On Friday, Japan experienced the worst earthquake in its recorded history (in world history, the fifth largest since 1900).
One thing getting a lot of attention is the situation at Japan’s nuclear reactors. 11 plants were shut down in the aftermath of the quake. However, generators one and three Fukushima I have encountering coolant problems post-shutdown, and hydrogen explosions (from vented coolant) have blown off the roofs of the generator buildings (note: not the reactor containment vessels). There are also worries of a meltdown at Fukushima I-2 and reported problems with several generators Fukushima II.)
Fortunately, those were not problems with the shutdown procedure itself, all the reactors were brought sub-critical. However, even with no fusion ongoing, the decay of existent radioactive isotopes releases enough heat to require a functioning coolant system for several days to prevent the fuel rods from melting. (Which would be a disaster: Newer Boiling Water Reactors (BWRs) have tertiary containment designed to contain a full meltdown (a “core catcher”), Fukushima I predates that design.) Unfortunately, venting of coolant steam during emergency cooling can result in the release of some radioisotopes: Some Cesium and Iodine (byproducts from the fuel rods if primary containment (cover on the fuel rods) is breached, Nitrogen-16 (from the oxygen in the water), Tritium (Hydrogen-3; from the decay of Boron-11 or Boron-12, from the boric acid used to suppress the fission reaction), and Carbon-14).
While some prognosticators are predicting none of the 11 reactors will come back online ever (which would mean really interesting things for Japan’s long-term energy situation), I’d bet that all but Fukushima I (and maybe II) will be up again after inspections and repairs. But that’s “relatively quickly” in nuclear reactor terms, so that still means that 20% of Japan’s current generating capacity is offline for months at least.
Further reading: Here’s a lengthy description of the sort of safety devices / procedures implemented at a BWR like the ones at Fukushima I. And here’s a more detailed analysis of the situation at Fukushima I specifically (I can’t verify the author’s identity or expertise, but the article makes some interesting (and specific) predictions; his assumptions about the worst-case scenario are too optimistic, though the post has now been moved to here and edited for correctness).
ETA: I may yet be forced to eat my words. Units 2 and 4 at Fukushima I have evidently also had explosions, and those have suffered actual breaches to the containment. Unit 4 wasn’t running before the earthquake, but it’s still filled with spent fuel. And unit 2 is probably in the middle of a partial meltdown with a ruptured containment vessel.
(Update again: Word now is that the inner reactor vessel is ruptured, not the outer containment. The design goes something like this: Fuel rod, casing, reactor vessel (the inner part of the “double boiler”), containment (the outer part of the “double boiler” and the last layer designed to hold in the core), building (not really designed to keep anything in, mostly there to keep the weather out). The fuel rods and casings are almost certainly damaged in reactors 1-3, the reactor vessel is damaged in 2, and the building is damaged in 1 and 3.)
The disaster is currently rated at INES Level 4 (“accident with local consequences”). Three Mile Island was 5, Chernobyl was 7. Earlier today, Intrade gave 50%, 38%, and 13% odds that it will be raised to 5, 6, and 7, respectively, before the end of March. Now those odds are at 95%, 46%, and 20%.