GWEN IFILL: We take a closer look now at some of the questions raised about the state of Japan's damaged nuclear reactors.
I'm joined by NewsHour science correspondent Miles O'Brien and David Brenner, director of the Center for Radiological Research at Columbia University.
Miles, I will start with you.
What is typical? What is supposed to happen inside of a nuclear reactor like this? And what is happening in Japan right now?
MILES O'BRIEN: Yes, let's do a little bit of boiling water reactor 101, shall we?
GWEN IFILL: Yes.
MILES O'BRIEN: Let's go -- go and look at a little graphic here to help people understand.
We have heard them talk about the fuel rods, for example. These are shafts of zirconium. Inside are little pellets of uranium that are encased in ceramic. Those rods go inside this reactor primary containment vessel. That's kind of the last line of defense.
You don't want that ever to breach. And we should be very clear that we have no indication that those primary containment vessels at those three stricken plants, reactors in Japan, have been breached at all. Now, what happens is, these rely on water for cooling. And these rods need to be bathed in water.
When there's a seismic activity, when there's some sort of earthquake, an accelerometer immediately begins what's called a SCRAM -- interesting acronym. It shuts down the plant in a quick manner. And you need additional power, auxiliary power, in order to keep the water over those fuel rods and keep them cool enough.
Now, let's look at the next line of defense, which is an important point as well, because this is what has been breached in those two explosions. The reactor is inside the primary containment vessel, and then there's that secondary containment vessel. What happened in this case was there was venting of hydrogen into the secondary structure -- think Hindenburg for just a moment -- a spark, and an explosion occurs.
And that is where we have seen, very likely, most of the release of the radioactive -- we have release of cesium, release of iodine. We will show you an image, by the way -- this comes from Digital Globe -- which gives you an idea.
On the left-hand side of the picture, that's one of the vessels which is still contained. And in the middle, you see one that has had the explosion because of the hydrogen, which exploded. So, at this point...
GWEN IFILL: Once -- once again, that's the roof and the walls, but not the containment -- not the nuclear reactor itself.
MILES O'BRIEN: Exactly. Just to be clear, that primary containment vessel remains intact. That's very important.
GWEN IFILL: So, when we talk about this kind of -- people -- we use the term meltdown very loosely as we talk about things like this.
MILES O'BRIEN: Yes.
GWEN IFILL: Do we have any indication of degree of damage that has been done when these rods were uncovered by water?
MILES O'BRIEN: Well, I have talked to quite a few experts today. And what they tell you is that some things have melted. The question is, has the fuel itself melted? Probably the rods, the zirconium, have. As a matter of fact, there's no doubt about that.
But has it reached the point where the fuel begins melting and becomes this molten mass, which creates that scenario we have heard in the movies, the China Syndrome. I don't know what you would call it on this side of the planet, the Cleveland syndrome or something.
Whatever the case may be, in theory, it becomes so hot, that it would melt its way out of this primary containment chamber. We haven't seen that yet, but this is still a very tense time. They're pouring in seawater into these vessels, as best they can, to keep it cool. Now, that means they have written off these plants. You will never be able to use them again. The seawater is modulated with boron to make sure the neurons aren't active.
And that means the plants are a write-off. And the question is can they continue keeping this seawater in at a proper level to keep the temperatures safe?
GWEN IFILL: And is what we just saw in those pictures, is that what we have here in the United States? Is it the same kind of setup?
MILES O'BRIEN: Well, basically, our structures are a little beefier than this.
And one important point which I should tell everybody is that the diesel fuel tanks for the auxiliary generators which keep the water pumping are all buried here in the United States. These tanks were above ground, for reasons that a lot of engineers can't fully understand.
This is, after all, a seismically active area, the Ring of Fire. And the Japanese, after all, invented the term tsunami. So, the fact that they had fuel tanks of diesel to run these generators, this last-resort generator, above ground has people mystified.
GWEN IFILL: And without the power, you can't keep it cool. And that's the problem we're facing tonight.
I want to turn to David Brenner and -- and ask for your assessment of, how serious a situation is this, potentially?
DAVID BRENNER, Columbia University: Well, it's very hard really at this point in time to give a clear answer to that question.
We -- we have some parameters that we can think about. And there have been two major nuclear power plant disasters in the world. There was the one in this country at Three Mile Island. There was the one at Chernobyl in the old Soviet Union.
And just to put perspective on those two, Chernobyl was the equivalent of a million Three Mile Islands. So, really, the question is are we closer to a Three Mile Island situation, or are we closer to a Chernobyl situation?
GWEN IFILL: And what's the answer to that question?
DAVID BRENNER: And I think the answer...
GWEN IFILL: Yes, go ahead.
DAVID BRENNER: I think -- I think the answer to that question is clearly that we're closer to a Chernobyl situation.
And if the event was to stop right now -- I'm sorry. I said that exactly wrong.
DAVID BRENNER: We're clearly closer to a Three Mile Island situation.
If the -- if the incident was to stop right now, the amount of radioactive releases is really very small. And the risk to the general population, just like it was in Three Mile Island, would be very small indeed.
GWEN IFILL: Now, understanding that you...
DAVID BRENNER: Of course, we await...
GWEN IFILL: I'm sorry. Pardon me.
Understanding that you don't, we don't -- none of us know tonight exactly how much radiation may have been released, we have heard some reports -- and we saw in some of the earlier taped pieces -- pieces people being wanded and checked for radiation exposure -- do we know, have any sense about how serious that might be?
DAVID BRENNER: I think we have a good idea at this point in time that the radiation exposure to the general public -- and I'm not talking about the nuclear power workers, but to the general public, at this point is clearly very small.
The radiation risks, correspondingly, are very small. The issue with the nuclear power workers, on the other hand, the people inside the plant, is very different. I think there's pretty good evidence that there have been some high-dose exposures to those folks, the folks who are actually fighting the -- the situation inside and are trying to get the water into the nuclear fuel.
I think we know there are some high doses associated with those folks.
GWEN IFILL: One of...
DAVID BRENNER: But, for the general population, so far, I think the doses are low.
GWEN IFILL: One of these reactors went online in 1971. Is the age of the plant significant in this case?
DAVID BRENNER: I think it is. I think it's actually key to the -- to the whole scenario.
You know, that's 40 years ago. The actual lifetime of this reactor was scheduled to be 25 years. And, really, what we see is, as you have just heard, that the backup systems were really not as good as they should have been. And, as time has gone on over the years, the newer plants have better and better and more and more backup systems in place.
This plant actually had only one backup system, the secondary generators. And when they failed, there was nothing. And that's not the case with any modern nuclear reactor. So, it is crucial that this was a 1971 machine.
GWEN IFILL: So, as we look over -- watch this for the next several days and watch to see whether the cores cool down, do -- what are we looking for? What are the signs we're looking for to find out whether things are getting better or -- or getting worse?
DAVID BRENNER: Well, there are really two issues which are going to determine the -- the public health significance.
And that's just how much releases we get in the next couple of days. And the other point actually is the wind. The wind is absolutely central here. Which way is the wind going to below? And among all the awful things that are happening here, the one bright sign is that -- is that the wind is actually blowing offshore.
So, the wind is actually taking whatever radioactive materials are being deposited in the atmosphere and moving them offshore, to the sea. We very much hope that that meteorology actually continues, and we continue to have offshore winds. That will really ameliorate the whole situation.
But it's the winds and how much radioactivity is actually emitted from these reactors.
GWEN IFILL: So, give me a best-case scenario and a worst-case scenario.
DAVID BRENNER: Well, the best case scenario is that, in the next day or so, they get the -- they get enough water into the fuels, and there's very little more release than we have right now.
And then we're going to be in a -- basically, a Three Mile Island situation, where, although there were releases, they were so small, that there was no good evidence of any public health risks to the surrounding population. So, that's the best-case scenario.
The worst-case scenario is, in fact, that there was -- there is a significant radioactive release in the next couple of days. And I think the next 48 hours is really crucial here. We still hope that most of the plume will get blown to sea. But the worst-case scenario is that the wind would change and the plume is starting to approach the population, which, as you know, has been evacuated some miles away now from the nuclear plant.
But, still, the plume will reach the population if the wind is blowing in the right direction.
GWEN IFILL: Of course. And we will be watching all of that.
David Brenner at Columbia University, Miles O'Brien, NewsHour science correspondent, thank you both very much.