactor itself is contained inside a 6-inch thick stainless steel “containment vessel”, but it still is not getting enough cooling water. To prevent a complete meltdown situation, the operators have begun to use a fire pump to pump seawater into the core to continue the cooling. This is the nuclear equivalent of a “Hail Mary” pass, because they have run out of other options. Seawater will render the reactor unrepairable. They are sacrificing the plant in order to minimize the potential for wider exposure.
“It remains to be seen if the cooling efforts will be sufficient. Some offsite radioactive contamination in the local area is unavoidable now. A meltdown is possible, but not likely. Breaching of the 6 inch thick reactor vessel and also the 6 inch thick “containment vessel”, which would release large amounts radioactivity into the local environment, is very unlikely. A simultaneous fire, like the one that widely spread the debris after Chernobyl, is not really credible. A huge catastrophe, nonetheless.
“Very unfortunately, the same scenario is developing for Fukushima I, Unit 3. They have core damage, hydrogen production, and possible partial fuel melting as well. They are venting hydrogen to their containment building with the same potential for a hydrogen explosion. They are also now cooling the core with seawater. To make matters worse, Unit 3 is apparently fueled with Mixed-Oxide Fuel (MOX), a mixture of uranium and plutonium fuel. If this reactor core is breached, the presence of plutonium could bring the contamination problem to a whole different level.
“Three of the four larger reactors at Fukushima II are also experiencing similar cooling problems, but perhaps due to newer designs, seem to be under better control. I haven’t seen any reports of core damage from there.
If you want to keep up on what’s really happening, someone is doing a good job of describing the problems on Wikipedia, under Fukushima Nuclear Power Plants.”
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