From fuel, the seeds of bombs


Uranium: It's an inefficient and dangerous business, but the stuff of nuclear weapons can be extracted from the material that creates peaceful atomic power.

January 19, 2003|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

It is an unavoidable hazard of the Atomic Age.

The same nuclear reactors that generate the electricity we use to heat soup will always convert a fraction of their uranium fuel into plutonium - the critical ingredient for building atomic bombs.

North Korea says it only wants to generate electricity when it restarts its Yongbyon reactor.

But American experts estimate that Yongbyon, while small by Western standards, is big enough to forge 2.2 pounds of plutonium in its fuel assemblies for every 40 days the plant is generating electricity.

That's sufficient for North Korea to build a new Nagasaki-sized atomic bomb every 10 months - or every five if its scientists are especially good at extracting the plutonium from the spent fuel, says Ivan C. Oelrich, a senior research associate at the Federation of American Scientists.

And that doesn't count the plutonium already available in the 8,000 spent fuel rods the North Koreans are believed to have cooling in storage pools. That stockpile could be reprocessed quickly into five or six bombs, intelligence officials say.

"That's the more immediate problem," Oelrich says. "If we could work some deal where we got the fuel rods under control next week, we could worry about the reactor later."

The North Koreans are already believed to have a fuel-reprocessing plant, which they call a "radiochemical laboratory." American intelligence officials suspect that before it was shut down in 1994 in an agreement with the United States, the plant recovered enough plutonium to build one or two nuclear weapons.

Now there is fear the Pyongyang government is preparing to resume plutonium production. Last month it disabled the international monitoring equipment installed at the reprocessing plant and expelled the inspectors who had kept watch on it.

Nuclear reactors everywhere are fueled primarily by uranium. It is a naturally occurring element found in such ores as uraninite and pitchblende. It is as common in Earth's crust as tin.

Uranium is found in 16 slightly different forms, called isotopes. They differ in the number of protons and neutrons in their atomic nuclei.

The most common isotope is uranium-238, which accounts for more than 99 percent of all uranium. It is very weakly radioactive, but its slow decay explains much of the heat at the center of Earth.

Uranium-235 constitutes only about 0.7 percent of the total in nature. But it is the isotope critical to electric power producers because its atoms are the most easily split when struck by a speeding neutron.

That atomic split-up, or fission, produces heat. It also spits out two or three more neutrons. Those neutrons crash into more U-235 atoms, which split and produce more heat and still more flying neutrons. And a nuclear chain reaction has begun.

The reaction is stopped, slowed or accelerated in a reactor by the movement of control rods inserted among the fuel rods to absorb the neutrons.

What power plant operators want from the chain reaction is the heat, which is used to boil water and to produce the steam that turns the electrical generators. (A pound of U-235 contains 100 times the potential energy in a ton of coal.)

But neutrons released by the U-235 also strike atoms of U-238, which is far more abundant in the fuel rods, as in nature. Some of the U-238 atoms split when they're struck, producing more heat and neutrons. But some absorb the neutrons, and that transforms the uranium into plutonium - Pu-239, suitable for weapons.

As the chain reaction continues, the amount of plutonium forged in the fuel rods increases.

Extracting the plutonium from the spent fuel in order to build an atomic bomb is a relatively simple chemical problem, Oelrich says. But it is an expensive and challenging engineering feat because of the highly radioactive nature of materials in the spent fuel and the large volume of waste the work generates.

In addition to the plutonium and the unused uranium, the fuel rods contain the highly radioactive waste, or "ash," produced by the splitting of the U-235 atoms. It includes deadly isotopes of cesium, iodine, strontium and other elements.

Working behind heavy shielding, reprocessing workers slice open the rods' metal cladding, cut up the fuel and dissolve it in hot nitric acid. The resulting liquid is put through a chemical process that separates the usable uranium and plutonium from the unusable and dangerously radioactive waste.

Just how much plutonium bomb-makers can get from spent reactor fuel depends on several factors. But a general rule of thumb, Oelrich says, is that you get a gram of plutonium per day for each megawatt of heat produced by the reactor.

The Yongbyon reactor would produce 20 to 30 grams of plutonium a day when it's running, Oelrich says, equal to the weight of seven to 10 U.S. pennies.

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