N. Korea says it's close to enriching uranium. What is that?

North Korea's boast that it is on the brink of mastering the dark art of uranium enrichment is not a good thing, insofar as world efforts to stop Pyongyang's nuclear program are concerned.

If the statement is true, it means the North Koreans will have developed a backup means of producing the fissile material that lies at the heart of atomic weapons.

And it is a backup that is easy to hide. Production of plutonium – North Korea's primary fissile material – requires huge nuclear reactors. In contrast, enriched uranium is produced by arrays of narrow spinning tubes, called centrifuges.

An enriched-uranium program "can be done in small unidentifiable facilities, no larger than a warehouse, which are difficult to identify," said Victor Cha, a North Korea expert at the Center for Strategic and International Studies (CSIS), on Friday.

In a letter to the United Nations Thursday, North Korea said that it has almost completed reprocessing into plutonium thousands of spent fuel rods from its Yongbyon nuclear reactor. This fissile material will be used for weapons, said the letter.

In addition, the letter said, North Korean scientists have successfully carried out uranium-enrichment tests. "That process is in the concluding stage," said the letter, which was published by North Korean official media.

This statement is significant because it is the first official public admission by the North, after seven years of denial, that it has had a secret uranium-enrichment program, according to Mr. Cha.

The United States has long suspected that Pyongyang had such a program.

Nuclear weapons made from enriched uranium tend to be larger and easier to make than those that use a plutonium core.

Enriching the uranium, however, is a tedious process. Raw uranium feedstock is converted into gas, then fed through an array of spinning tubes. The centrifugal force of the tubes separates the uranium gas, with the heavier molecules of an isotope called uranium 238 moving toward the outside and the lighter molecules of uranium 235 staying toward the center.

What you are after is a high concentration of the lighter isotope, U-235, which constitutes only about 1 percent of natural uranium.

Gas from the inside of a spinning centrifuge is sucked out, then fed into another tube, again and again, gradually increasing the percentage of U-235 to the 20 percent level. Then, the gas will constitute "enriched" uranium.

Until this week, experts thought North Korea was years away from developing enriched-uranium-based weapons, but now they are not so sure.

Raw uranium is plentiful in North Korea, points out Cha of CSIS.

"North Korea has sizeable deposits of high-quality uranium ore, estimated in the range of 4 million tons," he said in an analysis of North Korea's statement Thursday to the UN.