H-bomb or not, North Korea test still great concern: Hecker

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Seoul (AFP)

One of the world's foremost authorities on North Korea's nuclear programmes says speculating on whether Pyongyang's latest nuclear test was of a hydrogen bomb or not, rather misses the point.

"My greatest concern is not so much whether or not they actually tested a hydrogen bomb, but rather that they tested at all," Siegfried Hecker, the former Los Alamos Laboratory director, said in an interview published Thursday in the Bulletin of the Atomic Scientists.

"Since this test worked, they will have achieved greater sophistication in their bomb design – that is the most worrisome aspect," Hecker said.

"This is their fourth test -- with each test they can learn a lot," he added.

Pyongyang's claim to have successfully tested its first H-bomb on Wednesday has been largely dismissed by nuclear experts, who say the estimated yield of around six kilotons was similar to the North's last nuclear test in 2013, and far too low for a powerful thermonuclear device.

Hecker, who has visited North Korea seven times since 2004, agreed it was extremely unlikely to have been a true two-stage H-bomb -- a very sophisticated and difficult device that "is still likely beyond the reach of North Korea".

However, he noted that the true nature of Wednesday's test may never be known given the difficulties involved in retrieving any radioactive release from a deeply buried detonation.

Writing in the same publication, Jeffrey Park, a seismologist teaching at Yale University, said the seismic signals from Wednesday’s test bore an "uncanny resemblance" to the 2013 test.

"The simplest explanation is that ... (it was) a follow-up test of a 'gadget' with identical design," Park said.

Hecker, meanwhile, said North Korea's main focus was probably on miniaturising a weapon that could fit on the tip of a ballistic missile.

In order to demonstrate a credible deterrent, Hecker argued, the North needed to be able to threaten the US mainland or overseas US assets.

"At this point, what makes their nuclear arsenal more dangerous is not so much the explosive power of the bomb, but its size, weight and the ability to deliver it with missiles," he said.

North Korea said Wednesday's test was of a miniaturised device, but expert opinion is divided on just how far Pyongyang has come in mastering the miniaturisation technology.

And Hecker also pointed out that, even if it had the means to mount a warhead on a missile, Pyongyang had shown no evidence of a credible long-distance delivery system.

"North Korea is still a long way off from being able to strike the US mainland," he said.

"It has only had one successful space launch. It needs a lot more, but it has made a large effort in that direction," he added.