미국의 권위 있는 신문 시카고 트리뷴은 오늘 '과학자들은 북한의 핵실험이 그들에게 실망스런 것이었으리라고 추정한다'고 보도했다. 이 기사에서 미국의 MIT 工大의 핵전문가 짐 월시는 '북한은 그들이 만들려고 했던 핵폭탄을 만드는 데 성공하지 못한 것 같다'고 말했다. 이 신문은 핵실험의 폭발력이 1000t(TNT기준) 이하라고 해서 핵실험이 아니라 폭약을 쓴 가짜 실험이었을 것이라고 추정하는 건 무리일 것이라고 했다.

일부러 소형 핵폭탄을 만들어 실험했을 가능성은 더 약하다. 왜냐 하면 그 기술은 큰 핵폭탄을 만드는 것보다 더 어렵기 때문이다. 이 신문은 북한 과학자들이 폭탄의 설계를 잘못하여 起爆을 시킨 뒤 플루토니움의 연쇄반응이 제대로 이뤄지지 않아 당초 계산보다도 훨씬 약한 폭발이 되었다는 것이다. 폭탄으로서의 실험은 실패라고 하더라도 이번의 실패 경험을 잘 살리면 다음 실험에선 성공할 수 있기 때문에 완전실패라고 볼 수는 없다. 적어도 연쇄반응까지는 도달했기 때문이다.

이 정도의 실력으로는 실용성이 극히 낮은 큰 핵폭탄은 아직 만들 실력이 아니고 미사일에 실을 수 있을 정도로 소형화시키는 것은 더 먼 미래의 일이라는 점에서 核과학자들의 의견이 거의 일치하고 있다.
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Still, the appearance for now is that North Korea has not succeeded in making the bomb they wanted, said Jim Walsh, a nuclear proliferation expert in the security studies program at the Massachusetts Institute of Technology.





Scientists suspect North Korean nuclear test was a disappointment
By Jeremy Manier

Chicago Tribune

(MCT)

CHICAGO _When North Korea detonated what it said was a nuclear device last Sunday, the blast set off seismic sensors as far away as Wyoming and Norway, and sparked a round of urgent study by scientists around the world.

But determining just what caused the seismic spike is such a delicate art that after five days of intense work analysts still cannot say for sure whether the test was a success or a dud - and there is a remote possibility the blast was not nuclear.

The job requires analysis of data drawn from hundreds of monitoring stations worldwide, including seismic monitors so sensitive that they can detect rumblings from mine cave-ins halfway around the world. Getting an accurate read on the bomb's power is complicated by the fact that some forms of seismic waves are detectable only at a great distance.

One seismic clue the scientists may look for is whether the bomb caused its underground cavern to collapse. That could be one sign of a significant nuclear blast, said David Albright, president of the Washington-based Institute for Science and International Security.

Monitors are also watching air samplers that can pick out radioactive particles or gases released by a nuclear blast, though those devices are subject to the whims of the prevailing winds out of North Korea.

Scientists may not know the true nature of the explosion for weeks, if ever, in part because North Korea has a history of trying to conceal high-profile technological missteps.

But the effort to determine whether the bomb failed is urgent because it could help the United States and its allies weigh the immediacy of North Korea's nuclear threat. Given the available evidence, it appears North Korea still has 'a lot of homework to do' before it can menace far-away countries, said Charles Ferguson, a fellow for science and technology at the Council on Foreign Relations.

'The message to American citizens is, `Don't lose sleep that tomorrow North Korea will launch a long-range missile with a miniaturized missile that will land on Chicago,'' Ferguson said.

The first round of blast analysis has come from seismologists - a group whose entire research field is a partial outgrowth of the Cold War nuclear arms race. When nuclear tests went underground in the 1960s, seismic effects became an essential way to detect them.

'Seismology grew fairly slowly until the advent of underground nuclear testing,' said Bill Leith, an earthquake hazards specialist with the U.S. Geological Survey.

Despite initial confusion over the North Korean bomb's seismic effects, independent measurements by the U.S.G.S. and scientists with the Comprehensive Nuclear Test Ban Treaty Organization suggest the explosion registered about 4.1 or 4.2 on the Richter scale. The data from the test ban organization, headquartered in Vienna, is considered revealing because it includes readings from large seismic arrays attuned to detect nuclear explosions.

Those seismic readings indicate the explosion's force was less than one kiloton, or a thousand tons of TNT. By comparison, the atomic bomb that destroyed Hiroshima in 1945 was about 15 kilotons.

Distinguishing the North Korean blast from an earthquake was simple, experts said. All explosions, whether nuclear bombs or mine blasts, carry a seismic signature - a burst of strong compressional waves, followed by weaker readings that seismologists call shear waves. Earthquakes, in which massive amounts of rock slip past each other, emit strong, sustained shear waves.

'To mitigate earthquake hazards, we have to be able to separate earthquakes from mine blasts,' said Harley Benz, head of the USGS national earthquake center in Denver.

In addition to seismic disruptions, the test ban treaty organization also is trying to detect radionuclides, which are byproducts of the fission reaction inside a nuclear weapon. But much of that work depends on which direction the winds near Korea are blowing, said Lassina Zerbo, director of the treaty organization's Vienna-based International Data Center.

'We have to see if the wind is blowing toward our stations,' Zerbo said. 'If not, we would need more days for the particles to disperse.'

It's possible the blast was from conventional explosives, but experts say that's unlikely given the huge amount of explosives that would have required. Even a one-kiloton explosion, though small by nuclear standards, would be massive for a conventional blast.

North Korea may have intentionally made a nuclear weapon with a small yield, but experts say that's also unlikely. Such weapons can pose a greater technical challenge than a larger, Hiroshima-scale bomb.

The reason stems from the design of the plutonium-based bombs that North Korea is thought to be developing. Such bombs consist of a central, grapefruit-sized core with about 13 pounds of plutonium, surrounded by a basketball-sized shell of high explosives. The explosives must detonate with exquisite timing to collapse the plutonium and create an explosive nuclear chain reaction.

That precise sequence is difficult to pull off with a large weapon, but even harder with a small nuclear device. Besides, experts said, a large blast would have been better political theater.

Even so much as 'a stray neutron' could have started the weapon's chain reaction too soon and resulted in a dramatically smaller explosion, said Albright of the Institute for Science and International Security. But Albright said he thinks North Korea would learn to correct such problems.

Still, the appearance for now is that North Korea has not succeeded in making the bomb they wanted, said Jim Walsh, a nuclear proliferation expert in the security studies program at the Massachusetts Institute of Technology.

'It's hard for an objective observer not to conclude that this program is not as sophisticated or as free of problems as some have suggested,' Walsh said. 'And that's good news.'

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© 2006, Chicago Tribune.