Los Alamos National Laboratory has announced it was the first lab in the Western Hemisphere, and possibly the world, to use a radioactive material other than plutonium or uranium to achieve a nuclear chain reaction, which can lead to a nuclear explosion if it is not controlled.

"It is common knowledge that plutonium and uranium can be used in a nuclear weapon. It is not common knowledge, however, that neptunium can also be used to make a nuclear weapon," said Kevin Roark, a lab spokesman.

The experiment, 12 years in the making, was completed in late September and gives nuclear scientists for the first time a clear indication of how much neptunium is needed to create a critical mass, or a nuclear chain reaction.

Scientists now know it takes about 30 percent less neptunium than previously thought, or about 60 kilograms, to generate a nuclear chain reaction.

The criticality experiment was done at Los Alamos National Laboratory's Technical Area 18, the only site in the country capable of doing nuclear chain reaction experiments. The work supports the Department of Energy's Criticality Safety Program and the National Nuclear Security Administration's Non-proliferation and Emergency Response programs.

Lab scientists used neptunium-237, the most stable of 20 isotopes, or variations, of neptunium with a half-life of about 2 million years.

The whole experiment took only about four days, but the technical preparation, dealing with security issues and gaining clearance from the Department of Energy to do the work took about 12 years.

Scientists formed about 6 kilograms of neptunium into a sphere, which did not have enough mass in itself to sustain a chain reaction.

To help drive the reaction, scientists placed the neptunium sphere in a bed of several bowl-shaped shells of highly enriched uranium, which they slowly raised with a mechanized device toward another group of highly enriched uranium shells at a fixed position above.

The uranium helped drive the neptunium to a critical mass, and the scientists could carefully control the reaction's rate by raising or lowering the neptunium and uranium assembly away from the fixed uranium shells to prevent the reaction from going "supercritical." That's when nuclear chain reactions become explosive.

Roark explained that since the reactivity of enriched uranium is well established, the critical mass of neptunium can be calculated from measurements taken during the experiment.

He said the team of scientists plans to do more experiments with neptunium, using different configurations and materials, to establish the full range of masses capable of generating nuclear chain reactions.

The critical mass can vary depending on how many neutrons - the subatomic particle that splits atoms and releases energy in a chain reaction - get reflected back into the reaction to split more atoms and release more energy.

Rene Sanchez, one of the scientists involved in the experiment, said the purpose of the work was primarily to determine safety limits for technicians working with neptunium, a byproduct of nuclear power generation.

"We wanted to know how many kilograms can be put into a container before it goes critical," he said.

But the information is also essential for nuclear non-proliferation, because it defines more clearly how much neptunium might be required to make a nuclear weapon.

"That wasn't the intention of this experiment, but a byproduct, that it could have the potential to be used in a nuclear weapon," Sanchez said.

Roark said the lab was involved in getting the Switzerland-based International Atomic Energy Agency active in a global monitoring program of neptunium that began in 1999.

But no monitoring was in place before then.

"Who knows what has happened (with neptunium) during all those years," Sanchez said.

To get an idea of how much neptunium is out there, Sanchez said the 100 nuclear power plants across the country can produce about 12,000 kilograms of neptunium in a decade.

But in the United States, neptunium is not easy to obtain, because the country does not reprocess its spent nuclear fuel. Reprocessing is necessary to separate neptunium from other spent radioactive fuels, something many European and Asian countries do.

"There is a lot of neptunium out there in the world," said Steve Clement, a member of the experimental team.

In addition to providing information on the safe handling of the element, he said, "We want to be able to account for that material and track that material to be sure that it does not fall into the wrong hands."