Government scientists plan to conduct an underground nuclear experiment Tuesday at the Nevada Test Site, the National Nuclear Security Administration said.

The subcritical experiment, dubbed "Armando," will involve detonating high explosives around plutonium in a steel sphere while X-rays, radar and lasers chart the behavior of the radioactive element in a non-nuclear explosion.

Scientists from the Los Alamos National Laboratory in New Mexico are conducting the test in a tunnel 963 feet below ground at the Nevada Test Site, about 80 miles northwest of Las Vegas.

"Armando" is the 21st subcritical experiment at the test site, and the third in a series. Its predecessors, "Mario" and "Rocco," were conducted in August and September 2002.

Anti-nuclear groups criticize subcritical experiments as contrary to the spirit of the 1996 Comprehensive Test Ban Treaty on nuclear arms. The U.S. has observed a nuclear testing moratorium since 1992, but has not ratified the treaty.

Federal officials call subcritical experiments essential to maintaining the safety and reliability of the U.S. nuclear arsenal. The experiments technically do not violate the treaty because no critical mass is formed and there is no full-scale nuclear explosion.

The Bush administration and Congress last year reduced from three years to two years the time it would take to resume full-scale nuclear tests.

The last subcritical experiment, "Piano," was conducted Sept. 19, 2003, by scientists from the Lawrence Livermore National Laboratory in California

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On the Net:

National Nuclear Security Administration: http://www.nnsa.doe.gov

Nevada Test Site: http://www.nv.doe.gov/nts

Los Alamos National Laboratory: http://www.lanl.gov