First a very over simplified primer on how a nuclear "reactor" works so people can understand what we are dealing with here.
So...the way a nuclear power plant works is really pretty simple taken in broad strokes. It uses Uranium "fuel" to make heat, that heat boils water to make steam, the steam drives turbines much like fancy "Steam engines" and those steam engines drive generators which make electricity.
There ya go, you are now a nuclear power plant expert...lol..well..it's easy in broad strokes as I said, much like saying something like, "Put wings and a jet engine on it, grab the steering wheel and boom you're a pilot"...the actual real life operation is far more complicated, but the general underlying concept is just as simple as I put it.
So how and why does a nuclear power reactor work?
First the fuel, which is the central topic of discussion regarding the recent events. The fuel for the Japanese reactor and most all nuclear power plants on earth is Uranium. More specifically "Enriched Uranium" Uranium is a naturally occurring "metallic chemical element" found in the earth and we mine it pretty much like other metals, however as we all likely already know, uranium has some special properties.
Naturally occurring uranium is about 99.25% Uranium-238 isotopes or parts...this is more or less the dead stuff, the junk. It is also about 3/4% Uranium-235...this is the "good stuff" which creates the nuclear fission reaction. The beauty of U-235 is that it creates this fission reaction all by itself. It's a naturally fissionable isotope. What this means in simple terms is that hunks U-235 will "fight" with other hunks of U-235 by firing atomic particles at each other blasting off other atomic particles in a small explosion known as "Fission". That firing creates more fissionable "pieces" which get blown off and then those pieces blast into U-235 atoms which in turn creates more teeny explosions and carrying on the "chain reaction" known as fission...and that fission creates heat...lots of heat. All this takes place at the atomic level and it happens all by itself or naturally.
You get enough U-235 in some big pieces and slam them into each other?...you get a "critical mass" also known as a Hiroshima Bomb. You get some smaller pieces together to "fight"...you get nuclear fuel rods...pretty simple really and all it takes is "enriching" the level of the reactive U-235 to a level in the uranium metal where it can fight at the level you want. Enrich Uranium to about 2-5% you have nuclear fuel...enrich it a lot higher to say 90% or more?...you have "Weapons grade Uranium" for an atomic bomb. So now you know the entire core principal behind nuclear energy. Simple to say, but much harder to actually do, but that process is for another discussion....it's also quite dangerous as you have likely surmised, not only because those radioactive isotopes are deadly to humans, but also because that once the fission process starts?...it's virtually unstoppable...in fact the entire process system in a nuclear power plant is not really about starting the atomic fission at all, it's mostly all about controlling what happens naturally.
Just like sky diving isn't really about the falling part, that's easy, it's about the stopping part, lol, being under control, because if you don't control the falling part with a parachute?...well we all know what happens at the end of the sky dive ride down...Sudden contact with a medium sized planet and you die...much like a "melt down" in a nuclear power plant...it's the end of the line.
So now that you are a nuclear physist...put your knowledge of atomic science to work. One popular way is to build a nuclear power plant and make electricity. Simple concept as I laid out above, but the real life operation is very complicated. So to make it easy to understand, this all you need to do.
You have your nuclear fuel. Most commonly nuclear fuel is made into "pellets" or slugs of enriched uranium. Those pellets are inserted into long metal tubes and those tubes are then assembled into an array and that's your nuclear fuel "bundle" As you now know left to it own nature the fuel rods would be fighting as soon as they were near each other in the bundle. To prevent and control that natural fission you need "control rods" Control rods absorb the atomic particles fired back and forth by the U-235 fuel, they prevent the fission reaction by keeping the atomic particles the U-235 pellets fire off from reaching the other U-235 pellets.
Most commonly Control rods are graphite. So you insert your control rods between the nuclear fuel rods and you have a stable bundle...you remove the control rods?...the Uranium pellets begin launching particles and creating heat. So now you are in control of the whole process...you remove the control rods just a little bit?...you make a little heat. You remove them more?...you make more heat, simple.
So now you are going to put that heat to use in a nuclear power plant. You take your fuel and control rod bundle and put them in a vessel that can hold water, much like a pressure cooker. You put that "pressure cooker" in a containment building for safety, add water via various pipes and plumbing and close it off. Then use your atomic fission process to boil that water into steam and that's all there is to it. Use the steam to drive an "engine" or turbine and put it use...power an aircraft carrier or spin a generator and make electricity to sell. All in a neat self contained package that generates virtually unlimited heat energy until the U-235 metal runs out of particles to blast each other with, which is years. All to do with as you please....in theory...
So now that you know the entire nuclear fuel cycle it's easy to see why atomic power is so desirable even aside from the weapons aspect. Nuclear fuel rods will last for years and make a lot of usable energy, as long as they don't get out of control and break out of their atomic cage, which is what has apparently happened at the Fukushima reactor in Japan...
So what did happen and is happening at the crippled reactor in Japan...well the reports coming out aren't all very clear, but some things MUST have happened to create what we have seen and know as fact...and likely will see coming.
So as we all know Japan suffered a massive earth quake and the resulting tidal waves. This sent the nuclear power plants into emergency shut down mode...in essence slamming the control rods into the fuel bundle and "shutting down" the nuclear reaction. However you cannot just turn off a nuclear power plant like you shut off a water faucet...it's far more complicated than that. There is a lot of residual heat left in your nuclear reactor vessel and fuel bundle and it will stay hot for quite awhile. You must cool that entire assembly down via your plumbing...if you don't? the parts will burn and melt...and if the tubes holding the fuel pellets melt or if your control rods burn away?...the nuclear genie gets lose...and the naturally occurring atomic fight begins again...only now it's all broken free into a mess...a "melt down" and you aren't going to be able to put it all back into a neat package again because it's firing deadly particles all over the place in a wild rampage...causing more heat, breaking more U-235 fighters free..all getting worse and worse in a chaos of uncontrolled fission run amok.
If you cannot stop it?...the result is all your nuclear fuel and just about everything in contact with it is all melted into a super hot wad that will not stop it's fission process until it runs out of it's particles to happily fire away at itself and everything else around it. The resulting mess of nuclear fuel will create so much heat that it will melt everything in it's path...including the earth...which is where the term "China Syndrome" comes from. The theory that the meltdown wad could not be stopped until it melted through the earth all the way to China, it's debatable at best, but it illustrates the point pretty well too, that the chaotic fission mess mass is going burn so hot that once it gets going?...nothing known to man is going to stop it. As bad as that sounds?...well...it gets worse...
The whole time the U-235 mess is firing sub-atomic particles at itself and blasting away?..it's also breaking off other atomic hunks as nuclear waste...deadly to humans nuclear waste, not the least of which are radioactive Cesium and Iodine. So the whole mess is not something you want to let happen, in fact it's about the worst possible scenario that can happen during the operation of a nuclear power plant. So now you know what happens in a meltdown, so how close is this reactor to a full scale meltdown...
Well we know this...radiation has been released into the atmosphere. That is an incredibly bad sign because it means your once neat process is experiencing some very bad problems and it's beginning to bust lose, at least. The operators of the power plant decided to release radioactive steam in an attempt to decrease the too high pressure in the cooling system which was being over heated because the real cooling system and it's first line of defense backup system was broken down. The primary due to loss of power, the second line of defense due to the tidal waves knocking out the diesel generators. They were running on a limp along 3rd tier battery backup system that was only designed for the short term. Aside from possible other problems created by the historic earthquake and tsunamis that system seems to have been not capable of operating the cooling system long enough to sufficiently cool down the core to a safe level all by itself...and the plant operators ran out of time. The core began to overheat for whatever reasons, but there is no doubt that is what has happened and in fact we have been graciously allowed to hear of "cooling difficulties" by the "Mostly Shit Media" and the "No problemos doodsan" gov't officials.
The containment structure has exploded and no longer exists. There are not many ways that can happen as explosives are not kept in a nuclear reactor building. The virtually assured way that explosion did happen is that the cooling water was exposed to the nuclear fuel. The Uranium "cracked" the water molecules into the base elements of Hydrogen and Oxygen...as we all known hydrogen is explosive. Enough hydrogen accumulates and boom...Hindengurg bomb explodes in the containment building and destroys it as we have seen.
This is not only incredibly bad because it reveals a very seriously dangerous runaway nuclear condition is likely occurring, but it also removes a layer of protection between all of us humans and the "Core" which direct exposure to is deadly. You cannot have explosive gas being created in the containment building if the reactor core is still 100% intact, venting maybe, but not the latter...it's a very very bad sign.
At this point you can count on this. There is nuclear fuel exposed to what and where it not supposed to be. If and when that nuclear fuel is exposed to the atmosphere it not only spews forth deadly radioactive particles is also burns even hotter because it reacts with the oxygen. The more it burns the worse it gets, the worse it gets, the more will burn itself free repeating so on so forth in a runaway chain reaction and if it cannot be stopped?...the end result is a complete meltdown.
As bad as that is, this particular situation gets even worse...there is more than one reactor at this site...in fact there are four and we are already hearing about "cooling problems" in reactor number 2....
Any explosion at a place as complex as a nuclear power facility is going to create problems and further damage to necessary systems...and I don't care how well it designed against that happening, some damage is going to be done...even under the best conditions and obviously the conditions as a result of the disasters are nowhere near "best". There is radiation being bled off, how many nuclear engineers are going to stay on site and deal with the increasing problems under not only the hazards presented by the atomic reactors themselves, but all the problems created by the massive earthquake and Tsunamis. Obviously a lot of the tools those people would have under even standard "emergency conditions" are gone. There aren't too many tools left and some of those are going to deteriorate as time goes by.
They can to try to vent off steam and have done that...unfortuantely for us humans that steam is radioactive, not something you want to shoot into the air. They can basically poison the reactor with a fluid that "should" stop the fission reaction...I don't believe it's ever been done before and how that could be done with all the problems they are experiencing? I really do not know.
Considering that even the problems which they have admitted to are very high on the "Holy crap this is bad" scale? I myself would take into consideration that things are very likely worse than they are willing to admit, and they have admitted only after the detection of certain radioactive elements that their reactor has been "partially melting" they had virtually be "caught" into admitting even that. The presence of the hydrogen gas is also a very tell tale sign that the reactor core itself is very hot and that they may indeed be facing a virtually unstoppable nuclear monster.
They are already calling this the second worst nuclear disaster/accident in history, and it's far from over IMO and in the opinion of many experts as well. It's already bad enough with radioactive material being unleashed into the air...if it gets as bad as it could and gets even near a full blown "meltdown"?...it'll will get worse on an exponential scale with vast amounts of radioactive materials being spewed into the air and blowing around the world...and we all know what that will cause..the big C for cancer...it will only be a matter of how much and how bad at that point, but "no big deal" won't even be close to any terms used to describe that result.
The Stable Iodide pills (KI) are already being distributed locally, I would suggest that anyone who even thinks for a second that they are going to be in an area that's going to get any radioactive exposure at all that they get and take some too, they are really pretty harmless and won't hurt you if you take the proper amount and don't take them for years and years...I know I have some on hand, they are cheap and effective...I suggest everyone look into them.
The Japanese are performing a very delicate dance here guys...with an extremely fierce, erratic and deadly dance partner...and it's not Kabuki theater this time, it's very real...and if they trip....well...the results aren't going to be mere broken bones, it's going to be much much worse...so wish them luck...because we all need it on this one....
Some extra reading materials:
[link to english.kyodonews.jp
[link to en.wikipedia.org
[link to www.epa.gov