How much plutonium does North Korea have?
A physicist looks at the data and comes up with some answers
By David Albright
September/October 1994 pp. 46-53 (vol. 50, no. 05) © 1994 Bulletin of the Atomic Scientists
s July ended and this issue of the Bulletin was sent to the printer, the public was startled by the nuclear allegations of a North Korean defector.
Kang Myong Do, who said he was the son-in-law of Kang Sung San, North Korea's prime minister, was unveiled July 27 at a press conference by South Korean intelligence officials some two months after defecting through a third country.
According to the defector, the North Koreans not only have a nuclear weapons program--contrary to their frequent professions of innocence--but it has been wildly successful. The Pyongyang government, Kang said, has produced five bombs, and it intends to churn out five more. At some point, he added, North Korea would reveal the extent of its arsenal to the West, and use the weapons as leverage to gain concessions from the United States and to prop up the Kim Jong Il regime.
The immediate reaction of U.S. intelligence and State Department officials was one of intense disbelief. "There is a debate within our own intelligence community about the exact parameters of the North Korean nuclear program," noted Mike McCurry, a State Department spokesman. "But the information provided by this defector falls well beyond and well outside of those parameters."
A spokesman for the International Atomic Energy Agency (IAEA) dismissed the defector's words, saying they were not "plausible." The South Korean government itself said the defector's story was "groundless."
The consensus seemed to be that Kang was either repeating uninformed hearsay or that he was lying--either to spread disinformation or to ingratiate himself with the South Korean government.
At press time, it looked as if Kang's story would be just one more footnote in the continuing effort to untangle the secrets of the North Korean program. Meanwhile, on these pages, I summarize and interpret the best information so far available on the nuclear program.
U.S. intelligence discovered in the early to mid-1980s that North Korea was building a small nuclear reactor at Yongbyon, about 100 kilometers north of the capital of Pyongyang. The reactor was described as a gas-cooled, graphite-moderated model similar to those Britain and France used to produce electric power as well as plutonium for nuclear weapons. When Western nations expressed concern about the reactor, Russia pressed North Korea to sign the Nuclear Non-proliferation Treaty (NPT), which it did on December 12, 1985.
With the reactor apparently to be brought under international safeguards, Western interest in North Korean nuclear development waned, and little attention was paid even after the reactor began operating in 1986.
After signing the NPT, however, North Korea stalled on signing the required safeguards agreement that allows the International Atomic Energy Agency to inspect nuclear facilities. Then, in 1989, the press reported that a long, narrow structure at Yongbyon looked very much like a plutonium reprocessing plant. Such a plant would allow North Korea to separate plutonium from the other elements in spent reactor fuel, raising the possibility that the plutonium was destined for use in a weapon program.
Reports soon followed that the North was building a second, much larger gas-graphite reactor expressly to produce a large quantity of weapon plutonium. Some reports also claimed that high-explosive testing at Yongbyon constituted yet another element of a North Korean nuclear weapon program.
With international pressure building--and after years of diplomatic wrangling--the North and the IAEA finally concluded a safeguards agreement in January 1992. It appeared that the growing political crisis over the North's nuclear intentions was at last over.
The first inspections
On May 4, 1992, North Korea gave the IAEA its "initial declaration"--a statement of all its nuclear material subject to safeguards. The IAEA began inspections to verify the initial disclosure and to assess its completeness. Soon afterward, Hans Blix, the IAEA's director general, visited North Korea for the first time. Among the sites his delegation visited was the unfinished plutonium separation facility, which North Korean officials call the "Radiochemical Laboratory." North Korean officials told the IAEA that they had used the facility to separate a mere 100 grams of plutonium in the spring of 1990. They said the plutonium came from a few fuel rods that were removed from the reactor when their metal casing or "cladding" was damaged during reactor operations.
IAEA officials also toured the North's three gas-graphite reactors, only one of which was--and is--in operation. At Yongbyon, the IAEA delegation visited the small operational reactor as well as a 200-megawatt (thermal) reactor that was under construction. And the delegation flew to the construction site of an even larger reactor being built at Taechon in North Pyongan Province.
Shortly after this visit, Blix told a press conference in Beijing that the Radiochemical Laboratory building was about 80 percent complete but that only about 40 percent of the processing equipment was installed. North Korean officials told the IAEA that the rest of the equipment was on order but had not yet been delivered. When asked about the "Radiochemical Laboratory," Blix said, "I have no doubt that it would have been considered a reprocessing plant in our terminology." [1]
Discrepancies
During its initial inspections in the summer of 1992, the IAEA collected "samples" of material caught in the various processing steps as well as different types of nuclear waste generated during those separation processes. According to one IAEA official, the facility operators were willing--but not technically prepared--to take samples from the waste stored at the site. When inspectors asked for samples of highly radioactive fission-product waste, North Korean technicians had to improvise to get to the waste. (North Korean officials have since complained that this procedure resulted in some workers receiving greater-than-allowed doses of radiation.)
The samples were analyzed at the IAEA laboratory in Seibersdorf, Austria, and by affiliated laboratories in Europe and the United States. These analyses uncovered discrepancies that led the IAEA to suspect that the North had separated more plutonium than it admitted.
One set of analyses called into question the North's claim that plutonium had been separated only in March 1990. The IAEA took "smear" or "swipe" samples from the insides of glove boxes at the end of the separation process, where freshly purified plutonium is handled. They analyzed the samples for americium 241, a decay product of plutonium 241, to discover how much time had passed since the plutonium was separated. The decay-product test suggested that North Korea had in fact separated plutonium in 1989, 1990, and 1991.
In another test, the IAEA tried to verify that the plutonium declared by the North and the plutonium found in the waste came from the same source--the irradiated fuel rods that North Korean officials said had been damaged. The IAEA compared the ratios of plutonium isotopes in several waste and glove box samples to the ratios in the separated plutonium. Both the plutonium and the trace quantities of plutonium remaining in the waste should have had the same proportion of plutonium isotopes 239, 240, and 241. Instead, the IAEA found that the ratio of plutonium 240 in the waste samples differed from that in the separated material.
A third inconsistency involved the degree of irradiation of the fuel North Korea claimed to have processed during its single reprocessing campaign. The isotopic ratios of the separated plutonium and the plutonium recovered from the waste were not consistent with the North's report of the fuel's irradiation history.
One IAEA official familiar with the inspections said most of the inspectors concluded that the North had separated more plutonium than it had declared. But they had no way of knowing how much more. It could have been grams or kilograms.
When questioned, the North stuck to its original story, denying that it had separated more plutonium than it had declared, and either ignoring or challenging the technical basis of the IAEA's conclusions. North Korean officials said that the original fuel core was still in the reactor--and to have separated as much as kilograms of plutonium, the North would have had to unload much of the fuel.
Hidden waste sites
Meanwhile, the IAEA began to receive disquieting intelligence reports that the North had hidden two of its nuclear waste sites. A U.S. official said that this information showed that they had been camouflaged just before the start of the IAEA's inspections. The IAEA therefore asked to inspect the sites to see if they contained radioactive waste that had been generated during reprocessing.
U.S. satellite photos taken over many years show what appear to be two nuclear waste sites near the Radiochemical Laboratory; the sites are big enough to hold large quantities of both liquid and solid nuclear waste.
One set of photos shows a suspected outdoor waste site believed to be associated with an earlier Russian-supplied IRT (8-megawatt thermal) research reactor that has been under IAEA safeguards since 1977. In early photos the facility's layout resembles waste sites found near other Soviet-supplied research reactors. These sites have a distinctive pattern of round and square holes in an above-ground concrete structure that holds liquid and solid nuclear waste. One Western official said it closely resembled a waste site near a Soviet-supplied IRT research reactor in Iraq. Later photos show that the site has been covered and landscaped, effectively hiding it from inspectors and satellite surveillance. The waste site that the North did declare is located nearby; the new site is barely used.
The second hidden waste site is said to be in a building about 50 meters long that could be connected to the Radiochemical Laboratory by underground pipes. This building is about 150 meters east of the Radiochemical Laboratory and is separated from it by a small hill. Early photos show a two-story building with two trenches connecting it to the Radiochemical Laboratory. In more recent photos, however, the building appears to be of only one story because dirt has been pushed up around it, concealing the lower floor. The trenches have been filled in. The IAEA wants to know if the lower level of this building contains tanks that hold reprocessing waste.
Inspectors who visited this building in the summer of 1992 found no evidence of the lower floor, but could only conduct a visual inspection. But in late 1992 and early 1993, agency inspectors asked specifically for access to this building and permission to take samples from the space under the floor. North Korean officials refused to allow them to visit either site, claiming they were both exempt non-nuclear military facilities. (At the start of their visits, officials had told inspectors they could visit any site they wanted, even if it had not been mentioned in the North's initial declaration.)
At loggerheads
By February 1993, the IAEA and the North had reached an impasse; the IAEA announced that it could not confirm the North's initial declaration of its inventory of plutonium. Meanwhile, the North continued to refuse to cooperate. On February 25, 1993, the IAEA's board of governors gave North Korea until March 25 to agree to "special inspections" of the two hidden nuclear waste sites at Yongbyon. The North responded by declaring on March 12 that it would withdraw from the NPT. A few months later, in another bizarre twist, the North "suspended" its withdrawal; it now claims to have a "unique status" in relation to the treaty.
Over the next year, the North engaged in a series of negotiations with the IAEA and the United States on the conditions for inspections to resume. These negotiations did not end the crisis, reestablish safeguard inspections, or shed any light on the North's initial declaration.
On April 8, 1994, the North shut down the small reactor to prepare to start unloading the core in early May. U.S. Defense Secretary William Perry made headlines by telling the National Press Club on May 3 that the spent fuel would contain enough plutonium for four or five nuclear bombs.
The IAEA wanted to view the unloading to insure that the North did not divert any spent fuel. And the agency expected to set aside a few hundred of the thousands of fuel rods from a variety of locations in the core. An examination of the rods could determine whether it was the original fuel core as the North claimed. But by May 12 North Korea had started unloading the reactor--without the safeguards measures the IAEA had requested--precipitating yet another crisis.
Ultimately, North Korea allowed IAEA inspectors to observe the unloading, but it refused to allow the IAEA to select and secure any fuel rods for eventual testing. During consultations in Pyongyang in May, the North said the IAEA could sample the rods after they were placed in the nearby spent fuel ponds. IAEA inspectors refused this offer because they would not know what part of the core the rods had come from. "Without such identification," according to the agency, "future measurements would be meaningless and the agency's ability to verify non-diversion would be lost."
In a May 27 letter to the members of the U.N. Security Council, Blix reported that "the fuel discharge operation at the reactor was proceeding at a very fast rate which was not in line with information previously conveyed to the agency." Blix warned that if the unloading continued at the same rate, within a few days the agency would lose forever the opportunity to select fuel rods for later measurements.
When they first announced that they were unloading the fuel from the reactor, North Korean officials told the IAEA that the process would take two months. But Blix wrote in his May 27 letter that half the fuel was already unloaded. U.S. officials said the North was using a new, faster unloading machine that Western intelligence had not seen before. An IAEA official said in an interview that the new machine was delivered to the reactor a few weeks before unloading began. Another IAEA official said in an interview that the North was unloading 24 hours a day.
By June 2, Blix had concluded that it was too late to sample the fuel rods systematically. In a letter to the Security Council, he wrote that the IAEA's ability to determine past diversions had been "seriously eroded." He added that because the North had refused to allow special inspections of the suspect waste sites and because it was unloading the reactor core without the IAEA's required verification measures, the IAEA had lost the opportunity to "achieve the overall objective of comprehensive safeguards in [North Korea], namely, to provide assurance about the non-diversion of nuclear material."
At this point, a host of American commentators and members of Congress called for President Clinton to take decisive action to halt the North Korean program. However, former president Jimmy Carter defused the crisis--at least temporarily--with a trip to North Korea, where he conducted his own low-key brand of unofficial diplomacy.
Reconstructing past activities and developing adequate safeguards on the North Korean program requires more extensive North Korean cooperation than has been demonstrated so far. However, following Carter's mediation with North Korean leader Kim Il Sung in mid-June, such cooperation seemed to be forthcoming. The North agreed to "freeze" its nuclear program, which includes a pledge not to reload the small reactor with fresh fuel or to reprocess the discharged fuel while bilateral negotiations with the United States proceed. The North also agreed to allow two IAEA inspectors to remain at the reactor site to verify that reloading or reprocessing have not occurred.
A first priority of the negotiations is finding ways to delay reprocessing (see "Rust Never Sleeps," page 49). If these methods are instituted, the North's pledge could remain in effect long enough to find a satisfactory solution to this crisis.
Whatever happens, North Korea must account for its plutonium and abide by the NPT. The North will need to supply significantly more information about the past operation of its nuclear facilities, as well as its intentions for its nuclear reactors that remain under construction.
Production reactors
Technical reasons may make it difficult for North Korea to maintain its no-reprocessing pledge. The small reactor and the two others under construction use a design that depends on carbon dioxide gas cooling and graphite moderation. In the West, this type of reactor is called a magnox or gas-graphite reactor.
Britain and France developed this reactor type in the 1950s to make plutonium for nuclear weapons and to produce electricity. Its design is largely unclassified and the reactors are straightforward to build. The North appears capable of building them without significant foreign assistance.
The disadvantage of this type of reactor is that its spent fuel must eventually be processed, whether or not one wishes to recover the plutonium for weapon purposes. It is difficult to store spent magnox fuel safely for an extended period or to dispose of it in a geological repository.
North Korean fuel rods are encased in a magnesium alloy. This "cladding" breaks down if the rods are stored in water or exposed to moisture. If the uranium metal is exposed to air, radioactive material may escape and the uranium fuel oxidizes or "rusts." The uranium fuel can, under some conditions, ignite spontaneously if exposed to air. If it burns, a significant fraction of the radioactive materials can be released into the environment.
Although North Korea's choice of gas-graphite reactors increased suspicions about its interest in developing nuclear weapons, North Korean officials told the IAEA that they chose gas-graphite designs because they could develop them without foreign assistance. But this answer does not eliminate the fact that these reactors can produce weapon-grade plutonium relatively easily.
The plutonium calculus
Construction of the North's first reactor began in 1980. According to U.S. officials, the North began operating the reactor in 1986 and, for the first few years, experienced a number of start-up problems. However, by 1990 or 1991, these officials say, it was operating at 20-30 megawatts (thermal). One official added that U.S. intelligence agencies cannot determine with any accuracy the power output of the reactor during its first few years of operation.
The North describes the reactor as a 5-megawatt power reactor. This is an apparent attempt to direct attention away from its potential military purposes. The reactor's output of plutonium depends on its total energy output--its thermal power--not its capacity to produce electricity. (Since estimates of the reactor's thermal power range between 20 and 30 megawatts, I have used 25 megawatts in my calculations.)
Although the fuel remains in the reactor for several years, the total irradiation of discharged fuel is small. But the fuel is irradiated long enough to insure that enough weapon-grade plutonium has been produced to make reprocessing worthwhile. The level of irradiation is measured in terms of "fuel burnup" or the total amount of energy extracted per metric ton of fuel.
To replace the fuel in the core, the reactor must be shut down for unloading and reloading. The reactor core contains about 50 metric tons of natural uranium fuel in the form of short fuel rods. Each is about 50 centimeters long, 3 centimeters in diameter, and weighs about 6.2 kilograms. The core just discharged contained about 7,700 of these rods in 812 fuel channels. About 300 damaged fuel elements were removed from the reactor between 1989 and April 1994. Each channel can hold as many as 10 fuel rods, stacked one on top of the other. The reactor is loaded and unloaded through the top of the core.
How much plutonium does North Korea have? I derive the upper bound on plutonium production by assuming that the reactor has operated at full power about 80 percent of the time. Such consistent operation is probably at the upper limit of the reactor's capability. This estimate also relies on the belief that the fuel core was unloaded and the reactor refueled at least once. The reactor might have produced about 6.6 kilograms of weapon-grade plutonium a year. If the reactor had operated consistently at full power 80 percent of the time, by now it would have produced a total of 53 kilograms of weapon-grade plutonium. But very few analysts believe that the reactor has operated so well.
A 1989 refueling? A more credible worst-case scenario assumes that the North unloaded the core in 1989, but that the reactor has not operated as well as assumed in the above scenario.
In December 1993, the public learned for the first time that U.S. officials thought the first core had already been unloaded. Defense Secretary Les Aspin said on the December 7 MacNeil-Lehrer NewsHour that "in 1989 the North Koreans shut down their reactor for 100 days, and that would have given them enough time" to extract at least some of the fuel. He continued: "Depending upon how much plutonium they processed and their capabilities of putting that together into a bomb, they might have gathered enough plutonium for a bomb, maybe a bomb and a half."
A U.S. official said in a June 1994 interview that Aspin's 100-day statement was meant to be a crude estimate. The actual length of the shutdown was closer to 70 days.
A 70-day shutdown appears long enough to unload the entire core and put in new fuel, as events this spring have shown. Before this recent reloading, the North told the IAEA that it would take about two months to change the core. With two machines unloading the fuel, the North took less than one month to unload all the fuel. Thus, an upper bound is that the entire core was replaced in 1989.
But U.S. and IAEA officials are skeptical about a 70-day unloading in 1989. Based on operations at Britain's Calder Hall reactor, a 70-day shutdown might have provided only enough time to unload about half the core.
In summary, these estimates imply that between one-half and all of the core--roughly 25-50 metric tons of uranium fuel--could have been unloaded in 1989.
The plutonium in the samples taken by the IAEA had a range of isotopic compositions. The bulk of each sample was plutonium 239, and about 2.25-2.5 percent of the same was plutonium 240 and plutonium 241. Based on unclassified U.S. government studies of gas-graphite reactors, irradiated fuel that contains these fractions of isotopes contains about 0.27-0.30 kilograms of weapon-grade plutonium per metric ton of fuel. [2] (I use the mid-point of this range.)
If the level measured by the IAEA represented the average irradiation level for all the fuel in the core, the entire 50-ton core would contain a total of about 14 kilograms of weapon-grade plutonium. If 25-50 metric tons of fuel were removed in 1989, that fuel would contain about 7-14 kilograms of plutonium.
To produce that much plutonium, the reactor would have had to have operated about 55 percent of the time from 1986 to 1989. A 55 percent overall rate would be consistent with the statement of a U.S. official who said the reactor operated poorly during its first year and a half of operation, and only gradually approached its nominal power.
If, however, one believes the North's declaration that it removed only damaged fuel rods, how much plutonium would it have removed in 1989? The North said that the damaged rods it processed in the Radiochemical Laboratory in 1990 contained about 0.13 kilograms of plutonium, of which about .09 was recovered, with the rest remaining in the waste or processing equipment. Using the same assumptions, the North could have extracted this amount of plutonium from about 450 kilograms of uranium fuel, or about 70 rods. The North told the IAEA that it had removed several hundred rods in all, and the remainder remain in dry storage at the reactor.
The 1994 refueling. When the reactor was shut down for refueling in April, U.S. and IAEA analysts variously estimated that the unloaded spent fuel contained 20-30 kilograms of weapon-grade plutonium. What are these estimates based on?
The irradiation levels of the current fuel rods are believed to be higher than they were for any fuel discharged in 1989. If this is the original core, the fuel would have been in the reactor for eight years, which would mean a higher burnup. If it is the second core, the fuel would have been in the core since 1989. In any case, there has probably been a higher fuel burnup because the reactor's operation is believed to have improved. If the average burnup of the fuel approached the maximal weapon-grade plutonium production for this type of reactor, the core could contain 33 kilograms of weapon-grade plutonium. To achieve this level with the original core, the reactor would have had to operate at full power an average of 55 percent of the time between 1986 and 1994. On the other hand, if this is the second core, the reactor would have had to operate at full power 85 percent of the time. The latter is not considered credible.
At the other extreme, the core could contain as little as 17 kilograms of weapon-grade plutonium if the fuel burnup was typical of early gas-graphite reactors dedicated to weapon production. If this is the first core, the reactor would have had to operate at full power only 25 percent of the time from 1986 to 1994. If it is the second core, the reactor would have had to operate at full power about 45 percent of the time from 1989 to 1994. Based on information about the reactor's operation, both of these scenarios appear to underestimate the reactor's actual performance.
In summary, the midpoint of these estimates is 25 kilograms, with a range of 17-33 kilograms.
The new reactors. The North has worked intensely to finish the 200-megawatt thermal, 50-megawatt electric gas-graphite reactor at Yongbyon. The earliest possible startup date for this reactor is late 1995, although U.S. Defense Secretary William Perry said recently that it will probably be a few years before it is completed.
According to inspectors, the exterior of the reactor is largely finished but the interior requires more work. According to one U.S. official, the North added electricity-production capability just before the inspections began.
Many analysts believe that the North originally intended this reactor to be its main source of plutonium for a nuclear weapons program. They believe the North intended the small reactor to produce enough plutonium for the first few weapons, with the larger reactor the major source of plutonium thereafter. Based on the earlier U.S. estimates of plutonium production in magnox reactors, the larger reactor could eventually produce 40-53 kilograms of weapon-grade plutonium a year, enough for eight to 10 implosion-type weapons a year. (This estimate has a high degree of uncertainty. The reactor's actual thermal power could be higher or lower, and the plutonium production rate could vary.)
The 200-megawatt electric reactor at Taechon could be finished by 1996. The thermal power of this reactor is estimated at about 600-800 megawatts. If this reactor were operated to produce weapon-grade plutonium at a capacity of about 70 percent, it could produce between 140 and 180 kilograms of weapon-grade plutonium a year.
But few believe that the Taechon reactor is intended for weapon-grade plutonium production. This reactor will probably be optimized to produce electricity, meaning that it would produce plutonium that is not of weapon quality. However, it could serve as a backup production reactor if the other reactors did not produce enough weapon-grade plutonium or failed for some reason.
Separating plutonium
Producing plutonium is only the first step in making a nuclear weapon. The plutonium must next be chemically separated from irradiated fuel, and inevitably some of the plutonium is lost in the separation process. However, the North has worked on the processes associated with separating plutonium for many years and its knowledge of plutonium chemistry appears to be extensive.
Early efforts. In 1992 IAEA Director Hans Blix told the House Committee on Foreign Affairs that the North had conducted "experiments quite a number of years ago in which they identified plutonium." U.S. officials say the North conducted early laboratory-scale plutonium separation in "hot cells"--lead-shielded rooms with remote handling equipment for examining and processing radioactive materials. The Soviet Union supplied these hot cells in the 1960s or 1970s as part of the deal that included the IRT research reactor. According to IAEA officials, the North told the IAEA that it had separated only grams of plutonium in the hot cells. This separation campaign occurred before the IAEA safeguards were applied to the IRT reactor in 1977.
North Korea told the IAEA during its initial visit in May 1992 that it had scaled up from laboratory experiments to an industrial-size plant without building a pilot plant. North Korea said it often followed this course of action in industrial development. Although Western officials believe that the North could have jumped from hot cells to industrial-scale production, questions remain about the history of North Korea's plutonium separation program. Some analysts believe that the North operated a pilot plant that neither the IAEA nor intelligence services have yet discovered.
The Radiochemical Laboratory. The plutonium separation plant under construction at Yongbyon is large--180 meters long and six stories high. U.S. officials estimate that when it becomes fully operational it could theoretically process several hundred tons of spent fuel a year. They believe that it is large enough to handle the spent fuel that will be produced by all three gas-graphite reactors.
The North is believed to have received basic knowledge about reprocessing technology and chemistry many years ago from Russia and perhaps China. One inspector said that the Radiochemical Laboratory resembles a European reprocessing facility, the Eurochemic plant that operated in Belgium from 1966 until the mid-1970s. Information about this plant is largely declassified.
Some reprocessing chemicals probably came from abroad, and the Washington Post reported on April 2, 1994, that the North got stainless steel tanks from Japan. North Korea is also thought to have imported leaded glass for its hot cells. None of this implies, however, that the North has received significant foreign assistance.
The Radiochemical Laboratory had one reprocessing "line" in 1992, when inspections began. A line requires equipment to dissolve the fuel, to extract the plutonium, and to convert the plutonium into a pure form. According to one inspector, the facility's waste reduction processing section was not finished. (Plutonium separation facilities typically treat liquid waste to recover acids and other chemicals and to reduce the total volume of liquid waste that goes to the waste tanks.)
There is little hard information on the current capacity of this line to process spent fuel, but many believe that it could have processed all the fuel in the core of the small reactor before IAEA inspections began.
A preliminary estimate of its maximum annual capacity can be derived by assuming that it is sized to process all the fuel from both reactors at Yongbyon, or about 160 metric tons of fuel per year. If this estimate is accurate, it could reprocess 50 metric tons of spent fuel--an entire core--in as little as three months.
No plutonium separation process is 100 percent efficient. The North declared that it had separated 90 grams of plutonium and lost 40 grams in the waste. A loss rate over 30 percent is high, but possible, when first starting a plant. The Radiochemical Laboratory, however, should have been able to reduce its plutonium losses relatively rapidly. It seems more likely that any additional processing would have lost no more than 10 percent.
IAEA inspectors reported that a second line was nearing completion when they visited in March 1994. This line is nearly identical to the first and, when finished, will roughly double the plant's separation capacity. In addition to insuring sufficient capacity for all three reactors, the second line could also serve as a backup in case the first line fails. Such redundancies are common in newer plants.
The IAEA first learned about the second line during its initial inspections in 1992. One IAEA official said in a March 1994 interview that, during the most recent inspection, the inspectors were surprised to find that "a great deal of construction activity was going on." The IAEA did not think significant construction had taken place at the Radiochemical Laboratory since inspections began in 1992. He added, however, that the North had not allowed the IAEA to make an adequate inspection at the laboratory since spring 1993.
This official said that by March 1994 many components of the second line were installed but that it lacked the necessary instrumentation to monitor and operate the separation process. He thought the North had deliberately not finished or operated the second line.
Adding it all up
At the least, North Korea admits to having separated 100 grams of plutonium. At the most, the most believable worst-case estimate is that in 1989 North Korea removed irradiated fuel from its first reactor that contained 7-14 kilograms of weapon-grade plutonium. If fuel was unloaded in 1989 and processed during 1989-91, the North could have a total of 6-13 kilograms of separated plutonium.
A first nuclear weapon can require up to 10 kilograms of separated weapon-grade plutonium. This quantity is about twice the amount needed for the actual device, but plutonium is lost during each step in the weapon-manufacturing process. Most of the plutonium lost in these steps can be recovered and used in later weapons. The North might therefore have enough separated plutonium for one, or perhaps two, nuclear weapons.
How much more? In any case, the spent fuel unloaded this year contains an estimated 25 kilograms of weapon-grade plutonium. Using the same assumptions as above, North Korea's spent fuel contains enough plutonium for four or five nuclear weapons. However, it remains in the irradiated fuel and must be separated before it can be used. As of late July, no separation had taken place.
Bombs
Many experts believe that North Korea is capable of developing an implosion-type nuclear weapon, although there is little direct evidence that the country has done so. Although the CIA's worst-case assessment puts enough plutonium in North Korean hands to make a bomb, the agency has not claimed that North Korea actually has a nuclear explosive device. CIA officials say that North Korean scientists have not received training in nuclear weapon technologies from Russia or China. This statement implies that North Korea would have to develop its nuclear weapons largely on its own. Many believe that North Korea has as-yet-undiscovered nuclear weapons manufacturing sites.
One indication that North Korea may have a weapon program comes from the high-explosives testing conducted at Yongbyon. North Korean officials say they are using high explosives to shape metals, a technique several countries are pursuing for metals that cannot be shaped conventionally.
There are a few other signs that could point to a nuclear program. The North was reportedly interested in acquiring instruments for conducting non-nuclear tests associated with a weapons program. These tests usually involve high explosives.
U.S. intelligence agencies were quicker to suggest that the North may have a design for a first-generation implosion weapon. The mass of a device within the North's capabilities would probably be greater than 500 kilograms, but less than 1,000.
Little is known about how the North would deliver such a device. Delivery of such a device by aircraft is possible, but the substantial air defenses in North Asia would pose a serious threat to any North Korean aircraft. The North could deliver a nuclear device by ship or truck.
According to the intelligence assessments, any North Korean device would be too bulky to fit on a Scud missile. But it might fit on the Rodong, a missile the North is now developing. Although this missile, which was first flight-tested in late May 1993, has an estimated range of more than 1,000 kilometers, it is still a few years away from deployment.
Yes, based on the evidence, North Korea could have separated enough plutonium for a nuclear weapon. And it now has enough spent fuel cooling in ponds to produce enough weapon-grade plutonium for four or five nuclear weapons. However, that plutonium is useless unless it is separated from spent reactor fuel. The focus should be on preventing the North Koreans from separating it themselves and encouraging them to send it abroad for processing.
Any settlement also needs to include a way to insure that the IAEA can verify North Korea's past nuclear activities. The IAEA still needs to determine the amount of plutonium North Korea may have separated in the past.
1. International Atomic Energy Agency, "Transcript from the Press Briefing by Dr. Hans Blix, Director General of the IAEA," Beijing Hotel, Beijing, May 16, 1992.
2. S. E. Turner, et al., Criticality Studies of Graphite-Moderated Production Reactors. Report prepared for the U.S. Arms Control and Disarmament Agency, SSA-125 (Washington, D.C.: Southern Science Applications, Jan. 1980).
David Albright is the president of the Institute for Science and International Security in Washington, D.C., and a Bulletin contributing editor.
September/October 1994 pp. 46-53 (vol. 50, no. 05) © 1994 Bulletin of the Atomic Scientists
Sidebar: Rust never sleeps
In its extraordinary haste last May and June to unload fuel rods from its reactor, North Korea may have set the stage for yet another flare-up over its presumed nuclear weapons program. If so, the crisis may have more to do with North Korean incompetence--or intransigence--than with duplicity.
The North Koreans simply do not have the means to safely store their type of spent fuel for more than a few months. If it is not sent for reprocessing relatively soon after being extracted from the reactor, it becomes unstable and dangerous.
As of late July, North Korea had not yet begun to move its latest batch of fuel rods from the storage ponds to the Radiochemical Laboratory for plutonium separation. To do so would have violated its pledge made to the United States in June to "freeze" its nuclear program.
But according to Selig Harrison, a senior associate at the Carnegie Endowment for International Peace, North Korean officials told him in early June that--depending on the condition of the fuel rods--some of them might have to be removed after just two months in the ponds. The officials also said that complete unloading of the ponds would require two to three months.
Given that possible timetable, the United States and South Korea may have become embroiled in yet another crisis by the time you read this. If plutonium separation starts before inspection agreements are worked out, the North Koreans will say that they must reprocess for reasons of safety. Meanwhile, the United States may assume--once again--that it has been conned.
Spent fuel of the type used by the North Koreans--uranium metal in a magnesium alloy "cladding"--corrodes even under optimal storage conditions. But conditions at the North Korean reactor complex are far from optimal; they border on the primitive. Corrosion could be swift.
Given its inherent instability, the North Korean "magnox" fuel has to be reprocessed quickly. It cannot be left for long in the two storage ponds where it now sits. (In contrast, uranium-oxide fuel clad in zirconium alloys, which is used in the United States and in many other nations, can be safely stored in ponds for centuries.)
About 8,000 fuel rods--or about 50 metric tons of fuel--were placed in pond storage. (About 300 rods that appeared damaged were placed in dry storage.) Both the magnesium cladding and the uranium-metal fuel are vulnerable to corrosion when stored in water, humid air, or even in the presence of inert gases, if moisture is present.
If the cladding fails, large quantities of radioactive materials could leak into the pools and eventually get into the air or into groundwater, posing a risk to workers and the surrounding population.
More serious, however, is the possibility of fire. Once the magnesium cladding fails, the uranium metal itself can corrode, resulting--under the right conditions--in the formation of uranium hydride. When exposed to air--as it would be when the fuel rods are retrieved for reprocessing--uranium hydride ignites easily at room temperatures. And if a coating of uranium hydride ignites, it can touch off the uranium metal. A uranium fire fits anyone's definition of a bad accident.
Inspectors from the International Atomic Energy Agency (IAEA) report that the North Korean storage ponds (actually, large swimming-pool-like basins) lack adequate filtering or purification systems. The water is dirty and green with algae. Further, the IAEA says the North does not control the chemistry of the pond water.
Analyzing the chemistry of the water--and adjusting it accordingly--is essential to safe (but still temporary) storage. Even relatively small variations from optimal standards can significantly accelerate corrosion. Among the key steps that can be taken by the North Koreans are:
• Fill the ponds with demineralized water and keep chloride and sulfate concentrations to below 0.5 grams per cubic meter.
• Raise the pH levels to at least 11.5 by adding pure sodium hydroxides. (Current pH levels are about 11.)
• Reduce water temperatures (with portable chillers) to about 15 degrees centigrade to improve water clarity, retard algae growth, and reduce the rate of magnesium corrosion by roughly fourfold. (Current water temperatures are believed to be about 30 degrees centigrade.)
These relatively simple steps can extend fuel storage life for months--perhaps for as much as a year. But the corrosion rate can be further reduced--permitting wet storage of up to five years--by raising the pH level immediately around the fuel rods to 13 while maintaining low chloride and sulfate levels.
A pH level that high cannot be maintained in open ponds, because the pH level is lowered by dissolved carbon dioxide from the air. But Britain has developed magnox fuel canisters that can be lowered into the ponds, and which allow maintenance of the proper pH levels around bundles of rods.
Corrosion of the North's spent fuel has probably already occurred, perhaps to a serious extent. But Britain and France have extensive experience in reducing the rate of corrosion of magnox fuel, which they also use, and they stand ready to provide technical help if North Korea asks for it. The United States would help, too.
But will North Korea accept technical aid from outsiders? If it does accept help--soon--the fuel can remain in the ponds for many months or even for a few years. That would give everyone breathing room--time to work out a lasting solution to the on-again, off-again crisis.
--D. A.
he Democratic People's Republic of Korea has a population of nearly 20 million people, but only two citizens--and one of them is dead. 
n February 10, North Korea announced for the first time that it possesses nuclear weapons. [1] The claim grabbed headlines, but it is difficult to substantiate. In the early 1990s, the CIA concluded that North Korea had effectively joined the nuclear club by building one or possibly two weapons from plutonium it produced before 1992. [2] Yet North Korea has never conducted a nuclear test, and although it has extracted weapon-grade plutonium, it has never conclusively demonstrated that it possesses operational nuclear warheads. (Nor has the United States been able to verify it.) It is known, however, that Pyongyang has a nuclear program. By cataloging the program's capabilities and quantity of separated plutonium, it is possible to estimate how many nuclear weapons Kim Jong Il's country might have.
ho failed to live up to the Agreed Framework between the United States and the Democratic People's Republic of Korea? A better question might be, who didn't?
ovember 16, 1994: The cold morning sky was clear as we took off from the Pyongyang airport in a dark green military helicopter bearing the distinct red-star emblem of the Democratic People's Republic of Korea. As the vintage 1960s chopper flew with surprising quiet over the rough mountain terrain, Dr. Li Sang Gun, director of the "Radiochemistry Laboratory" at Yongbyon, and I made an attempt at small talk. 