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Japan Radiation Risks: FAQ

Answers to questions about health risks posed by Japan's damaged Fukushimi Daiichi nuclear power plant.

Is it safe to visit Japan?

 That depends on whom you ask and what areas of Japan you’re talking about.

As of March 30, the U.S. State Department still advised people to defer non-essential travel to a number of regions beyond just the area of the earthquake, tsunami and radiation crisis, including Tokyo and Yokohama.

Other popular destinations, such as Kyoto, Okinawa and Osaka, are “outside the regions of concern,” according to the State Department’s warning. (For an updated alert, go to the State Department's web site.)

On the other hand, experts tell WebMD that they would not hesitate to travel to areas of Japan outside the disaster zone.

“I was supposed to go to Japan next month, but the meeting was cancelled because the organizers understandably have other things to do,” Williams says. “But I was perfectly happy to go, and I’m a bit sad that I’m not because I’ve never been to Japan. I would be absolutely fine to go.”

What does it mean for a nuclear energy plant to melt down?

 "Meltdown" is not a technical term, but it vividly describes the worst-case scenario for a nuclear reactor.

Nuclear reactors generate power via controlled nuclear fission, which occurs when enough radioactive material is gathered into a critical mass. Control rods can separate the radioactive material, thus ending the nuclear reaction.

That's exactly what happened when the March 11 earthquake hit Japan. Control rods effectively halted the nuclear reactions.

This means there is no danger of a nuclear blast from Japan's damaged nuclear plants, even if fuel rods melt inside one of the containment vessels and pool into critical mass. 

But the nuclear materials inside the reactors remain radioactive, which means they give off a great deal of heat. It takes days to cool these materials down, and spent fuel rods must be kept submerged in a cooling bath until their radioactivity decays and their intense temperature goes down.

If not cooled, these materials will melt. In a worst-case scenario, they could possibly melt right through the thick metal shield that contains the reactor, spilling highly radioactive materials into the environment.

When the tsunami hit Japan's Fukushima Daiichi nuclear plant, three reactors lost power. That meant serious trouble, as the pumps needed to cool the overheated reactors with water stopped running when their battery backups were exhausted.

Nuclear reactors have two main containers that keep radioactive materials from spreading outside the plant. One container is the thick-walled building surrounding each reactor. The other is a thick metal vessel that makes up the outer wall of the reactor itself.

As water cools the fuel rods in the innermost container, steam is created. The intense heat also releases hydrogen from the water. If the pressure inside the container gets too high, the steam has to be vented. This releases some radioactivity. It also releases hydrogen, which can build up inside the building.

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