Here are some more resources and updates (as of March 14, 2011, 0700PM EST) about the status of Japanese nuclear plants:
Of immediate concern is the prospect of a so-called “meltdown” at one or more of the Japanese reactors. But part of the problem in understanding the potential dangers is continued indiscriminate use, by experts and the media, of this inherently frightening term without explanation or perspective. There are varying degrees of melting or meltdown of the nuclear fuel rods in a given reactor; but there are also multiple safety systems, or containment barriers, in a given plant’s design that are intended to keep radioactive materials from escaping into the general environment in the event of a partial or complete meltdown of the reactor core. Finally, there are the steps taken by a plant’s operators to try to bring the nuclear emergency under control before these containment barriers are breached.
The Three Mile Island disaster, in 1979, is often described as a “partial core meltdown.” In that case, the reactor vessel, which houses the reactor behind thick walls of steel and concrete, was not breached. The Chernobyl disaster, in 1986, resulted in a rupture of the reactor vessel and the wide dispersal of radioactive particles. However, since Russian reactor design is very different from American (and Japanese), most experts argue that the Chernobyl accident does not offer much information that is useful outside of Russia.
The fuel rod exposure at Fukushima Daiichi number 2 reactor is potentially the most serious event so far at the plant. A local government official confirmed the fuel rods were at one point largely, if not totally exposed; but we do not know for exactly how long. Without coolant around the rods, temperatures can rise to hundreds of degrees Celsius, almost certainly resulting in some melting. This opens the possibility of a serious meltdown – where molten, highly radioactive reactor core falls through the floor of the containment vessel and into the ground underneath. However, engineers appear to have restored some water flow into the reactor vessel and if they are successful, temperatures will begin to fall again rapidly.
The technicians and experts eventually decided that the controlled venting of gases was not good enough and so they flooded the whole reactor with a mixture of seawater (presumably because they’re next to the ocean and it’s available) and boron (sometimes they’ll say boric acid, but that’s just the compound they’re using). Boron absorbs neutrons and is sometimes used to make control rods, so by flooding the reactor with boron they’re essentially increasing the number of control rods to slow down the reactions even more.
They cannot add water indefinitely, however, because that increases the pressure; water becomes steam and if the steam has nowhere to go, the building pressure could damage the reactor vessel. So, they continue to vent gases, some of which goes into containment structures (especially the turbine hall) and some has to go into the atmosphere. A number of processes going on lead to the build-up of hydrogen gas, including the application of heat to water which sometimes splits the atoms apart rather than changing phases to steam.
Also, Craig explains how it is possible to know if a partial meltdown occurred even without a direct access to the inside of the reactor:
As far as anyone can tell, the fuel rods have been damaged to some degree, which makes this a partial meltdown. We can’t know for certain what’s going on inside the reactor until it’s cool enough to physically inspect, but we have a good idea. The main way we know that there’s been fuel damage is the presence of cesium-137 and iodine-131 outside of the plants, as far away as 60 miles. These are byproducts of uranium fission but should not be released in ordinary functioning. The damage to the fuel means that the byproducts can mix with the water in the reactor and are getting dispersed into the air in low doses when the steam is vented in order to keep the pressure down.
Previously on Aphelis: Status of Japanese Nuclear Plants: Facts and Resources (March 13, 2011)
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