Source: NBC

BRIAN WILLIAMS, Anchor: And we have more now on this nuclear risk. And beyond the immediate area in Japan, a lot of Americans are asking a lot of questions. To that end, with us here in our New York studio tonight, physicist James Acton with the Carnegie Endowment.

Thank you very much for being with us. Let's talk about the risks and consequences, because we are starting to throw around terms like meltdown. What does that mean in terms of danger to humans, both in the area and a great distance away?

JAMES ACTON, Carnegie Endowment for International Peace: Well, good evening, Brian. And when Americans have been looking at the wind maps over the Pacific and they see that the wind blows from Japan to the United States, it's understandable that they get worried. But what they have to remember is two things.

Firstly, as the radiation gets taken and blown out, it gets more and more dilute. And secondly, we are also subject to radiation the whole time. It's in the air, it's in the rocks around us. So whereas this is a very serious situation, the White House has announced today that based on its modeling it doesn't believe there's a threat to people in the United States, and I have no reason to doubt that conclusion.

WILLIAMS: And when you hear they're pouring seawater on a core to cool it down, that's obviously the last resort. They've decided this is just going to become a hot tomb. For how long does that just have to stay inert, untouched?

ACTON: Well, a lot is going to depend on the scale of damage in the core. But you see, as soon as the reactors were shut off, they continued to produce heat. And over the last three days, the engineers running the plant have been in a desperate race to cool those reactors down. And the news for reactors one and three looks like they might, and I stress might, be coming through. But reactor two still appears to be in significant trouble.

WILLIAMS: So a worst-case scenario here, if we lose one, two or, God forbid, all three of these, this remains a deeply local crisis; but thankfully, in a hyper-prepared nation where people have gotten away from the immediate area and now must stay away for a long period of time, I'm guessing.

ACTON: I think that's right. I mean, if there is--if there is substantial core melting, then there is the risk of a large release of radiation into the environment. There's no certainty of a large release of radiation. At Three Mile Island there was extensive core melting and relatively small release of radiation into the environment.

But if there is a large release of radiation into the environment, then the Japanese really have done everything they can. Everybody has been evacuated from around the site, so the immediate injuries due to radiation should be relatively low. The real long-term problem, if there is a large release of radiation, is in long-term illnesses, particularly cancers.

WILLIAMS: All right, James Acton with Carnegie, thank you very much for answering our questions tonight.

ACTON: Thank you.