So, yeah, this is pretty much why people worry about nuclear energy. Great when it's working, but when it goes wrong,
holy shit does it go wrong. This is an unfolding situation and I still have hope that the heroic efforts of the reactor workers and their helpers will pay off. But many of signs I've seen are pretty catastrophic, and I expect this to get worse before it gets better.
The good news is that even the worst Fukushima meltdown would not be a second Chernobyl. The choice of RBMK reactors for the Chernobyl power plant was incredibly stupid, even for the time, and was a product of even more unbelievably stupid cold war thinking. Virtually all Soviet industry in that era had to theoretically serve dual civilian and military purposes: car plants had to be able to be quickly converted to produce tanks, and nuclear power plants had to be able to produce mass amounts of plutonium for nuclear weapons. Why anyone thought it made sense to apply the same thinking to nuclear weapons production (estimated maximum length of a nuclear war: 2 hours) as conventional weapons production (estimated maximum length of a conventional war: 10+ years), I will never know. But that's hindsight for you.
Because of their stupid design, the Chernobyl reactors were made primarily with graphite, which burns. After the reactor massively spiked (read: exploded), the graphite was exposed to oxygen and ignited into a towering pillar of white-hot flame with a base that surrounded the still-critical reactor, shooting the equivalent of a 12-megaton bomb's worth of radiation into the atmosphere alongside the smoke. Furthermore, the plant was built without a concrete-and-steel vessel that would be capable of containing the radioactivity even in the event of such a meltdown. In this sense, the disaster could have been much worse: by dumping tons of neutron-absorbing material onto the hole in the building by helicopter, the Soviet army managed to cool the reaction (both in the thermal and radioactive sense) before it burnt its way into the groundwater.
(Fun Fact #1: there were actually FOUR reactors at Chernobyl, and the other three continued to operate for years after the accident.)
In contrast to Chernobyl, the Fukushima reactors are water-moderated and built within an intelligently-designed containment structure. Even if the temperatures got so high within the core that the fuel rods melt (which is looking more and more likely), there isn't any graphite to ignite and the containment structure would still have a fighting chance of keeping most of the radioactivity out of the groundwater and atmosphere. In theory, the torus-shaped shell underneath the reactor would spread out the fissionable material sufficiently to keep it from going supercritical and producing so much heat that it would melt through and escape into the ground underneath.
(Fun Fact #2: there is actually a name for the radioactive lava-like substance that is produced when a reactor melts down. It's called
Corium.)
Thankfully, the reactor scrammed (shut down) successfully during the earthquake, and most of the heat in the core is caused not by chain-reacting nuclear fuel but by the remaining decay products from when the reactor was operating normally. As the short-halflife (ie, highly radioactive) byproduct isotopes decay into long-halflife (ie, less radioactive) isotopes, and thus do not decay (and release heat) as often, the excess heat problem will take care of itself. In this sense, it's a race against time that the rector operators will win if they can just hold on long enough.
That said, increasing radiation levels and additional fires and explosions can only mean one thing: the situation basically looks terrible at Fukushima right now, and is only getting worse. The fact that there have been hydrogen-caused explosions in the reactor buildings is actually a way worse sign than the explosions themselves: the most likely cause of hydrogen production under these conditions is the reaction between the casing of the fuel rods with the air at much higher temperatures than normal reactor operation. In other words, the fuel rods inside the reactor appear to have been exposed to the air for some time, at temperatures higher than 1000 degrees...
and the resulting contaminated steam was vented outside the core.
Furthermore, pumping salt water into the cores is an absolutely last-ditch effort to stem a catastrophic meltdown, and is essentially an admission that the reactors will never operate again (at an instant loss of billions of dollars). The additional salt water produces additional contaminated water that either has to be contained or released into the environment, as well as introduces additional (and somewhat unpredictable) elements into the reactor core that can potentially produce additional, dangerous radioisotopes. The fact that salt water cooling appears to have failed at at least one of the reactors is an abysmal sign. There's not all that much more that can be done to save the reactors.
Now here's the real, real shit. As I've said, most Fukushima meltdown scenarios aren't globally, or even nationally, catastrophic: the possibility of true failure is on everyone's mind, but the reactor containment systems are reasonably well-designed and could probably keep a complete meltdown from seriously contaminating the surrounding environment. More worrisome are the on-site cooling pools that house spent fuel and byproducts from the reactor. This waste is flammable, highly radioactive, and largely unprotected - which is to say,
tremendously vulnerable and dangerous in a situation like this.
Because they aren't going to be subjected to the same insane pressures and temperatures of a nuclear reactor core, the cooling ponds aren't protected with the same sorts of containment structures. And because they are home to highly radioactive substances, they require the same sort of cooling mechanisms that have failed at the reactors themselves. The timeframe is longer than for the reactors, but if the pumps remain off for too long and the heat level in the ponds rises sufficiently (particularly if reactor leaks render the area unsafe for work), a massive release of radioactivity is possible. This is before we even begin to consider the possibility that they have been damaged by the (ongoing?) hydrogen explosions.
Anyway, this is an incredibly complicated and evolving issue. I've skipped a lot of background info in this post, and there's still an enormous amount that I don't know myself. Ask questions in the comments, and I'll do my best to answer!
