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Of the three nuclear tests that North Korea has conducted, the latest one is by far the most powerful. The weapon could be in the range of the bomb that devastated Hiroshima during World War II. It's much harder to verify North Korea's claim that it tested a miniature nuclear weapon. If true, that would mean the country is closer to putting a nuclear warhead on the tip of a missile. NPR's Richard Harris explains.

RICHARD HARRIS, BYLINE: It's much easier to make a large nuclear bomb than a compact one. So countries trying to start a nuclear weapons program tend to begin by making large devices. Jeffrey Lewis at the Monterey Institute of International Studies says big bombs aren't that useful as a threat.

JEFFREY LEWIS: It doesn't do the North Koreans much good if they have to put it in a truck.

HARRIS: The threat level obviously rises as the bombs get more compact. And James Acton, at the Carnegie Endowment for International Peace, says that's apparently, the news with this third North Korean test.

JAMES ACTON: KCNA, the Korean news agency, has come out and said it was a smaller and light design, I believe were the exact words that they used.

HARRIS: It may be possible to glean more information about that claim if gases from the underground test have escaped into the atmosphere. Sniffer aircraft will scoop up air samples that might be used to learn more details about the design of the explosive.

ACTON: Even if it's possible to infer some design details, any conclusions are only likely to be fairly tentative. So although I do think it may well have been a miniaturized device, we actually may never know.

HARRIS: The United States undertook hundreds of nuclear tests before developing reliable small warheads. But Acton says the North Korean program has been designed from the start to produce a compact weapon. That means it could have made some real strides, even though this is just the third test.

ACTON: There's also the possibility that the North Koreans have received outside assistance. That could make their task of producing a miniaturized warhead easier.

HARRIS: And even if it has produced a compact warhead, North Korea doesn't yet have the ability to strike the United States with it on an intercontinental ballistic missile. They have demonstrated an ability to launch missiles into space, but so far, North Korea hasn't shown that it can build a warhead that is rugged enough to re-enter the earth's atmosphere at hypersonic speeds and blistering temperatures.

ACTON: Not only do you have to have a weapon that's light enough to fit in the nose cone and small enough to fit in a nose cone, you also have to have a re-entry vehicle and design combination that is durable enough to survive the rigors of re-entry.

HARRIS: That said, North Korea may have the ability to threaten its neighbors with a warhead on a shorter-range missile, which moves much more slowly and can carry a bigger warhead. Corey Hinderstein is at a nonprofit called the Nuclear Threat Initiative.

COREY HINDERSTEIN: Some of the missiles that they have, the shorter range missiles, they might only be looking at a payload of about three feet, in which case we'd have to believe that that's doable.

HARRIS: And North Korea could build a handful of weapons. It is apparently no longer producing plutonium, and there's probably enough on hand for about a dozen warheads. Some of that plutonium could have been used in the latest test or...

HINDERSTEIN: The real interesting question is whether this used highly enriched uranium.

HARRIS: It's harder to build a compact nuclear warhead with uranium, compared with plutonium, but it is possible. And it's possible to enrich uranium in secret. No reactor is required.

HINDERSTEIN: At least publicly, they have declared that they are not producing any highly enriched uranium for nuclear weapons, that all of their interest in uranium enrichment has been in support of their nuclear energy program.

HARRIS: Hinderstein says if the sniffer aircraft detect a uranium bomb, that would be a very significant finding about the North Korean program.

HINDERSTEIN: It doesn't necessarily change their capability, but it may change the number of weapons they could have at their disposal in the near future.

HARRIS: She says it's nothing to panic about, but clearly with each advance the situation becomes more uncomfortable. Richard Harris, NPR News.

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