By Fredrik Dahl
VIENNA (Reuters) - North Korea, believed to be preparing for a third nuclear test, would probably be able to make and explode a uranium device for the first time after earlier relying on plutonium, a former chief U.N. inspector said.
If it were to do so, that would show North Korea had developed the technology to produce highly enriched uranium (HEU), putting it in a position to build up larger stocks of weapons-grade material.
"This assumes that the North Koreans have succeeded in producing HEU, in sufficient quantities as well, and have a bomb design," Olli Heinonen said in a paper he sent to Reuters on Friday. (http://tinyurl.com/c8x8etv)
A uranium enrichment facility of the type seen by a U.S. expert in late 2010 could be easily modified to produce HEU, said Heinonen, who headed safeguards inspections worldwide for the International Atomic Energy Agency (IAEA) until 2010.
North Korea, which tested plutonium devices in 2006 and 2009, has almost completed preparations for a third nuclear test, according to a senior source with close ties to Pyongyang and Beijing.
With only limited plutonium stocks, North Korea admitted two years ago that it was working on enriching uranium.
The smuggling network of Pakistani nuclear proliferator A.Q. Khan, which sold nuclear secrets to North Korea and others, had design drawings of a uranium device developed in the mid-1990s, Heinonen said.
And in any case, he said, "due to the fact that they were able to make a plutonium device, they should also be able to make a uranium one."
RAPIDLY EXPANDING ARSENAL
Siegfried Hecker, the U.S. expert who saw the North's enrichment facility, believes the country has 24-42 kg (53 to 95 pounds) of plutonium, enough for four to eight bombs.
But the North's plutonium-based programme at its Yongbyon complex was suspended under a now-defunct 2005 international disarmament deal.
"North Korea's plutonium stockpile is only sufficient to produce a handful of weapons and, given the dilapidated state of its plutonium production infrastructure, producing more would be slow-going and very noticeable," James Acton of the Carnegie Endowment for International Peace said.
In contrast, the enrichment programme appears capable of "significantly expanding North Korea's arsenal," he added.
It is easier to design a bomb with uranium than plutonium, but harder to make a warhead with HEU to mount on a missile.
With IAEA inspectors thrown out, nuclear experts find it hard to assess how far North Korea has progressed in its uranium enrichment programme.
If commissioning of the enrichment site had been successful, North Korea would now have at least 3.5 tonnes of low-enriched uranium, Heinonen said, with a fissile concentration still well below the 90 percent level required for bombs.
To produce weapons-grade material, it could install 1,000 more enrichment machines to convert an annual production of 1.8 tonnes of low-enriched uranium to 40 kg of HEU, he said.
"This is an amount sufficient to generate the necessary fissile material for one to two additional nuclear bombs per year," Heinonen, now a senior fellow at Harvard University's Belfer Center for Science and International Affairs, said.
For North Korea, a test would showcase its technological skills, helping to impress a hard-line military at home and buyers of North Korean weapons, one of its few viable exports.
It would come as Kim Jong-un, the third of his line to rule North Korea, seeks to cement his grip on power.
North Korea has yet to show, however, that it can make a bomb small enough to fit on a missile. A test-launch on April 13 of a long-range missile also failed.
(Editing by Myra MacDonald)