All so-called "fission-fusion-fission" weapons (i.e., all conventional modern thermonuclear warheads) employ the additional step of "jacket fissioning," using fusion neutrons.

Enough plutonium has been produced for 35,000 to 45,000 modern warheads.

The bomb detonated that day was miniaturized with a core, the spy said, in the distinctive shape of an ovoid, indicating China had begun to master the art of making modern warheads.

Supplies of the gas in modern warheads must be periodically replenished or the bombs cease to function.

The spy's report arrived as American intelligence agencies were examining a separate suspected Chinese espionage coup: the theft of designs of America's most modern nuclear warhead, the W-88.

During World War II, the precision of the charge's construction and its detonation mode were both inferior to modern warheads.

This reflects the fact that even modern warheads may not always destroy a target such as an aircraft, missile or main battle tank.

In 2005, the average will be 20 years, the planned lifetime of many modern warheads when they were designed.

For months, American intelligence experts have been investigating the possible acquisition by China in the 1980's of the designs of the United States' most modern nuclear warhead.

The W-69 warheads were designed with a chemical explosive that was less resistant to fire or crashes than more modern warheads.