The past couple of pages have revealed a lot of misunderstandings, misinformation and out-of-date information on all sides. I’m going to try and clarify a few things, for what it’s worth:
The reactors are each contained in big steel pressure vessels - basically thick-walled steel boilers. Water fills these “nuclear boilers” and leaves through a pipe as steam, passes through turbines to generate power, passes through a condenser (a tube cooled on the outside by seawater) to turn it back into liquid water, and returns back to the reactor. This is a closed, sealed loop and in all the Japanese reactors IT HAS NOT BEEN BREACHED. The reactor fuel is still sealed in heavy metal pressure vessels. At Three Mile Island all cooling was lost to the reactor and the fuel rods melted down to liquid and slumped to the bottom of the pressure vessel, which must have been glowing red-hot for days, but nothing escaped the pressure vessel. Then the pressure vessel itself is in a Containment Building which is also fully sealed and super-strong. Which leaves the big question, why did any radioactive material escape at Three Mile Island? Why has any radioactive material escaped in Japan?
From one point of view, just letting a Three Mile Island-style meltdown occur in the first place is actually the safest thing to do. Drain all the water and let the thing rip. The containment will have to remain sealed off for decades and the plant is a write-off but at least nothing gets out.
Generally people will try and put fires out rather than let them burn themselves out in a safe manner, and reactors are no different. The reactor operators prevent a meltdown by circulating cooling water through the reactors and removing the heat. Unfortunately in Japan, the multiple redundant cooling systems were either not functional or had limited functionality after the quake and tsunami. (It’s not just that the emergency power-generating diesels were swamped: - there were two other emergency cooling systems that were powered by steam pressure (of which there would remain plenty for days) and one driven by direct-drive diesels independent of the power diesels. A LOT of stuff had to punk out for what we’ve seen to occur.) The problem comes with water and a heat source you can’t turn off, together in a sealed system.
The reactor operators have been trying to keep the reactors cool but their ability to circulate water has been limited or absent. Instead they’ve been letting the reactors cool by boiling off the cooling water. This means of course that the steam pressure builds up in the vessel and loop, so the steam has to be periodically vented to the outside world. At first this steam was only radioactive from neutron activation: it didn’t contain any radioactive material from the reactors but had itself been made radioactive due to neutron bombardment while passing through the fissioning reactor. Venting this steam is a release of radioactive material, but not a dangerous one. Its radioactivity only lasts a few tens of minutes. If you have plenty of ultra-clean water to replace that being boiled off, you could do this for some time. However, the boiled-off coolant water has reportedly been topped up using seawater, which might mean the ultra-pure make-up water tanks were cracked by the earthquake or swamped by the tsunami. Seawater is full of salts and suspended solids which can be activated by neutrons much more readily than the water itself. This is a smaller issue than you might think though because the reactors are no longer fissioning - there’s a not a lot of neutron radiation inside the pressure vessels any more.
Unfortunately the boiling-off method of cooling has not been successful in keeping the reactors immersed in water. The water level dropped in all three reactors, “exposing” the fuel rods. (Of course they were still contained in the pressure vessel and Containment Building.) This let the fuel rods overheat and almost certainly rupturing their zirconium claddings. Exactly how hot the rods have got isn’t known - they may have just warped a bit and split their claddings, or they may have melted at their tops and dripped liquid zirconium like wax into the bottom of their pressure vessels. Hence all the arguments about whether “meltdown” has occured or not. Nobody knows. All we really know is that the closed-loops of the pressure vessels still hold pressure so they haven’t leaked, and that the water-level instruments in the pressure vessels are still giving readings.
The problem with the ruptured fuel rods is that now, other radioactive elements can enter the cooling water, and the steam still needs to be periodically vented, carrying those elements with it. Fission by-products like iodine, cesium and strontium, which have appreciable half-lives and are biologically available. They can become incorporated into people’s bodies (e.g. strontium 90 can substitute for the calcium in our bones) and remain there, irradiating cells and acting as carcinogens. This of course is a Bad Thing but you should keep in mind that we already have radioactive elements incorporated into our bodies, e.g. potassium, and so quantity is still important. A small amount of strontium 90 incorporated into your bones can still be insignificant compared to what’s already there. This is important to keep in mind since we can identify strontium 90 and other isotopes in miniscule amounts by the gamma-ray spectra of their decay, and you WILL be detecting radioactive strontium, iodine etc. from this incident in the USA and you WILL be eating and drinking it. In tiny amounts, but it will happen. Read about bananas and keep a sense of perspective: http://chemistry.about.com/b/2010/03/08/bananas-are-radioactive-2.htm
As it happens, the radioactive steam wasn’t vented directly into the atmosphere. It was bubbled through the wetwell first - a donut-shaped, half-full tank of water, which scrubs a lot of the radioactive material out of the steam. They could also try to time the venting operations for when the wind was blowing out to sea. Finally, venting wasn’t directly into the air but into the top section of the reactor building, for reasons I don’t know. It may have been an effort to contain the radioactivity by allowing the steam to condense on the steel sheet walls and run to the storage pool floor, limiting the contamination to the reactor site. This failed rather spectacularly when hydrogen in the steam (possibly generated by chemical reaction between steam and the fuel rods’ zirconium cladding) explosed and blew the top parts of three of the reactor buildings apart. Another explosion in the wetwell of reactor 2 has stopped it from holding pressure and means that any steam venting from reactor 2 is probably no longer being scrubbed by the wetwell.
So: to re-iterate, all the reactors are still entirely contained within their pressure vessels, which are still entirely contained within their Containment Buildings (drywells), which are themselves contained in somwhat damaged Secondary Containments (the concrete reactor buildings.) All radioactive material releases FROM THE REACTORS have been a result of venting steam from the boiled-off water being used to keep those reactors cool.
