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What next for North Korea's nukes?

The May 25 test provided the world with a host of data about North Korea's bomb-making abilities. But only Pyongyang knows if the test was a success.

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"The missing piece of information is what they were trying to achieve," says Paul Brannan, a senior research analyst at the Institute for Science and International Security.

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Many experts believe that North Korea's nuclear design is an implosion type, as was the Nagasaki weapon. Implosion bombs squeeze a round core of plutonium to produce their explosive power. North Korea is known to have plutonium – an estimated six to eight bombs' worth, produced from spent nuclear reactor fuel.

By contrast, gun-type nuclear bombs need uranium. They create a critical mass by firing one cylinder of highly enriched uranium into another. They are much easier to engineer, but Pyongyang has long denied that it has any highly enriched uranium, despite a few hints to the contrary puzzled out by US intelligence.

The yield produced by the recent test was within the range of scientific expectation for a first-generation implosion bomb, according to Charles Ferguson, senior fellow for science and technology at the Council on Foreign Relations.

"So I believe that North Korea is probably happy with this result ... this is a bomb that can wreak massive destruction," says Mr. Ferguson.

It's still possible that the device did not produce an explosion as big as intended, of course. And a nuclear "device," cobbled together for an underground test, is in any case a less-precise item than an actual warhead, engineered to be small enough to fit on top of a missile.

That is a level of accomplishment that Pyongyang has yet to achieve, Ferguson says.

"There is still considerable doubt that North Korea can field such a weapon on a long-range ballistic missile," he adds.

As to North Korea's plutonium stockpile, it was an estimated 46 to 64 kilograms (101 to 141 pounds) of material prior to the test this week, according to an analysis by the Institute for Science and International Security (ISIS).

Of that, about 28 to 50 kilograms (62 to 110 pounds) were thought to be in a separated form and usable for weapons.

This amount of fissile material would produce one 5 kilogram (11 pound) test device, plus a hypothetical arsenal of three missile warheads, and three heavier bombs.

But is North Korea now gearing up to separate more plutonium? That's a question raised by reports in the South Korean press, which suggest that North Korea may have resumed reprocessing spent nuclear fuel at its plutonium separation plant at Yongbyon.

These reports cite US intelligence imagery showing steam rising from the facility.

In one sense, it would not be surprising for North Korea to fire up the plant again, given that Pyongyang vowed in April it would do just that.

But commercial imagery obtained by ISIS in late May does not show any smoke rising from the chimney of the coal-fired stem plant at Yongbyon, nor any plume rising from the stacks at the reprocessing plant.

"Steam or no steam, we doubt they are actually doing plutonium separation at the reprocessing plant now," says Paul Brannan of ISIS. "It is too soon."