Building a nuclear weapon is not easy -- nor as difficult as you might hope.

Although technical details of building nuclear weapons are classified, basic information on designing a simple fission weapon has been available for many years in open literature.

Uranium 235 and plutonium 239 are the most efficient materials for making a nuclear bomb. Sophisticated nuclear weapons use material enriched to 90 percent U-235 -- a level considered weapons-grade.

Plutonium, which does not occur naturally, must be produced in a reactor.

"The existence of simple designs for nuclear weapons is what puts nuclear weapons within the reach of terrorist organizations that are able to obtain fissile material," four nuclear scientists wrote in Avoiding Nuclear Anarchy.

Bomb expert Robert F. Mozley, a retired Stanford University particle physicist, said that "even a weapon that is poorly constructed or is made of a large amount of lower-grade uranium could be a very messy and inefficient but incredibly destructive bomb."

A kiloton (equivalent to 1,000 tons of TNT) nuclear-weapon detonation above ground would create massive destruction:

At a distance of 600 feet from ground zero, winds would blow at 675 m.p.h. and all life would perish almost immediately.

One-quarter mile, or 1,320 feet, from the center, winds would blow at 220 m.p.h., all brick buildings two stories or higher would be destroyed, and reinforced concrete buildings would be heavily damaged.

Half the people exposed to radiation 900 to 1,300 feet from the explosion would die within 60 days, even with medical treatment. The remainder would suffer various degrees of radiation sickness or long-term health consequences.

To build a crude nuclear bomb would require at least one person with knowledge of physics and several with welding and other toolmaking skills. A simple HEU device could be constructed by a half-dozen people, said physicist Stephen Fetter of the University of Maryland. Because plutonium is more radioactive and has more barriers to easy construction, as many as 10 people may be needed, Fetter said.

A beginner with limited scientific knowledge would need as little as 3 kilograms of plutonium for a kiloton yield, or 6 kilograms for a 20-kiloton bomb. Eight kilograms of HEU would be needed for a 1-kiloton bomb, or 16 kilograms of HEU for a 20-kiloton weapon.

Someone who is technically proficient would need less material -- as little as 3 kilograms of plutonium or 5 kilograms of HEU to produce a 20-kiloton yield, according to the Natural Resources Defense Council in Washington.

© 1998 Philadelphia Newspapers Inc.