Twitter is ablaze with news of nuclear meltdown being imminent in Japan and some tweets are along the lines of "OMG! the reactors are going to explode! God save the world". This sounds a little extreme, and I thought I'd share what I found out in extremely non-tech terms for anyone to get an overall "panic-rating" kind of grip on the subject.
The Japanese reactors are light water reactors. Without getting into the scientifics of it, in the words of an expert, Naoto Sekimura, a professor at the University of Tokyo, a major radioactive disaster is unlikely.
"No Chernobyl is possible at a light water reactor. Loss of coolant means a temperature rise, but it also will stop the
reaction," he said.
"Even in the worst-case scenario, that would mean some radioactive leakage and equipment damage, but not an explosion. If venting is done carefully, there will be little leakage. Certainly not beyond the 3 km radius."
IAEA seems to agree on the whole, though they are concerned and actively monitoring.
Nuclear reactors use radioactive fuel to generate electricity. In the process, the fuel gets hot, and much like say, the engine of your car, it needs to be kept cool. This is a big deal, because unlike your car, if this stuff explodes, we can forget using that area of the earth for a long, long time because its radioactive contents will get scattered in the blast. Thus, the fuel is kept at desired temperatures and prevented from overheating. There is massive planning and enginnering around this, with several methods used simultaneously, each capable of cooling the core independently. In addition to that, each method has back ups and fail safes till a mind numbing redundancy is achieved. This is in order to set things up so that once the reactor is in operation, there is absolutely no possibility that there is a failure in cooling it down. The fuel needs to be cooled for a day or two after shutting a nuclear reactor down.
Another factor is the pressure. Evaporating coolant can create high pressures that can threaten the integrity of the containment dome. This can be released by venting, which is a management mechanism and not procedure, since it means that some quanitites of radiation can get released along with the steam. This isn't radioactive materials, but the water itself absorbing neutrons from the cooling process, which are shed off quickly. This radioactivity isn't supposed to last long because the water used is specially demineralized for the purpose, thus making it extremely resistant to this kind of radioactivity. Not that the core cares what cools it, but the water not being radioactive makes it easier for managing the plant.
Sometimes, this system can fail, like it is failing in Japan, right now. When the earthquake happened, the reactors were shut down. This was pretty much instantly. Well before the tsunami. It means the control rods came and fitted in between the fuel rods, so that the ricocheting neutrons had less space to move and less power and less targets, till it would finally wind down and stop in a day or two. This is normal. Nuclear reactions are like that.
And the backup generators took over the cooling since the reactor was no longer producing electricity (and there were back up generators for the ones in operation too). This worked well for about an hour till the tsunami hit and took out all the generators. This was unfortunate, because the cooling system needs constant power. However, the third line of back up kicked in and the generator switched to battery power, which would last for 8 hours or so. Post this point, things seemed to go into chaos. For some reason, they were not able to use the time provided by the battery to rig up yet another power source, and when the battery was exhausted, the reactor started heating up. Without a circulation mechanism for the coolant, the whole thing is overheating and pressure is increasing from the evaporating water.
There are plans to let off steam. US has flown in coolant. Japan has been extremely transparent and proactive in dealing with the exposure to people. The area was evacuated well before any radiation could be found.
My suggestion would be to not panic. Yes, it can blow up, like the US Pennsylvania's Three Mile Island Meltdown. Mushroom cloud and all. Nothing is impossible. The venting could run into problems (though I don't see how). Everything to prevent a disaster could fail. But more likely it won't.
The domes that are so characteristic of nuclear reactors are basically built to contain any meltdowns. You can read about the architecture/engineering of a nuclear reactor facility, but I have, and without boring you with the details, there are vastly reassuring quantities of steel and very, very thick leak proof concrete structure. Its purpose is to contain any explosion/radiation that may occur. And there is no evidence that this integrity is breached.
Japan is a country with a reputation for engineering and efficiency. It has survived the only two atom bombs explosions in the world. I think its fair to say that they aren't going to give this up without a good fight. And, their expertise, ready aid from the world and the inherent safeguards built into every aspect of a reactor are on their side.
So, like the Hitchhiker's guide says, "Don't panic".... if the worst happens, I promise you you will have plenty of time.
Update: there has been an explosion OUTSIDE Japan's nuclear power plant at Fukushima 1. Doesn't seem to be a nuclear explosion, but building is damaged. 2killed, 4 workers injured. Uh... don't freak out just yet. Unlikely that a nuclear blast will result in 4 injuries at ground zero. More likely to do with the pressure building up or steam from some drastic cooling measure or hydrogen exploding from the venting. Let's wait for news. The only thing I am worried about is the radioactive stuff outside the containment - like spent fuel. Building gone means that is exposed, right? Or worse - exploded? But nothing in the news, so obviously I don't know as much as I imagine.
Update 2: Yep, like I said - steam. People within 20km asked to evacuate. Radiation leaking from damaged building. Residents advised to remain indoors, not drink tap water, and to cover their faces with wet towels (? for how long with covered face - but I guess for as long as it takes to get a green signal ?)
Update 3: Early, unconfirmed tweets on mushroom explosion spotted over reactor, but from an Australian, from the look of it. Could be a reaction to earlier blast, or something new? Scientists didn't seem to have invested much belief in the explosion idea for a light water reactor (like these are). Wait n watch - you'll get to panic or breathe a sigh of relief soon.
Update 4: Stray initial tweets about pressure having been successfully released from the reactors, but paranoid Tweeters on and on about "Japanese reactor just exploded, OMG!!!" The links provided are all to the video footage of the earlier explosion OUTSIDE the reactor that damaged the building and *possibly* raised leakage. Nothing remotely like a nuclear explosion has happened yet, nor is it scheduled.
Robin Grimes, Professor of material physics at Imperial College, London
Despite the damage to the outer structure, as long as that steel inner vessel remains intact, then the vast majority of the radiation will be contained.
Professor Paddy Regan, Nuclear Physicist from Britain's Surrey University
"If the pressure vessel, which is the thing that actually holds all the nuclear fuel ... if that was to explode -- that's basically what happened at Chernobyl -- you get an enormous release of radioactive material.
"It doesn't look from the television pictures ... as though it's the vessel itself.
Update 5: For those who absolutely must follow microdetails (like me), a better source is http://www.tepco.co.jp/en/press/corp-com/release/index-e.html you will get all the techy things like timings for different things done, status of reactors, worker accident status, etc.
Update 6: News of problems at reactor 3 at Daichi.