IN recent months, United States officials and proliferation experts around the world have become increasingly concerned about two suspected nuclear waste sites near Yongbyon, North Korea, sites that are not hidden particularly well.
One of the suspect sites is a building that once had two stories. Now it has one, as dirt has been piled around the outside to turn the old ground floor into a basement. The other is a field that once showed pits characteristic of nuclear dumps. It has been buried and re-landscaped.
"It's got grass, and trees, and bushes," says a US official. "They say it's a military park."
The sites may contain traces of plutonium that could prove North Korea has a secret nuclear weapons program. When international inspectors asked to visit them earlier this year, Pyongyang refused - and said it was quitting the Nuclear Non-Proliferation Treaty (NPT) instead.
The result has been a quiet crisis which arguably represents the most dire threat to US security today. North Korea may yet decide not to walk out on the NPT. It may decide a nuclear weapons program is not worth economic isolation. But if it really wants to acquire The Bomb there may not be much the US can do about it.
"I don't think you can stop them," says a Pentagon consultant who works on the issue.
CIA director James Woolsey has identified North Korea as his most grave proliferation worry. Besides a nuclear effort at its Yongbyon complex, North Korea has advanced missile programs - and seems willing to sell any military system to anyone with cash.
North Korea's ostensibly civilian nuclear program apparently dates from the late 1950s, when it began sending young science students to Moscow for training in nuclear physics. In the early '60s, the Soviet Union presented its Korean ally with an assortment of research equipment, including a small reactor that was installed at the new Institute of Nuclear Physics in Yongbyon.
Near this site in 1980 North Korea began building what it calls an "experimental power reactor" of 5 megawatts. Designed without outside help, this reactor is of the gas-graphite variety. Fueled with natural uranium, it is a type considered obsolete in the West - but unlike newer designs it produces weapons-grade plutonium.
The 5 MW reactor is thought to have started up sometime in the mid 1980s. The North Koreans claim they have had trouble getting it to run right. That may be true - or the reactor may have already produced the 10 kilograms or so of plutonium needed to build a nuclear weapon.
"It's starting to stretch people's beliefs that they've had 7 years of start-up problems," says David Albright, head of the Institute for Science and International Security and an expert on proliferation technology.
A 200 MW gas-graphite reactor is currently under construction near Taechon. But perhaps North Korea's most worrisome piece of new nuclear infrastructure is being built back at Yongbyon: a plutonium reprocessing facility. The 600 foot-long building is termed a "radiochemical laboratory" by North Korea, and is thought by the West to be about 80 percent complete.
North Korea has said it needs a plutonium reprocessing facility for technical reasons related to electric power production. Western experts are not convinced.
"We do not see any good, solid reason why they would need the plutonium for the ... purposes that were mentioned," Hans Blix, director of the International Atomic Energy Agency, told Congress last year.
The North Koreans have admitted separating a small amount of plutonium at their unfinished reprocessing facility. But the IAEA already has evidence that the amount of material involved has been greatly understated.
IAEA inspectors were allowed into Yongbyon last year after Pyongyang agreed to fully implement the Non-Proliferation Treaty, which it signed in 1985. From traces of decaying radioactive elements inspectors collected at the reprocessing facility, the IAEA determined that plutonium had been separated there on at least three occasions - not just once, as North Korea claimed.
To make sure they were right, IAEA officials asked to visit the camouflaged dump sites, which had been pointed out to them in satellite photos by US intelligence. To this point North Korea has refused entry. The sites were not declared as nuclear in the original list of facilities submitted to the IAEA, and North Korean officials claim instead that the areas in question have conventional military use.
Perhaps the IAEA's technical snooping ability is greater than North Korea had realized. Thus the IAEA's ability to deduce clues from mere swipes in the reprocessing facility was a wake-up call.
"Suddenly the North Koreans are choking and saying, 'my goodness, we can't let them near this stuff'," says Peter Zimmerman, a physicist and senior fellow at the Center for Strategic and International Studies.
Once a potential proliferator has enough fissile material, the other aspects of building a bomb are relatively easy. They are also easy to hide from satellites.
There are benign explanations for many of North Korea's suspicious activities. Perhaps they built a gas-graphite reactor because it could be fueled by natural uranium mined within the country, rather than for its plutonium-producing abilities.
But the little things add up to one big conclusion for most Western officials and experts. "Underneath all this is a weapons program," says Mr. Albright.
* Next: What the rest of the world can do.