The United States detonated a thermonuclear device 1,500 feet beneath the desert Friday, some 40 air miles from a base camp housing a group of visiting Soviet scientists.

"It is the first time in history that we have conducted a nuclear test while Russians were within the boundaries of the Nevada Test Site," federal Energy Department spokesman Chris West said.A half dozen Soviets scientists, on the Nevada Test Site in preparation for a joint nuclear verification experiment later this year, were confined to Mercury at blast time. Mercury is a Nevada Test Site base camp, about 40 air miles from ground zero at Yucca Flat.

The third announced nuclear test of the year went without incident, shaking the remote desert site 85 miles northwest of Las Vegas and registering 5.0 on the Richter scale at the National Earthquake Information Center in Golden, Colo.

"It was pretty good-sized, but some register as much as 6.0," said Bill Schmieder, a geophysicist at the center.

The weapons-related test had a yield of up to 150 kilotons, the largest allowed under existing treaties with the Soviet Union.

The Energy Department said the test went smoothly, with no apparent release of radiation.

"It appears to be a success," said department spokeswoman Barbara Yoerg. "Everything was contained underground."

Yoerg said ground motion from the explosion was not felt at the blast's command center, 13 miles from ground zero.

The Soviet scientists were not allowed to monitor the blast, the spokeswoman said.

The Energy Department had warned managers of high-rise buildings in Las Vegas, about 85 miles away, to keep workers out of precarious positions because of possible ground motion.

The weapon was buried 1,500 feet beneath the surface of Yucca Flat when detonated at 8:35 a.m. It was classified as having a yield of between 20 and 150 kilotons.

The test was initially set for Wednesday but was delayed a day because of winds. It was postponed again Thursday after a security safeguard designed to prevent accidental explosion froze and scientists were unable to detonate the weapon.

Five demonstrators gathered at a boundary to the site, about 45 miles from ground zero, to protest the test. Ms. Yoerg said the demonstration was peaceful.

The blast, code-named Shellbourne, was the third announced test of the year at the nation's nuclear proving grounds and the 678th since testing at the desert site began in 1951.

The Richter scale is a measure of ground motion as recorded on seismographs. Every increase of one number means a tenfold increase in magnitude.

A ground quake of 3.5 on the Richter scale can cause slight damage in the local area, 4 moderate damage, 5 considerable damage, 6 severe damage. A 7 reading is a "major" quake, capable of widespread heavy damage; 8 is a "great" quake, capable of tremendous damage.