Fukushima's nuclear disaster: Worse than Japan thought?

A new report from Tokyo Electric Power Company (Tepco) paints a surprisingly dire picture of the damage at its Fukushima Daiichi nuclear power plant. Just how bad is the situation, and what is Japan going to do about it? Here, a brief guide:

What does Tepco's report say?

It's now clear that three of the six reactors (not just one, as originally thought) experienced at least a partial meltdown — and odds seem high that, in each reactor, molten fuel rods breached the containment vessel, letting radiation leak into the environment.

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How much radiation has leaked out?

It's not clear yet. But if the containment vessels were breached, even partially, more radiation could leak from Fukushima than Chernobyl. Still, a worse-than-Chernobyl leak remains unlikely, says Tepco general manager Junichi Matsumoto. Most, but not all, of the molten fuel is believed to have stayed inside the containment vessels. And in Reactors 2 and 3, the rods remained under water, which should reduce the damage.

Where does this information come from?

It is based on Tepco's recently completed computer simulations of the disaster — the plants are still too dangerous to enter. (The reactors got as hot as 3,000 degrees Celsius — 5,400 degrees Fahrenheit — at one point, but they are now at a stable 100-170 degrees C, and slowly dropping.) Several outside experts have also come to similar conclusions.

What happens next?

A team from the International Atomic Energy Agency arrived in Tokyo Tuesday to conduct an investigation into what went wrong at the Fukushima plant and what lessons can be drawn to make nuclear plants in Japan and elsewhere safer. Japan also approved its own independent investigation into the government's handling of the disaster. That inquiry will be headed by Yotaro Hatamura, an engineering professor who has made his name studying failure.

Sources: New York Times, Bloomberg, Wall St. Journal, Daily Yomiuri, Washington Post