NEVADA TEST SITE - At ground zero for the nation's nuclear testing, the stewards of the atomic stockpile stopped creating mushroom clouds and craters more than a decade ago.
Now they devise complex underground experiments using radar, laser and X-ray imaging to explore the finer points of how plutonium performs in an explosion.
Scientists call the experiments "subcritical" because they don't set off thermonuclear blasts like those that rocked the Nevada desert northwest of Las Vegas from 1951 to 1992.
"When you had the nuclear test, what was the proof? It exploded," said James Danneskiold, a spokesman for Los Alamos National Laboratory in New Mexico, where plutonium triggers for bombs are produced.
"Now, you have to ask the necessary questions to show that the weapon still functions as it was designed," Danneskiold said, "that it's safe, reliable, and will work when needed."
The experiments subject plutonium "pits" like those at the core of nuclear weapons to stresses that scientists say only high explosives can produce. They don't detonate the radioactive element, but collect data about how it would explode.
Contamination is a risk. Darwin Morgan, spokesman for the National Nuclear Security Administration, the agency that oversees the site, said no plutonium detonations have occurred and no radioactivity has been released during 20 previous subcritical tests.
Remoteness was a factor when the Nevada Test Site was established in 1950 by President Harry Truman. The site encompasses 1,375 square miles, nearly the size of Rhode Island, and is surrounded on three sides by the 4,562-square-mile Nellis Air Force Base bombing range. Combined, the federal reservation is larger than the state of Connecticut.
A short distance from the underground test laboratory is the Frenchman Flat dry lake bed, where the first of 1,021 Nevada nuclear weapons tests was conducted. After 14 atmospheric and five underground tests, Frenchman Flat remains strewn with structures built in the 1950s to measure the effects of primitive nuclear blasts.
Steel reinforcement bars from a crumbled concrete dome curl like hair blown back. Rusting pens mark where pigs dressed in Army uniforms were subjected to shock, heat and radiation waves. Warped wooden benches sit on a knoll where VIPs watched detonations from only nine miles away.
Before boarding a steel cage elevator for the 75-second descent down a mine shaft to the lab, Ghazar Papazian, Los Alamos project director at the test site, characterized the safety zones of the laboratory as a "nested bottle concept."
"If the first cork leaks, the second can contain it. If the second leaks, the third can contain it," he said, pointing to escape routes on a three-dimensional mock-up of one mile of underground tunnels.
At the underground lab, a vault 300 feet deep is filled with concrete where the 20 kiloton Ledoux underground nuclear test was conducted in September 1990. The bomb dropped on Hiroshima, Japan, in World War II measured 16 kilotons.
Other sealed vaults entomb most of the 20 previous subcritical experiments, and quarter-inch steel doors can be closed to seal tunnel sections like compartments in a submarine. Rubber-soled shoes squeak on painted gray cement floors.
"Armando," an upcoming test, is the third in a series. Its predecessors, "Mario" and "Rocco" in August and September 2002, were conducted in six-foot diameter wells drilled 35 feet deep beneath the tunnel floor of one finger of the complex.
The upcoming detonation 963 feet underground will involve high explosives inside a steel sphere that would fit in the back of a pickup truck. Cameras, lasers and a special high-intensity X-ray focusing 2 million volts of electricity will beam through the sphere to measure temperature, pressure and velocity, and any microwaves the plutonomium gives off.
The tunnels will be cleared of workers, while diagnostic equipment shielded in tractor-trailer sized containers collect data. After the test, the area will be swept for radiation before anyone returns.
"Armando" is designed to answer questions about how plutonium ages and whether weapons triggers produced by milling or casting processes perform the same, Papazian said.
Production of weapons-grade plutonium was suspended in 1989 at a mill in Rocky Flats, near Denver. A cast process at Los Alamos, N.M., is expected to resume producing 10 plutonium pits a year by 2007, Danneskiold said.
Tests also seek to determine "if things designed for 20 to 30 years can last for 40 to 60 years," Papazian said.
Experiments can cost up to $40 million each, compared with $90 million apiece for full-fledged underground nuclear tests. Papazian said he did not know what "Armando" would cost.
Test site officials call the program "stockpile stewardship" - essential to the U.S. policy of nuclear deterrence. Critics call it unnecessary.
"They're still doing bomb testing," complained Peggy Maze Johnson, director of Citizen Alert, a Nevada anti-nuclear advocacy group.
The number of U.S. warheads is classified. The Natural Resources Defense Council, which has monitored nuclear issues since 1970, estimates the United States has about 10,400 warheads - about half the nuclear weapons in the world.
Morgan said nuclear tests are strictly defined by international treaty.
"We do experiments," he said. "There's no sustained nuclear reaction."
The work has taken on new emphasis with the Bush administration seeking to cut the lead time needed to resume full-scale underground nuclear testing from in half, to 18 months. The United States has observed a nuclear testing moratorium since 1992, but has not ratified the 1996 Comprehensive Test Ban Treaty.
