In the late 1980s, U.S. analysts became concerned over intelligence evidence indicating that North Korea may have been trying to build a nuclear bomb. Satellite photos revealed new construction at the Yongbyon Nuclear Center, north of Pyongyang. It appeared that the North Koreans were building a plutonium reprocessing plant, a facility that would give them the ability to create fuel for a nuclear weapon.
Before it can be used, the plutonium must be extracted from the irradiated reactor fuel. Fuel rods are removed from the reactor, cooled in a storage pond, and taken to a reprocessing facility where plutonium is separated out from the irradiated fuel.
The extensive Yongbyon nuclear complex now includes a five-megawatt graphite-moderated research reactor and a working reprocessing facility.
In January 1992, under U.S. pressure, the North Koreans agreed to allow a team of International Atomic Energy Association (IAEA) inspectors visit the Yongbyon facility. Subsequent official inspections were repeatedly blocked, but the IAEA gathered enough information to raise suspicions that North Korea was trying to hide some of their nuclear activities. The Nuclear Nonproliferation Treaty (NPT), which North Korea signed in 1985, requires signatory nations to negotiate "safeguard agreements" with the IAEA. Inconstancies in the Pyongyang's reports to the IAEA, combined with physical evidence gathered by the inspectors, led them to suspect that the North Koreans had separated more plutonium than they had declared as required by the safeguards agreement. Inspectors could not determine, however, exactly how much more plutonium existed.
Pyongyang's provocative move brought the simmering nuclear crisis to a head; the U.S. began reviewing plans for military strikes on the Yongbyon complex and speculating on the North Korean response to punitive U.N. sanctions. Simultaneously, former President Jimmy Carter went to Pyongyang to try to broker a peace deal. At the eleventh hour, he announced that he had come to an agreement with Kim Il Sung that North Korea would freeze all activities at the Yongbyon plant and return to the negotiating table.
Subsequent negotiations between the two countries resulted in the Agreed Framework of 1994, in which North Korea agreed to shut down the Yongbyon complex and cease construction on two larger reactors, one at Yongbyon and one at another site, Taechon. In return, the U.S. promised to provide two modern light-water nuclear reactors, which would provide North Korea with the energy the country desperately needed, but was harder to use for nuclear weapons development. In addition, the U.S. committed to delivering 500,000 metric tons of heavy fuel oil annually to North Korea, purportedly to offset the loss of energy from the closing of the nuclear reactor.
When the Yongbyon faculties were closed, 8,000 fuel rods containing about 50 metric tons of uranium were removed. from the five-megawatt reactor to a storage facility. This spent fuel was estimated to contain 25 to 35 kilograms of plutonium.
It is still uncertain exactly how much plutonium was extracted before the Yongbyon facility was shut down. An unclassified January 2003 CIA report estimated that North Korea "probably has produced enough plutonium for at least one, and possibly two, nuclear weapons."
In December 2002, following the U.S. discovery of its uranium enrichment program, the North Koreans turned off all monitoring equipment at Yongbyon and expelled the IAEA inspectors. Three months later, they restarted the five-megawatt reactor. At its current rate, the reactor would have to operate for almost a year to produce enough plutonium for a nuclear weapon.
If the North Koreans begin reprocessing the spent fuel that was removed in the 1994 shutdown, however, they could have enough plutonium for five or six bombs within months. If they complete construction on the two larger reactors that were halted in 1994, some estimate that within several years Pyongyang could be producing 30 to 50 plutonium-powered nuclear weapons annually.
Besides plutonium reprocessing, an alternative method for obtaining fissile material for nuclear weapons is the enrichment of naturally occurring uranium. In its natural state, uranium is consists of two isotopes: U235 and U238. Normally, uranium consists of approximately .7 percent U235 and 99.3 percent U238. "Highly enriched uranium," or HUE, is produced by increasing the percentage of the U235 isotope to between 20 to 90 percent.
There are a variety of ways to enrich uranium. One method is by spinning it in specially designed gas centrifuges, made of high-strength metal that can withstand significant heat and pressure. In this process, the heavier U238 molecules are separated from the lighter U236 molecules.
Over the summer of 2002, mounting intelligence convinced the U.S. that North Korea was pursuing a covert uranium enrichment program. U.S. intelligence officials believe that Pyongyang may have traded ballistic missiles and their blueprints to Pakistan in exchange for gas centrifuges and other equipment. They also saw evidence that the North Koreans were seeking to obtain quantities of high strength aluminum -- a key ingredient used to make gas centrifuges.
In October, Assistant Secretary of State James Kelly traveled to Pyongyang to confront North Korean officials about the HUE program. Although the Agreed Framework did not explicitly address uranium enrichment, it did stipulate that North Korea would remain a party to the Nuclear Nonproliferation Treaty, which clearly prohibits the development of any type of nuclear weapons program. Apparently surprised by how much the U.S. had learned, the Pyongyang officials admitted to the existence of the HUE program, in direct violation of the NPT.
The sudden and startling admission prompted U.S. President George Bush, along with South Korean President Kim Dae Jung and Japanese Prime Minister Junichiro Koizumi, to issue a statement demanding that North Korea dismantle the uranium program in a prompt and verifiable manner. The next month, Bush ordered all shipments of heavy fuel oil to North Korea stopped until North Korea undertook verifiable steps to dismantle the program.
The extent of the HUE program and the number of centrifuges the North Koreans have is unknown. Unlike the plutonium program, which requires large distinctive nuclear reactors that are easily recognizable in satellite photos, uranium enrichment could be taking place in dispersed or underground facilities. Most experts agree, however, that North Korea could have a working uranium enrichment program within the next few years, at the latest. In March 2003, U.S. Assistant Secretary of State James Kelly told the Senate Foreign Relations Committee that the North Korean HUE program could be up and running within months.