10 Critical Questions About Japan's Nuclear Crisis

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What you need to understand about the situation as it unfolds

As Japan's nuclear crisis worsens at the Fukushima Daiichi plant 160 miles northeast of Tokyo more questions than answers have arisen about just how damaged the plant is and how much danger residents are in. This seesaw story has swung rapidly between peril and promise and back again, or as a colleague of mine once said, between no hope (catastrophe) and new hope (it's under control).

The challenge for the press and for government, seldom achieved in this crisis thus far, is to sound an appropriate alarm for those who are at greatest risk -- workers and local residents -- while calming those at little or no risk. (The challenge is intensified when instant global communication turns everyone into an observer, reporter, and worrier all at once.)

So how worried should we be about the unfolding disaster at Fukushima? Here are some key questions to ask in the hours and days to come.

1. What is the current state of damaged reactors at Fukushima Daiichi?

This has been a moving, and confusing, target and it's been hard to plot a trend line. Tokyo Electric Power Co. reported that fire had broken out again early Wednesday morning in Japan (Tuesday evening in the U.S.) in the building housing reactor 4. A hydrogen explosion had also occurred earlier at reactor 2. That brings the number of crippled units to four of the plant's six reactors. As with the pot that boils over on one burner of a stove when you're stirring another, the initial concerns focused on units 1 and 3, in which hydrogen explosions occurred over the weekend and had seemed somewhat more stable. But new concerns about reactor 3 containment have now arisen. Units 4, 5, and 6 were not operating at the time of the earthquake, but experts are now worried about storage of pools of spent nuclear fuel rods heating up.

2. Is this more like Three Mile Island or Chernobyl?

While the question is unavoidable (and I've been asking it myself, having covered TMI), we still can't answer it yet. The cascade of reactor crises, and uncertainty about bringing them under control, has increased the degree of concern that Fukushima could become a more serious catastrophe than it originally appeared. As Matthew Bunn of Harvard's Belfer Center for Science and International Affairs pointed out: "This is the worst nuclear accident since Chernobyl, and in some respects worse than Three Mile Island (TMI)."

At TMI, operators managed to avoid a hydrogen explosion and there was only a modest release of radiation as a result. In the case of Japan, there have been hydrogen explosions that damaged buildings at the plant and signs of significant radioactive releases (reports conflict about the amounts)., According to an international rating scale, Chernobyl (which involved a completely different type of reactor without a containment vessel and resulted in a significant radioactive release) was ranked 7 out of 7, while TMI was ranked 5. Initially, Japan's nuclear crisis was given a preliminary rating of 4, but this is sure to change over time once the outcome becomes more clear.

3. What do we know about possible "meltdowns" involving Fukushima's reactors?

The short answer is: very little. At least "partial" meltdowns (which occurred at TMI) may have occurred in several reactors. The key issue is the degree to which crucial containment vessels housing the nuclear fuel cores remain intact. There were fears that an explosion at reactor unit 2 may have damaged the containment vessel, which increases the risk of release of hazardous radioactive materials. While reactor 4 was shut down at the time the earthquake occurred, there is concern there and at the other reactors about the pools containing used or spent uranium fuel rods. Again, it is all about cooling and containment and it ain't over yet. Scientific American has a good online package, while this New York Times graphic shows what would happen in a meltdown.

4. What's happening with the water?

Ironically water--or lack of it--has been the real story at Fukushima for the past four days. The nuclear cores need water to cool them down, and the tsunami swamped Fukushima and initially cut off electricity powering the cooling systems. Then various backups failed, which forced plant operators to pump sea water into the reactors to try to cool them down. The Times initially reported that helicopters might be used to drop water on the pools of spent fuel that are too hot. (Later the idea was discounted.) In short: follow the water.

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Cristine Russell is a senior fellow at Harvard Kennedy School of Government and the president of the Council for the Advancement of Science Writing.

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