A satellite image taken in May of tje Pyongsan Uranium Concentration Plant near what is believed to be North Korea’s largest uranium mine. (Centre National d’Études Spatiales/AP)

North Korea has been stepping up its capacity to mine and mill uranium, new satellite imagery shows, raising fears that Kim Jong Un’s regime is trying to expand its stockpile of nuclear weapons.

The images show that a major mill that turns uranium ore into yellowcake, a first step toward enriching uranium, has recently been refurbished, said Jeffrey Lewis, director of the East Asia nonproliferation program at the Middlebury Institute of International Studies at Monterey.

“Pyongyang appears to be modernizing a key facility associated with the production of uranium yellowcake,” Lewis wrote in a new report for 38 North,a Web site run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies. The facility is near a uranium mine outside Pyongsan, in the south of the country near the border with South Korea.

“This suggests that North Korea intends to mine and mill a significant amount of uranium that could serve as fuel for expanding its nuclear weapons stockpile,” Lewis wrote. However, he added that the fuel could also be used in light-water reactors, which generate electricity, which North Korea may be planning.

As with all reports about North Korea’s nuclear program, the latest study is impossible to verify. But Lewis is a respected nonproliferation expert, and other recent reports have also suggested a renewed uranium processing push.

North Korean leader Kim Jong Un. (Kcna/Reuters)

In a separate report, IHS Jane’s, a defense publication, said its analysis of satellite imagery suggests that North Korea is now running a second hall of uranium-enrichment centrifuges at its Yongbyon fuel fabrication plant, north of Pyongyang.

After conducting its third nuclear test in February 2013, North Korea rebuilt and restarted its five-megawatt plutonium-producing reactor at Yongbyon and doubled the size of its known uranium-enrichment facility there. Uranium is the essential fuel for nuclear reactors that produce plutonium, and it can also be enriched to produce fissile material for nuclear weapons.

Diplomatic efforts by the United States and by North Korea’s neighbors to persuade North Korea to give up its nuclear weapons program have ground to a halt, with Kim Jong Un insisting that his country be accepted as a nuclear-armed state.

Since succeeding his father at the end of 2011, Kim Jong Un, the 30-something third-generation leader of the world’s only communist dynasty, has been promoting “byungjin” — a dual-track approach that contends North Korea can advance its economy and its nuclear program at the same time.

American officials have negotiated with North Korea for years on the premise that these things are mutually exclusive.

But due to creeping marketization, the North Korean economy appears to be growing modestly; South Korea’s central bank estimated it expanded by 1 percent last year. And the latest satellite analysis suggests that North Korea is making progress on the nuclear front, too.

Using commercial satellite imagery, Lewis analyzed the layout of the uranium mine and mill near Pyongsan, believed to the most important in North Korea.

The mine is connected to the mill by a conveyor belt that brings uranium ore into the mill for processing. The mill is connected to a large pond where tailings, the waste products of uranium processing, are dumped.

While it is difficult to estimate how much uranium has been processed, Lewis was able to conclude that North Korea seems to be accelerating uranium production.

“Since 2013, most of the buildings have received new roofs. The terminus of the conveyor belt was demolished and rebuilt,” he wrote in the report. “Other buildings appear to have been gutted and are now in the process of being rebuilt with new roofing. The significant investment in refurbishing the mill suggests that North Korea is expecting to process significant amounts of uranium, either from the Pyongsan mine or other uranium mines.”

The uranium produced at Pyongsan could be used at North Korea’s main nuclear site at Yongbyon, the focus of American denuclearization negotiations. The site houses a five-megawatt plutonium reactor, used for producing electricity, but the rods could also be processed to weapons grade. Analysts have also speculated that it could be home to a uranium-based nuclear weapons program.

IHS analysts noted that the snow on the roof of a second centrifuge hall at Yongbyon had melted over the winter, suggesting that heat resulting from operational centrifuges was coming from inside the buildings. The second hall probably started testing in January, 2015, and may have become operational by early February, their report said.

Like Lewis, the IHS analysts said North Korea could be processing uranium for electricity production or to produce highly enriched uranium for nuclear weapons.

Washington continues to try to encourage North Korea to resume negotiations to end its nuclear weapons program, holding up last month’s agreement with Iran as an example of the kind of deal that foes can broker.

"The Iran deal demonstrated the value and possibilities that negotiations bring," Sydney Seiler, the U.S. envoy on the North Korean nuclear issue, told reporters in Seoul last month. He urged Pyongyang to "choose a different path." North Korea has said it's not interested in an Iran-style deal.

Concerns about the North’s nuclear program come amid new tensions between North and South Korea following an incident in the Demilitarized Zone between the two countries last week.

Two South Korean soldiers were seriously injured when they stepped on land mines on the southern side of the border. One soldier lost both legs, while the other lost half of one leg. The South has accused the North of deliberately planting the mines on a known patrol path.

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