A narrow dirt road climbs through the mountainous North Korean countryside, meandering past shabby villages and down a long stretch that is oddly devoid of human habitation.
The landscape betrays no clues that anything of significance lies beyond, until the road comes to a guard post and checkpoint and then dead-ends in front of 10-foot-high slabs of concrete.
Here, nestled discreetly in the mountains is a walled city that does not appear on maps and officially does not exist. It is the Yongbyon Nuclear Research Center, the centerpiece of North Korea's decades-long quest to become a nuclear power.
In its heyday, Yongbyon was home to about 50,000 people who labored secretly on this project of the utmost national import -- making it, in effect, the Los Alamos of North Korea. The population was reduced to an estimated 10,000 after the communist regime agreed in 1994 to freeze its nuclear development.
Today, with North Korea's decision to restart its nuclear program, Yongbyon is enjoying a resurgence and is again raising anxiety levels around the world. On Saturday, North Korean diplomat Son Mun San in Vienna said the reactor will be ready to operate again "in a few weeks, not a few months."
Most of what is known about Yongbyon comes from a handful of North Korean defectors and from satellite photographs.
Access to the sprawling compound by international inspectors has been severely limited, and with the expulsion late last month of the International Atomic Energy Agency's three most recent inspectors, the activities in Yongbyon have again lapsed into secrecy.
Sean Tyson, a former U.S. Department of Energy employee and one of the few Americans who has spent much time at Yongbyon, describes an atmosphere of extreme paranoia at the site.
"We were only allowed to walk in the building where we worked," said Tyson, who, after the 1994 freeze, managed a project to ensure that fuel rods from Yongbyon's nuclear reactor were safely stored. "If anybody tried to talk to us, they would have been arrested and interrogated. We were always monitored."
Yongbyon is about 55 miles north of Pyongyang, the capital, wedged between the twisting Churyong River and Yaksan, a mountain extolled in Korean poetry for the beauty of its azaleas. The complex sprawls over 2,224 acres and contains about 390 buildings, according to the Center for Nonproliferation Studies in Monterey, Calif.
Besides the 5-megawatt reactor at the center of the current controversy, there are laboratories, research institutes, housing and schools for workers and their families, a museum of revolutionary history and an office of the ruling Korean Workers' Party.
The nuclear compound was developed with Soviet technical assistance in the early 1960s under orders from North Korea's founder, the late Kim Il Sung, who was anxious not to fall behind archrival South Korea's efforts to produce nuclear power. The reported cost was $500 million.
For the North, Yongbyon was a development of unsurpassed national importance, say defectors familiar with the project.
"People in North Korea took great pride in the fact that they were developing nuclear capabilities. It was something that only the superpowers had access to, and they were proud of that fact," said Cho Myong Chol, a high-ranking defector and economist, whose father was minister of construction and oversaw the building of Yongbyon.
"Nobody has been able to calculate it, but it was clear that an enormous part of the national budget went into Yongbyon."
Kim Tae Ho -- who worked in the nuclear program for eight years, two of them living inside Yongbyon with his family -- said workers were given the best that North Korea had to offer, with a condition.
"You had a total lack of freedom," Kim said. "Nobody could visit you in Yongbyon, and you couldn't tell anybody outside what your work was. But you got the highest priority of treatment in North Korea."
He recalled that Kim Il Sung visited in 1991 and was so impressed, he ordered that Toshiba color televisions with remote controls be given to scientists at Yongbyon.
"People were thrilled. They had never seen a remote control like that before," said Kim Tae Ho, who as a bureaucrat received only a North Korean-made black-and-white television.
As Kim recalled, workers reached Yongbyon by a special limousine bus, which in itself was luxurious by the standards of North Korean transportation.
"Once you are inside, it's like you've arrived in a city. There's everything from colleges to elementary schools," Kim said. "But a lot of the facilities are closed themselves, surrounded by barbed wire."
Kim held various jobs, at one point managing a nuclear waste dump outside the complex, at another time working in public relations -- which in North Korea meant writing propaganda slogans and songs to inspire fellow workers.
"Let's open the gates of [Korean] unification with nuclear development" and "Nuclear power is self-determination" are samples of his craft.
From its inception, Yongbyon appeared to be designed for weapons production rather than nuclear power.
Its 5-megawatt reactor -- built in the early 1980s and known as the "February enterprise," after a month in which Kim Il Sung paid a visit -- is too small to produce meaningful quantities of electricity and is not even connected to a power grid.
"This type of reactor is no longer used for energy production," said Kim Tae Woo, a nuclear specialist at the Korea Institute for Defense Analyses in Seoul, who says the 1950s design was obsolete even before construction began. "When North Korea argues they are using it to produce electricity, that is clearly a lie."
What the reactor could produce, however, is plutonium-239, a key ingredient in nuclear bombs. The reactor uses fuel rods of natural uranium, which the North has in abundant supply, to create a controlled nuclear reaction. The spent fuel rods later can be removed and plutonium extracted from them.
In fact, the North Koreans appear to have a reprocessing facility at Yongbyon designed for just such a purpose. It is an imposing oblong building, six stories high and the length of two football fields, which the North Koreans call a radiochemical laboratory or "December enterprise," after another Kim Il Sung visit.
In 1990, North Korea reported to the International Atomic Energy Agency that it had produced about 3 ounces of plutonium on an experimental basis. The agency was skeptical that the regime would have built such a huge laboratory merely to dabble in academic experiments.
Satellite photographs suggest that the North Koreans closed down the reactor on at least three occasions, each time long enough to remove some fuel rods to extract plutonium. International inspectors concluded from studies of plutonium samples that the North produced as much as 30 pounds of weapons-grade plutonium.
That is a quantity sufficient to build two nuclear bombs, each as large as the one that destroyed Nagasaki in 1945.
The inspectors' efforts in 1992 and '93 to investigate more closely set off a chain of events that very nearly prompted the Clinton administration to order preemptive strikes on Yongbyon in 1994. The crisis was settled by that year's agreement, under which the North promised to mothball its nuclear program. In exchange, the United States and its allies started shipping fuel oil and building light-water nuclear reactors -- a design that could not be used for weapons -- to ease North Korea's chronic energy shortages.
The deal virtually collapsed last October, when the United States confronted North Korea with evidence that the communist regime was cheating with a program to secretly enrich uranium -- another method of producing nuclear weapons.
Since then, the situation has rapidly deteriorated.
In recent weeks, the North has expelled the three international inspectors, taped over the lenses of their surveillance cameras and started to load fresh fuel rods into the Yongbyon reactor in preparation for restarting it.
An even bigger fear is that the regime will start extracting plutonium from about 8,000 spent fuel rods that were removed from the reactor in 1994.
Those rods are stored in canisters in what is essentially a large indoor swimming pool next to the reactor; video footage given by the North to the inspection agency shows men who look like chefs standing around the rim of the pool, peering into murky waters.
If the North Koreans processed these spent fuel rods, they could get enough plutonium for five or six bombs, in addition to the one or two that the country might already have.
"After you extract the plutonium, it is just a matter of a few weeks before you can make the bomb," said South Korean nuclear physicist Kang Jung Min.
"North Korea already has several nuclear weapons, we believe," said a senior U.S. military official in Seoul who requested anonymity. "There is a concern that if they get more, they will sell them to anyone."
In the intelligence and diplomatic community, there is some debate about North Korea's technical capabilities, given the overall state of decrepitude in which the impoverished country finds itself.
Tyson, the former Energy Department employee, said Yongbyon was in terrible shape in 1998 when he last visited.
"Everything was broken, everything was just disintegrating. The barbed wire was covered in rust and falling down in places. It was pathetic," Tyson said.
He recalled that the North Koreans were unable to make simple infrastructure repairs because their cement was substandard.
Even the pool for storing fuel rods was leaking.
"If they are pouring their resources into nuclear, it is not happening at Yongbyon," he said.
Tyson allows that it is possible that some of the activity at Yongbyon is happening underground and out of sight.
When he was there, he said, he noted with curiosity that in just a few days the North Koreans tore up a plaza and planted the area with gardens, a pond and a bridge -- suggesting that they might have been hiding something underneath.
"It was an unbelievable transformation in two or three days. They only do things like that when they have to," Tyson said.
In recent years, suspicion has focused on underground activity at more remote sites. About 25 miles north of Yongbyon, a large underground complex of tunnels and pipelines at Kumchangri was inspected in 1999 by U.S. officials. They found nothing.
Another facility, buried under Mt. Chonma near the Chinese border, is suspected of being used to enrich uranium.
Some nuclear specialists believe that although North Korea is poor, its leadership desperate and its technology obsolete, the country is no less dangerous as a nuclear power.
"Even if North Korea has only a primitive bomb, you must remember what the most primitive nuclear bombs in history did to two Japanese cities in 1945," said Kim of the Seoul defense institute.
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North Korea developed the Yongbyon compound in the early 1960s to produce nuclear power. Dormant since 1994, the sprawling complex has recently been reactivated. In addition, international inspectors have been expelled and surveillance cameras disabled.
Why U.S. officials are worried
1. 5-megawatt reactor: Recently restarted, it is the centerpiece of the current controversy. Newly loaded fuel rods raise fears that the reactor could produce plutonium-239, a key ingredient in nuclear bombs.
2. Potential 50-megawatt reactor: Though not completed, it could produce plutonium in far higher quantities than the smaller reactor.
3. Radiochemical laboratory: It has the potential of extracting weapons-grade plutonium from spent fuel rods. About 8,000 such fuel rods are stored at the complex.
4. Fuel fabrication complex: Fuel rods can be made here out of natural uranium, which North Korea has in abundance.
Sources: Institute for Science and International Security, GlobalSecurity.org
Times staff writer Alissa J. Rubin in Vienna contributed to this report.