If President Trump actually meets Kim Jong-un in the next few months — an encounter that many American officials still doubt will come to pass — his challenge will be much larger than merely persuading North Korea to give up its nuclear weapons.

Mr. Trump must also get Pyongyang to give up the factories, reactors and nuclear-enrichment facilities that produce the nuclear fuel needed to build more weapons — even as new satellite evidence suggests that North Korea is expanding its production.

Feb. 25, 2018 N Main entrance Emissions from stack Reactor Control room Turbine generator building Reactor entrance New reactor Electrical yard N Main entrance Emissions from stack Reactor Control room Turbine generator building Reactor entrance New reactor Electrical yard Image from DigitalGlobe via Jane’s Intelligence Review

The satellite image above shows a new North Korean reactor that appears to be coming online now, after years of construction, according to analysts. It sits in the Yongbyon nuclear complex, where the North began its nuclear program in the 1960s. Today, the site boasts hundreds of buildings that lie along a loop of the Kuryong River and cover an area of more than three square miles.

North Korea insists the reactor is intended to produce electricity for civilian use. But the new reactor can also make plutonium, one of the main fuels used in nuclear arms. It can thus supplement the output of the aging, existing facilities at Yongbyon.

Making bomb fuel in reactors is seen as easier to do than perfecting missiles that can hurl nuclear arms around the globe. While experts clash over how soon the North will develop warheads that can survive the blistering heats of re-entry, they agree that the North has already mastered the art of using reactors to make plutonium.

Factoring in Nuclear Fuel

The new reactor could be a central issue in the Trump-Kim talks, if the goal, as the United States insists, is complete denuclearization. Even if Mr. Kim agrees to a freeze on nuclear and missile testing, he would still be able to accumulate more bomb fuel for a larger arsenal as long as the negotiations dragged on.

This was a critical issue in the Iran negotiations, where President Barack Obama negotiated a freeze on new production of significant quantities of new nuclear fuel, though it expires in 13 years. It is unclear whether Mr. Trump could extract a similar halt in production from North Korea.

But if the talks fail, or simply drag on, the reactor could also be part of the justification for military action — at least if the past arguments of Mr. Trump’s newly appointed national security adviser, John R. Bolton, prevail. In March 2015, just before the Iran deal was struck, Mr. Bolton argued in a New York Times op-ed that neither negotiations nor sanctions would stop Iran from bolstering its nuclear and weapons programs. He has since made similar arguments about North Korea.

“The inconvenient truth is that only military action like Israel’s 1981 attack on Saddam Hussein’s Osirak reactor in Iraq or its 2007 destruction of a Syrian reactor, designed and built by North Korea, can accomplish what is required,” Mr. Bolton wrote. “Time is terribly short, but a strike can still succeed.”

Before and after the announcement of Mr. Bolton’s appointment last week, the National Security Council did not respond to several requests for comment on the evidence that North Korea’s new reactor is being started up. Mr. Bolton assumes his new role on April 9.

An Increase in Activity

The image of the North Korean reactor above, from Feb. 25, shows what look like emissions from a smokestack. That suggests that preliminary testing may have begun at the new reactor, according to a report by Jane’s Intelligence Review and the Center for International Security and Cooperation at Stanford University. The plant is called the experimental light water reactor.

It has the potential to make 25 to 30 megawatts of electricity, enough to power a small town. The plant could also potentially produce about 20 kilograms of weapons-grade plutonium each year, according to the Institute for Science and International Security, a private group in Washington that tracks nuclear weapons.

This would be more than four times the amount made annually by the North’s only other large reactor, which has long supplied the country with plutonium for its nuclear arsenal.

Imagery analysts at Stanford found that activity around the new reactor increased significantly in 2017, suggesting that the North has been rushing toward its full operation.

N New reactor Personnel on pathway N Vehicle and objects at reactor entrance N Personnel, equipment and vehicles at reactor entrance N Vehicle and objects at reactor entrance N Shadow of crane Tower crane at reactor entrance Image from Airbus DS via Jane’s Intelligence Review

Throughout 2017, analysts observed what appeared to be major work to complete a river cooling system, shown below, for the new reactor.

Nov. 26, 2017 N Existing reactor New reactor Transport road Small dam for reactor’s water intake Cistern Possible hot water discharge line Cold water intake channels River cooling system for new reactor Embankment Kuryong River N New reactor Transport road Small dam for reactor’s water intake Cistern Possible hot water discharge line Cold water intake channels River cooling system for new reactor Embankment Kuryong River Image from Airbus DS via Jane’s Intelligence Review

Analysts also found some evidence that could support North Korea’s assertion that the new reactor would be used for power generation. Satellite images appeared to show that power lines and a transmission tower had been erected around the site.

“There are a number of objects that have been put in place that lead me and a number of experts to the conclusion that this might be used for production of electricity,” said Allison Puccioni of the Stanford team. She cautioned against assuming that North Korea sees the reactor as a way to make more fuel for nuclear weapons.

But the potential is there, and the North long ago banned international inspectors who carefully monitor what happens to the plutonium in used reactor fuel.

Oct. 28, 2017 N Existing reactor New reactor Power transmission tower Kuryong River Power line N New reactor Power transmission tower Power line Kuryong River Image from Airbus DS via Jane’s Intelligence Review

A History of Nuclear Development and Talks of Denuclearization

North Korea first began operating a nuclear reactor in the 1980s at Yongbyon, according to declassified C.I.A. documents and a report by Siegfried S. Hecker, a former director of the Los Alamos laboratory in New Mexico who has visited the Yongbyon complex multiple times. The image below highlights not only the two reactors but the plutonium reprocessing plant, where the North mines spent reactor fuel for the precious radioactive metal that can power nuclear arms.

Jan. 17, 2018 Existing reactor New reactor Kuryong River YONGBYON Plutonium reprocessing facility Fuel-fabrication and uranium-enrichment facilities Image from DigitalGlobe via Institute for Science and International Security

In 1986, North Korea began operating the five-megawatt reactor, which some analysts say has produced the nation’s entire supply of plutonium.

A 60-foot cooling tower, which carries waste heat away from the old reactor by emitting steam, was one of the most visible parts of the nuclear fuel operations at Yongbyon.

N Existing reactor Steam from cooling tower Dredging Dredging activity in river Steam from cooling tower Dredging activity in river Steam from cooling tower Dredging activity in river Steam from cooling tower Dredging activity in river Image from DigitalGlobe via Google Earth

After six-nation nuclear talks in 2007, North Korea agreed to shut down all facilities at the sprawling Yongbyon complex, and in 2008, the 60-foot cooling tower, one of the most visible reactor structures, was demolished. Video of the event was broadcast around the world.

“As a gesture of good faith,” said Ms. Puccioni, “they destroyed the cooling tower, ostensibly to show the world that they are no longer going to use the five-megawatt reactor.”

In truth, the destruction of the cooling tower was mostly a symbolic step that did little to undo the vast enterprise at Yongbyon.

In 2010, satellite imagery showed signs that the North was beginning construction of a new reactor.

N N Construction began for new reactor New roads More signs of construction Additional structures Footpint of reactor dome Buildings completed Footprint of reactor dome Additonal structure Piping Reactor dome completed Building completed Construction appears completed A new fence or wall Image from DigitalGlobe via Google Earth

By 2013, the exterior of the new reactor appeared to be completed, and activity around it was relatively stagnant after that, according to the Stanford group.

Dr. Hecker, with two other nuclear engineering experts, wrote in the Korea Observer in 2016 that North Korea was still developing the technology needed to start the reactor.

Over roughly the same period, the country began taking steps to get its old reactor running again, despite earlier promises to abandon the plant.

In 2013, satellite images revealed a new trench connecting the reactor to the Kuryong River. It would become part of a new cooling system to replace the destroyed cooling tower. After that, analysts observed periodic discharges of hot water from the reactor into the river.

N Existing reactor Possible trench for hot water discharge Dredging activity in river Water discharge suggests activity at the reactor Dredging activity in river Water discharge from reactor Dredging activity in river Possible trenches for new reactor Water discharge from reactor Image from DigitalGlobe via Google Earth

In a satellite image from Jan. 17, 2018, steam is visible from the existing five-megawatt reactor’s turbine building, and hot water appears to be melting snow at a discharge pipe. The evidence suggests that the reactor could again be in active use.

“The five-megawatt reactor has been in continuous operation more or less for the entirety of 2017,” said Ms. Puccioni, who has been studying satellite imagery of Yongbyon for almost a decade.

Jan. 17, 2018 N Existing reactor New reactor Steam from reactor turbine building Power transmission tower Small dam built for reactor’s water intake Hot water discharge Kuryong River N Existing reactor New reactor Steam from reactor turbine building Power transmission tower Small dam built for reactor’s water intake Hot water discharge Kuryong River N Existing reactor New reactor Steam Hot water discharge Kuryong River Image from DigitalGlobe via Institute for Science and International Security

The development and operation of the two reactors at the Yongbyon site threatens to complicate any talks on a freeze of the North’s activities, and on the ultimate goal of denuclearization.

Yet the issue is not insurmountable. The usual approach is to rely on inspectors who ensure that no spent reactor fuel gets mined for plutonium. The International Atomic Energy Agency did so before at Yongbyon, before its inspectors were expelled, and could surely do so again.

Trump administration officials say the denuclearization inspections, however, would have to cover the entire country, because there are suspected undeclared uranium enrichment facilities outside of Yongbyon.