Pursuing the Bomb in North Korea

North Korea has produced enough nuclear weapon material for six to eight atomic bombs. This is the conclusion of U.S. intelligence, which has watched a small reactor operate for four years at a place called Yongbyon, 60 miles north of Pyongyang. Each year in its uranium fuel, U.S. intelligence analysts say, the reactor has created about two bombs’ worth of plutonium, the grey metal that destroyed Nagasaki in 1945.

These conclusions are based on overhead photographs and environmental sampling. From these observations, the reactor’s power level has been estimated at 30 thermal megawatts, a level quite close to that confirmed recently by the North Koreans, who have told the International Atomic Energy Agency that they designed the reactor to operate at 25 thermal megawatts. Either of these two thermal megawatt levels could easily provide the 5 electrical megawatt power level reported in the press.

The North Koreans have confirmed that the reactor is cooled by gas, fueled by natural uranium, moderated by graphite, and modeled on the United Kingdom’s reactor at Calder Hall.

The attached table shows the operating characteristics of three such reactors. All are well-known designs that have been used to produce plutonium. For all three, the amount of plutonium produced in the uranium fuel is directly proportional to the megawatt-days of operation. Also, the amount of fuel burn-up for all three reactors is optimized for plutonium production.

Applying the data in the table to North Korea leads to the following conclusions:

* The North Korean reactor appears to have produced six to eight bombs’ worth of plutonium.

The table shows that the reactor would have produced some  40 kilograms of plutonium in normal operation over four years, enough for 8 bombs at 5 kilograms per bomb. This size bomb would have the same power as the one dropped on Nagasaki. The table assumes off-line refueling for all types of graphite production reactors. In other words, the reactor would shut down for at least some time (probably a couple of months) each year. This would mean on-line operation about 80 percent of the time.

The actual quantity of plutonium produced depends upon how often the reactor is on line and its real power level. If it were on line only three-fourths as often as the table assumes, or if its power level were only three-fourths as great, 30 kilograms of plutonium would have been produced since 1987, enough for 6 bombs. If one assumes a real power level as high as 50 megawatts, which has been reported in the press, some 66 kilograms of plutonium would have been produced, enough for 13 bombs.

U.S. intelligence has observed nearly continuous operation since 1987 at high power levels. Thus, six to eight bombs’ worth of plutonium is a reasonable estimate.

* It is likely that plutonium-bearing fuel has been discharged in North Korea and is being hidden.

It is likely that one or more complete core discharges have been made from this reactor. The table shows that graphite production reactors are operated at low burn-up, meaning that the fuel is left in the reactor for only a short period of time. If the fuel is left in for a longer time, it builds up undesirable plutonium isotopes. Even intermittent operation for four years would require fuel discharge if the low burn-up levels in the table were not to be exceeded.

The reactor’s discharged fuel could easily be hidden. The table shows that a 30 megawatt reactor would discharge only 94 tons of uranium fuel per year, a total of less than 400 tons in four years. While this might sound like a large quantity, it would fit into a standard-size swimming pool. It could also be stored dry in one of the numerous deep tunnels the North Koreans are reported to have built.

The reactor has a pool designed to hold discharged fuel, but the International Atomic Energy Agency reports that the water in it is colored opaque green. On its first visit to the reactor in May, the Agency’s team was not able to see whether the pool contained any discharged fuel. This raises the possibility that discharged fuel has been moved elsewhere, and that its plutonium is being extracted for the production of nuclear weapons. One way around the visibility problem is to sample the water, which should indicate whether any discharged fuel has been stored in the pool.

* North Korea’s claims about its nuclear program are implausible.

North Korea claims that the small reactor did not work when operators tried to start it in 1987, and has been virtually inoperable since. As a result, North Korea also claims that all the uranium fuel originally loaded into the reactor is still there. This story directly contradicts U.S. observations, which show continuous operation at high power. In addition, Pyongyang is busy building two larger graphite-moderated reactors exactly like the small one. One doesn’t scale up a failed design.

The controversy over the small reactor’s operation can be put to rest by on-site inspections. Inspectors from the International Atomic Energy Agency, who are now inspecting the reactor, can trace its operating history by analyzing its internal parts. They can do the same for the reactor’s fuel. They can then draw their own conclusions about how much plutonium the reactor has made. If their conclusions match the observations of U.S. intelligence, they can ask the North Koreans to submit the undeclared plutonium to Agency inspection.

The North Koreans have also argued that the small reactor, as well as two larger 50 and 200 megawatt reactors that they are building, are limited to the peaceful production of electrical power. This is implausible. It is not economical to build 5, 50, or 200 megawatt reactors for power generation. They are too small to achieve the economies of scale that make nuclear reactors attractive. Nor is their graphite design the technology of choice for electrical power. The small, low-temperature graphite reactor has historically been used to produce plutonium for atomic bombs. Indeed, graphite reactors have been the hallmark of a nuclear weapon program. North Korea has also built these reactors without visible transmission lines. In January, CIA director Robert Gates told Congress that the reactors’ “sole purpose is to make plutonium.”

North Korea is planning to process the plutonium into weapon-ready form. To prepare plutonium for use in a bomb, it must first be extracted from discharged reactor fuel. This is a dangerous, expensive, messy process done behind heavy shielding. North Korea, a poor country, has built an extraction plant the size of an aircraft carrier, big enough to handle all three reactors’ discharged fuel and produce more than twenty bombs’ worth of plutonium per year.

North Korea says that it will use the plutonium to fuel future power reactors, but it is planning no reactors that could use plutonium fuel. Also, it calls the giant extraction plant a “laboratory,” which it obviously is not.

The concern is that North Korea probably would not have built such an expensive extraction plant without first building a successful prototype. U.S. analysts fear that the prototype, still hidden, could already have extracted enough plutonium for bombs.

* North Korea’s plutonium is a threat to world peace.

According to South Korea’s president, intelligence monitoring has picked up evidence of North Korean efforts to build the high-explosive parts for a nuclear warhead. Also, a North Korean defector is said to have reported on a secret nuclear weapon research site near Yongbyon. These reports support the claim that North Korea intends a military use of its plutonium.

North Korea has also developed nuclear-capable missiles at the same time it has developed its nuclear program. By upgrading the Soviet SCUD-type design, North Korea has produced a 350-mile missile that covers all of South Korea. More alarming is North Korea’s plan to field a 600-mile version this year that would reach Japan. This missile would make Japan vulnerable to long-range nuclear attack as soon as North Korea can put plutonium into compact warheads.

There is also the question of where North Korea’s plutonium might wind up. The cash-strapped North Koreans have sold everything they have produced. Their first batch of SCUD missiles was shipped in 1987 to Iran, which paid all the development costs. A subsequent batch went to Syria in 1991. Libya, too, has been a major buyer and funder of North Korean SCUDs. If foreign money is also behind the nuclear program–which seems likely–the world could soon see the first black market sales of renegade-made A-bombs, or the plutonium to fuel them. Libyan or Iranian bombs could then be smuggled into American cities.

* The public should be aware of the true risks presented by North Korea.

When inspectors from the International Atomic Energy return from a visit, they usually say little or nothing about what they found. They have a policy of holding their inspection results confidential.

This is ill-advised in this case. In order to build the international support needed to deal with North Korea, the inspectors should disclose what they have found.

If North Korea refuses to reveal the location of all of its plutonium, it will breach the Nuclear Nonproliferation Treaty and be subject to U.N. sanctions. If the inspectors are not satisfied with North Korean cooperation, they should publicly protest, and should bring North Korea before the United Nations Security Council. The world should then rally behind the inspectors and force North Korea to fulfill its obligations.

Representative Reactor Concepts
Thermal power, Mw 30 250 400
Coolant Air CO2 H2O
Number of fuel channels 1418 1892 2155
Moderator and reflector Graphite Graphite Graphite
Total graphite, mt 989 1550 2260
Fuel type Nat. U-metal Nat. U-metal Nat. U-metal
Cladding Mg or Al Mg Al
Base plants, previously constructed and operated Brookhaven, Marcoule G1 Calder Hall, Marcoule G2 Hanford, Soviet
Approximate Pu production 1 rate, kg/full-power yr 10 80 134
Uranium requirement, mt/full-power yr 1, 2 94 147 274
Fuel burnup, 1Mwd/mtU 115 620 520
Fissile Pu content, 1 % 99.2 95.5 96.2

1 Assuming annual fuel cycle.
2 Also equal to the total loading and to the quantity of uranium to be reprocessed in recovery of the plutonium.