North Korea’s Nuclear Weapons: Latest Developments
Released: 2007-03-13T00:00:00

Background

In the early 1980s, U.S. satellites tracked a growing indigenous nuclear program in North Korea. A small nuclear reactor at Yongbyon (5MWe), capable of producing about 6kg of plutonium per year, began operating in 1986. Later that year, U.S. satellites detected high explosives testing and a new plant to separate plutonium. In addition, construction of two larger reactors (50MWe at Yongbyon and 200MWe at Taechon) added to evidence of a serious clandestine effort. Although North Korea had joined the Nuclear Nonproliferation Treaty in 1985, the safeguards inspections that began only in 1992 raised questions about how much plutonium North Korea had produced covertly. In 1994, North Korea pledged, under the Agreed Framework with the United States, to freeze its plutonium programs and eventually dismantle them in return for several kinds of assistance. At that time, Western intelligence agencies estimated that North Korea had separated enough plutonium for one or two bombs; other sources estimated four to five bombs.

Weapons Production Milestones

Acquiring fissile materialâplutonium-239 or highly enriched uranium (HEU)âis the key hurdle in nuclear weapons development. Producing these two materials is technically challenging; in comparison, many experts believe weaponization to be relatively easy. North Korea has industrial-scale uranium mining, and plants for milling, refining, and converting uranium; it also has a fuel fabrication plant, a nuclear reactor, and a reprocessing plantâin short, everything needed to produce Pu-239. In its nuclear reactor, North Korea uses magnox fuelânatural uranium (>99%U-238) metal, wrapped in magnesium-alloy cladding. About 8000 fuel rods constitute a fuel core for the reactor.

When irradiated in a reactor, natural uranium fuel absorbs a neutron and then decays into plutonium (Pu-239). Fuel that remains in the reactor for a long time becomes contaminated by the isotope Pu-240, which can "poison" the functioning of a nuclear weapon. Spent or irradiated fuel, which poses radiological hazards, must cool after removal from the reactor. The cooling phase, estimated by some at five months, is proportional to the fuel burn-up. Reprocessing to separate plutonium from waste products and uranium is the next step. North Korea uses a PUREX separation process, like the United States. After shearing off the fuel cladding, the fuel is dissolved in nitric acid. Components (plutonium, uranium, waste) of the fuel are separated into different streams using organic solvents. In small quantities, separation can be done in hot cells, but larger quantities require significant shielding to prevent deadly exposure to radiation.

North Korea appears to have mastered the engineering requirements of plutonium production. Its 5MWe nuclear reactor operated from 1986 to 1994, restarting in January 2003. North Korean officials claimed to have separated plutonium in hot cells and tested the reprocessing plant in 1990, and to have reprocessed all 8000 fuel rods from the 5MWe reactor between January and June 2003. The January 2004 unofficial U.S. delegation reported that "All indications from the display in the control room are that the reactor is operating smoothly now....However, we have no way of assessing independently how well the reactor has operated during the past year." The same delegation reported that the reprocessing "facility appeared in good repair," in contrast to a 1992 IAEA assessment of the reprocessing plant as "extremely primitive." In the end, however, significant growth in North Korea's arsenal depends on the completion of the two larger reactors and progress in the reported uranium enrichment program.

In January 2004, North Korean officials showed an unofficial U.S. delegation alloyed "scrap" from a plutonium (Pu) casting operation. Alloying plutonium with other materials is "common in plutonium metallurgy to retain the delta-phase of plutonium, which makes it easier to cast and shape" (two steps in weapons production). Dr. Siegfried Hecker, a delegation member, assessed that the stated density of the material was consistent with plutonium alloyed with gallium or aluminum. If so, this could indicate a certain sophistication in North Korea's handling of Pu metal, but without testing the material, Hecker could not confirm that the metal was plutonium or that it was alloyed, or that it was from the most recent reprocessing campaign.

There is no reliable information on North Korean nuclear weapons design. Although the U.S. Director of National Intelligence confirmed that a nuclear test was conducted on October 9, 2006 in the vicinity of P'unggye, the sub-kiloton yield of the test suggests that the weapon design or manufacturing process likely needs improvement. Environmental clues suggest that the device used plutonium. By comparison, a simple plutonium implosion device normally would produce a larger blast, perhaps 5 to 20 kilotons. The first nuclear tests conducted by other states range from 9 kt (Pakistan) to 60kt, but tests by the United States, China, Britain and Russia were in the 20kt-range. Implosion devices, which use sophisticated lenses of high explosives to compress fissile material, are generally thought to require testing, although the CIA suggested in 2003 that North Korea could validate its weapons design using extensive high explosives testing. It is possible that Pakistani scientist A.Q. Khan may have provided North Korea the same Chinese-origin nuclear weapon design he provided to Libya. If so, this might help North Korea develop a reliable warhead for ballistic missilesâsmall, light and robust enough to tolerate the extreme conditions encountered through a ballistic trajectory. Although former DIA Director Jacoby told the Senate Armed Services Committee in April 2005 that North Korea had the capability to arm a missile with a nuclear device, Pentagon officials later backtracked from that assessment.

Estimating Nuclear Material Production

Most estimates of nuclear weapon stockpiles are based on estimated fissile material production. Factors in plutonium production include the average power level of the reactor; days of operation; how much of the fuel is reprocessed and how quickly, and how much plutonium is lost in production processes. According to North Korea, the 5MWe reactor performed poorly early on, unevenly irradiating the rods. There is no data on the reactor's current performance or the reprocessing facility's efficiency. North Korea told the IAEA that during the 1990 "hot test," it lost almost 30% of the plutonium in the waste streams. A key consideration is whether or not the reprocessing plant can run continuously, since frequent shutdowns can lead to plutonium losses. According to North Korean officials in January 2004, the plant annual throughput is 110 tons of spent fuel, about twice the fuel load of the 5MWe reactor. A final factor in assessing how many weapons North Korea can produce is whether North Korea's technical sophistication enables it to use more or less material than the international standards of 8kg of Pu and 25kg for HEU per weapon. North Korea's abilities here are unknown.

What Does North Korea Have Now?

Secretary of State Powell stated in December 2002 that "We now believe they [North Koreans] have a couple of nuclear weapons and have had them for years." On February 10, 2005, North Korea announced that it had manufactured "nukes" for self-defense and that it would bolster its nuclear weapons arsenal. In June 2005, Vice Foreign Minister Kim Gye Gwan told ABC News that "We have enough nuclear bombs to defend against a U.S. attack. As for specifically how many we have, that is a secret." Kim also said North Korea was building more bombs and when asked about delivery systems, said "our scientists have the knowledge, comparable to other scientists around the world." Some Members of Congress interpreted then-CIA Director Porter Goss' statements in March 2005 on a "range" of nuclear weapon estimates to confirm that North Korea's arsenal has multiplied. In December 2005, the North Korean foreign ministry stated that it would "increase [its] self-reliant national defense capacity, including nuclear deterrent." In October 2006, North Korea's Foreign Ministry said that "nuclear weapons will serve as [a] reliable war deterrent."

Has North Korea Reprocessed the Existing Spent Fuel?

On July 13, 2003, North Korean officials told U.S. officials in New York that they had completed reprocessing the 8000 fuel rods on June 30. On January 8, 2004, North Korean officials told an unofficial U.S. delegation that the reprocessing campaign began in mid-January 2003 and ended at the end of June 2003. In all, they reportedly reprocessed 50 tons of spent fuel in less than six months, which tracks with earlier estimates that North Korea could reprocess about 11 tons/month, roughly enough plutonium for one bomb per month.

An unofficial U.S. delegation in January 2004 concluded that the spent fuel pond no longer held the 8,000 fuel rods and surmised that they could have been moved to another storage location, but not without significant health and safety risks. The delegation was not allowed to visit the Dry Storage Building, where the fuel rods likely would have been stored before reprocessing. The delegation also did not visit waste facilities. Reprocessing the 8,000 fuel rods from the 5MWe reactor would yield between 25 and 30kg of plutonium, perhaps for four to six weapons, but the exact amount of plutonium that might have been reprocessed is unknown. In 2004, North Korean officials stated that the reprocessing campaign was conducted continuously (four 6-hour shifts). U.S. efforts to detect Krypton-85 (a by-product of reprocessing) reportedly suggested that some reprocessing had taken place, but were largely inconclusive.

Adding to the Arsenal

Make New Plutonium in 5MWe Reactor

On February 6, 2003, North Korean officials announced that the 5MWe reactor was operating, and commercial satellite photography confirmed activity in March. In January 2004, North Korean officials told U.S. visitors that the reactor was now operating smoothly at 100% of its rated power. The U.S. visitors noted that the display in the reactor control room and steam plumes from the cooling towers confirmed operation, but that there was no way of knowing how it had operated over the last year. In April 2005, the reactor was shut down, and on May 11, 2005, North Korean officials stated they harvested fuel rods for weapons. According to commercial satellite images, the reactor resumed operations in August 2005.

A common estimate is that the reactor generates 6 kg of Pu per year, roughly one bomb per year, but the reactor would likely be operated for several years before fuel is withdrawn. One estimate is that the reactor held between 10 and 15 kg Pu in April 2005, and that North Korea could have reprocessed all the fuel by mid-2006. From August 2005 to 2006, the reactor could have produced another 6 kg of Pu; in total, there could be enough separated plutonium for another three weapons.

Complete Other Reactors

The reactors at Yongbyon (50MWe) and Taechon (200MWe) are likely several years from completion. U.S. visitors in January 2004 saw heavy corrosion and cracks in concrete building structures at Yongbyon, reporting that the reactor building "looks in a terrible state of repair." The CIA estimates that the two reactors could generate about 275kg of plutonium per year. In August 2005, another unofficial U.S. delegation to Pyongyang was told by North Korean officials that they planned to finish building the 50MWe reactor within two years. Commercial satellite images in 2005 and 2006 showed little progress.

Produce Highly Enriched Uranium for Weapons

A 2002 unclassified CIA working paper on North Korea's nuclear weapons and uranium enrichment estimated that North Korea "is constructing a plant that could produce enough weapons-grade uranium for two or more nuclear weapons per year when fully operationalâwhich could be as soon as mid-decade." Such a plant would need to produce more than 50kg of HEU per year, requiring cascades of thousands of centrifuges. The paper noted that in 2001, North Korea "began seeking centrifuge-related materials in large quantities." Pakistani President Musharraf revealed in his September 2006 memoir, In the Line of Fire , that "Doctor A.Q. Khan transferred nearly two dozen P-1 and P-2 centrifuges to North Korea. He also provided North Korea with a flow meter, some special oils for centrifuges, and coaching on centrifuge technology, including visits to top-secret centrifuge plants." However, the United States has not been able get direct confirmation from Khan.

Questions have been raised about whether U.S. estimates were accurate. In a hearing before the Senate Armed Services Committee on February 27, 2007, Joseph DeTrani, the mission manager for North Korea from the Office of the Director of National Intelligence, was asked by Senator Reed whether he had "any further indication of whether that program has progressed in the last six years, one; or two, the evidenceâthe credibility of the evidence that we had initially, suggesting they had a program rather than aspirations?" DeTrani responded that "the assessment was with high confidence that, indeed, they were making acquisitions necessary for, if you will, a production-scale program. And we still have confidence that the program is in existenceâat the mid-confidence level."

How to Verify North Korean Claims?

Information about North Korea's nuclear weapons production has depended on remote monitoring and defector information, with mixed results. Satellite images correctly indicated the start-up of the 5MWe reactor, but gave no details about its operations. Satellites also detected trucks at Yongbyon in late January 2003, but could not confirm the movement of spent fuel to the reprocessing plant; imagery reportedly detected activity at the reprocessing plant in April 2003, but could not confirm large-scale reprocessing; and, satellite imagery could not peer into an empty spent fuel pond, which was shown to U.S. visitors in January 2004. Even U.S. scientists visiting Pyongyang in January 2004 could not confirm North Korean claims of having reprocessed the spent fuel or that the material shown was in fact plutonium. Verifying those claims would require greater access to the material and North Korean cooperation. This is particularly true in the case of uranium enrichment; U.S. intelligence officials have said they do not know where the uranium program is and more recently, have shown less confidence about what the scope of the program might be. Although seismographs registered the October 9 th detonation, and environmental sampling confirmed radioactivity, there is still no information on what North Korea intended to accomplish with the test, from technical, security, political, and diplomatic perspectives. More data is necessary to project what this "new" capability might mean for North Korea, the region, and the United States.