By Joyce Lee and Kyu-seok Shim
SEOUL, April 15 (Reuters) - North Korea has made "very serious" advances in its abilities to turn out nuclear weapons, with the probable addition of a new uranium enrichment facility, as it stepped up activity at a key complex, IAEA chief Rafael Grossi said on Wednesday.
Enriching uranium can provide an alternative, and experts say, a more effective, path to acquiring weapons-grade material in addition to reprocessing spent plutonium extracted from a nuclear reactor.
Speaking in Seoul, the head of the International Atomic Energy Agency confirmed a rapid rise in activity at the five-megawatt reactor, the reprocessing unit, a light water reactor and other facilities at the North's Yongbyon nuclear complex.
NORTH'S NUCLEAR PROGRAMME ESTIMATED AT FEW DOZEN WARHEADS
North Korea's nuclear programme was estimated at a few dozen warheads, he told a news conference, citing signs of activity such as the operation of a light water reactor and activation of other facilities besides Yongbyon.
"All of them point to a very serious increase in the capabilities of the DPRK in the area of nuclear weapons production," Grossi said, using the initials of the North's official name, the Democratic People's Republic of Korea.
The watchdog had observed construction of a new facility similar to Yongbyon's uranium enrichment halls, he said, adding that analysis of external features showed a significant expansion of enrichment capacity.
Grossi told a meeting of the agency's governors this month that it was monitoring a new building at Yongbyon with similarities to an enrichment facility at Kangson, another key nuclear site near the capital, Pyongyang.
SATELLITE IMAGERY SUPPORTS IAEA ASSESSMENT
Satellite imagery from April supported the IAEA's assessment, the U.S.-based Center for Strategic and International Studies said on Monday.
It indicated completion of a suspected uranium enrichment plant, capable of producing weapons‑grade material, the centre said in a report.
On Wednesday Grossi said the agency had not seen any evidence of Russian technology being used in North Korea's nuclear weapons programme.
References in a cooperation pact both countries signed last year appeared to be limited to civilian nuclear projects, though it was too early to draw firm conclusions, he added.
"Moving towards nuclear weapons would never give any country increased security," Grossi said, but could instead trigger proliferation.
SOUTH KOREA'S NUCLEAR SUBMARINE PLANTurning to South Korea's programme to build nuclear-powered submarines, Grossi said he invited Seoul to work closely with the agency to avert proliferation risks, with formal talks to begin on the matter.
Naval reactors pose special challenges as nuclear fuel on submarines can go uninspected for long periods during missions.
"It is essential that this activity is not conducive to proliferation of nuclear weapons," Grossi said, adding that the IAEA would seek an "ironclad guarantee" against any diversion of the material.
South Korea's submarine ambitions advanced after President Lee Jae Myung and U.S. President Donald Trump finalised joint steps on trade and security last November, in which Washington approved its ally's plan to build the nuclear-powered vehicles.
(Reporting by Kyu-seok Shim and Joyce Lee; Editing by Ed Davies and Clarence Fernandez)
