North Korea said on Monday, Oct. 9, it conducted a successful nuclear weapons test, forcing an emergency meeting of the U.N. Security Council. Experts discuss the alleged test, proposed responses and implications for the region. CISAC visiting professor Siegfried Hecker talked with PBS's Margaret Warner about North Korea's nuclear technology.

MARGARET WARNER: And . . . we go to Siegfried Hecker. He's the former director of the Los Alamos National Laboratory. He's twice been to North Korea. And he joins us now from Stanford University, where he is a visiting fellow at its Center for International Security and Cooperation.

Mr. Hecker, thank you for joining us.

How persuasive do you find the evidence or the signs that, in fact, this explosion was nuclear?

SIEGFRIED HECKER, Former Director, Los Alamos National Laboratory: Good evening, Margaret. Thank you for having me.

At this point, it's still very early to tell. Until we get a full analysis of the seismic signals, it's difficult to tell. But what we know at this time is that the estimates from the scientific stations close to North Korea would peg this at perhaps half-a-kiloton to one-kiloton yield, in which case I would certainly put it into the realm of a real nuclear explosion, although quite of low yield.

MARGARET WARNER: Now, if we say it's a nuclear explosion, what does that really mean? Does that mean they have a nuclear bomb, or is it some other sort of device?

SIEGFRIED HECKER: Well, one sometimes differentiates between a device and a bomb of the bomb being deliverable, and that is it's actually made it to a delivery vehicle, in other words, being able to be put into a plane or put on a missile. But the bottom line is, at one kiloton, it would be a bomb. One kiloton set off in Manhattan would be a catastrophe.

MARGARET WARNER: And when you say "deliverable," would something of the nature that -- let's say it's one kiloton, for the purposes of argument. Is that deliverable in some way that is less sophisticated than a missile, for example, on a tanker?

SIEGFRIED HECKER: Well, it's not at all clear, you know, how sophisticated their device is. One would have expected them to test something that's quite primitive on the order of, let's say, the plutonium bomb used in Nagasaki. There we're talking about 10,000 pounds. Perhaps they've made it somewhat smaller than that, but nevertheless that's more for deliverable on a tanker or perhaps a big plane, not on a missile.

MARGARET WARNER: Now, explain to us how there can be such a disagreement about even the size of this explosion. As you said, I think the South Koreans are saying, it's about a kiloton. I think the Australians, that was their estimate. The Russians were saying five to 15. What are these different seismologists looking at? And how can they come up with such different analyses?

SIEGFRIED HECKER: I haven't seen any of the actual data, but from the scientific networks, one has heard half a kiloton from South Korea, one kiloton or so from Australia, and one kiloton from France. The Russian message was delivered by a politician and not by a scientist. I'm not quite sure how authoritative that is.

At the half- to one-kiloton, that would not be that unusual a variation. And, of course, since what we're talking about in a seismic signal is we're listening, and that means the closest listening point would have the best opportunity to pick up the signal, and that would be South Korea. But in all fairness, we still need to wait a few days until all the data can be analyzed.

MARGARET WARNER: Now, that's what U.S. officials have been saying, that it will take intelligence analysts several days to figure out what really happened. What will they be looking at? What evidence will they have that they don't have now?

SIEGFRIED HECKER: Well, it's just a matter of doing the analysis of the seismic signal itself to make sure that one understands the seismic signal and whether one can tell where there's some specific indication as to whether this might have been, indeed, a nuclear test. So analyzing the signal would be the first important aspect.

The second one is, you have to translate the seismic signal into what the energy of the explosion was -- in other words, what the yield was --and that takes a good understanding of the geology of where the test was actually conducted.

And so it would not be surprising that scientists would differ, and so I would expect scientists to compare their notes, analyze the seismic signal, use their models to make a yield prediction.

MARGARET WARNER: Now, if it were one kiloton, is it feasible, is it possible that that could be done by just thousands of tons of conventional explosives?

SIEGFRIED HECKER: That gets on the high side, but it still would be possible to do.

MARGARET WARNER: It would?

SIEGFRIED HECKER: Yes.

MARGARET WARNER: Now...

SIEGFRIED HECKER: But at one kiloton, we would be looking at something that really looks more like a nuclear explosion than a conventional. Of course, the other aspects, what one could look for is whatever evidence we may have had ahead of time as to what actually went into the areas where the nuclear explosion took place.

MARGARET WARNER: Now, finally, how good, how developed is their missile technology now?

SIEGFRIED HECKER: I'm sorry. Could you repeat that?

MARGARET WARNER: How well-developed is the North Koreans' missiles technology? Their last test was in July.

SIEGFRIED HECKER: Right. Of course, the missile tests are a very different issue. The question is, you know, could they have developed a device that actually fits on a missile? My own opinion of that is that, whereas they could have some reasonable confidence in a primitive device, and that is a big device of the order of Nagasaki, it would be very difficult to have confidence in a small device that you can fit on a missile.

I would also find it surprising, if they tested such a small, you know, miniaturized advanced design first. And so in most likelihood, at least what one does in mirror imaging, is that it was a primitive device at low yield. There's no indication at this point whether they've done anything with an advanced design.

MARGARET WARNER: All right. Siegfried Hecker, thank you so much.