greyautumnrain said I should post about the Fukushima reactor accident. Right now, what seems relevant is a comparison with other major commercial reactor accidents, namely Three Mile Island and Chernobyl.
At Three Mile Island, neither the primary containment - the vessel and pipes holding the reactor and its cooling water - nor the secondary containment - the building housing the primary containment - was ever breached. There were small releases of radioactive gas to relieve pressure, mostly of noble gases that, like the radon in many homes, isn't dangerous unless it's confined. A little of the radioactive release was water soluble iodine, but water soluble radioactives also dilute widely in the environment and so little was released that it didn't measurably increase background levels of radioactivity near or far from the plant. The expected number of radiation deaths was less than one. It was only a disaster for the power company that owned the plant, and only from the financial standpoint of losing a major investment.
At Chernobyl, both the primary and the secondary containment were completely destroyed. In addition, the reactor was graphite moderated, and the graphite caught fire, causing all of the fission products in the reactor to be released as particulates. These particulates spread in a plume to the northwest across Russia, northern Europe, and Scandinavia before falling out of the atmosphere; they included a lot of insoluble radioactives that would stay concentrated in amounts large enough to be dangerous - and the total amounts were also huge. Estimates for the number of deaths range from the thousands to the hundreds of thousands.
Fukushima is a water cooled reactor, so at least its core won't burn up in a firestorm like Chernobyl. However, the accident is clearly already worse than Three Mile Island, with detectable increases in radiation outside the secondary containment. In fact, part of the secondary containment of the number 2 reactor was destroyed by an explosion early on - possibly because the operators were initially denied permission to vent the plant. It's likely that much of the gaseous and water soluble fission products in the core will eventually be released - probably enough for a significant health effect. However, it's likely that most of the insoluble fission products will stay on site, so it won't be the huge public health disaster that Chernobyl was. While the current focus is on the nuclear plants, it's likely that the other deaths and damage from the earthquake will remain much larger.
Moral of the story: use pressurized water reactors like Three Mile Island, which have the thickest metal in their pressure vessels and the rest of their primary containment, use secondary containment buildings that can withstand a few atmospheres of internal pressure, as with the one at Three Mile Island, and when someone at a nuclear plant asks to vent the plant, even though it will release a little radioactivity, let them do it.
Personally I wouldn't eat any food from Japan for a few months at least, until any radioactive Iodine could decay, though that's not really an issue for me as I don't generally eat anything from Japan in the first place. If you frequently eat sushi from Japan, pelagic fish are probably still fine, since the ocean is a pretty big place for dilution.
I would not expect much in the way of longer term issues as have resulted from Chernobyl as most of the strontium and cesium will likely stay on site. However, that's a preliminary assessment as they don't yet have things under control and the situation could get worse. I'm a little concerned that the use of seawater for direct cooling could have some unpredictable effects, and also about any smoke that escaped from the zirconium fires in the spent fuel pools.
It doesn't appear that radioactives will be carried to the upper atmosphere, which is good since a Chernobyl style accident would probably have resulted in a lot of fallout in the northwestern U.S. given the jet stream routes. In this case, the only way that could happen would be if the concrete catches fire.
It depends on just how much damage the earthquake and tsunami did. Continuing to run the plant would require an operating power turbine and generator, and I understand that the tsunami flooded out electrical gear, located in the lower levels of the plant. That's what caused the failure of the diesel generators that normally provide power when the plant is shut down, and it likely would have prevented the plant from operating too, so continued operation of the reactors was probably not an option.
Just wanted to add some pictures and videos.
Picture and video of unit 2 explosion:
Picture and video of unit 3 explosion:
Both were reported as hydrogen explosions, but I think the unit 2 explosion looks more like a steam explosion.
Edited at 2011-03-17 05:11 pm (UTC)