What is termed a "subcritical" experiment involving plutonium is to be carried out at the Nevada Test Site today, drawing criticism from a Salt Lake activist.

"We don't believe that subcritical tests are necessary," said Steve Erickson, director of the anti-nuclear Citizens Education Project. Such experiments tend to blur the distinction with actual nuclear detonations and could be an international problem, he said.

The NTS is located northwest of Las Vegas and upwind of Utah. The latest experiment at the site is about 85 miles from Las Vegas.

When above-ground nuclear detonations took place there in the 1950s and early '60s, radioactive clouds swept across Utah, dropping fallout. Above-ground tests were halted in 1963 after a test ban treaty, but underground testing continued.

However, sometimes underground tests vented into the atmosphere, sending radioactive material into the air. Since 1992, the United States has observed a moratorium on nuclear detonations. But it has not ratified the Comprehensive Test Ban Treaty.

Besides the subcritical test, a new radiological defense complex is planned for the test site. Erickson said these could be part of a pattern leading to the resumption of full-scale underground nuclear testing.

On Sunday, the U.S. Department of Energy posted a press release saying the test site would conduct a subcritical experiment called Armando. The experiment, set for today, is to examine the behavior of plutonium as it is shocked by conventional high explosives.

"Subcritical experiments produce essential scientific data and technical information used to help maintain the safety and reliability of the nuclear weapons stockpile," says the DOE press release. Because the experiments are subcritical, "no critical mass is formed and no self-sustaining nuclear chain reaction can occur."

Because of that, it adds, "there is no nuclear explosion."

The complex where the experiment was to be performed is called the U1a Complex, which is "designed to contain these experiments in a safe and secure environment." The experiment was planned for an underground lab of horizontal tunnels about 960 feet below the desert floor.

The Armando test is part of a series. The most recent experiment before today was Rocco, on Sept. 26, 2002. So far, 20 subcritical experiments have been carried out at the test site, says the release.