A thermonuclear weapon buried 1,200 feet beneath the desert was detonated Saturday - the second announced U.S. nuclear test within a month of the President Reagan's May 30 summit date with Mikhail Gorbachev in Moscow.

Energy Department spokesman Jim Boyer said the nuclear detonation had a yield of "less than 150 kilotons," putting it within the threshhold of a superpower treaty negotiated in the 1970s by President Nixon but never ratified by the Senate. The atomic bomb dropped on Hiroshima was a 13-kiloton weapon."We apparently have another successful test both from a safety standpoint and otherwise," Boyer said. "There was no leak of radiation into the atmosphere."

Boyer, who was eight miles from ground zero, said, "There was considerable earth motion here and it seems like I felt two separate waves."

Boyer said the detonation likely was not felt in Las Vegas other than being recorded on seismic instruments scattered in highrise buildings throughout the gambling resort. Residents reported feeling no earth movement.

The test, code named "Laredo," originally was scheduled for 8 a.m. Thursday but was delayed until 3:30 p.m PDT Saturday because of adverse winds and technical problems. It was the second nuclear weapons test by the United States this month and the fourth this year.

The test conducted Saturday, as well as one detonated May 13, occurred while seven Soviet scientists were on the Nevada Test Site in preparation for a joint verification nuclear test expected later this year.

The Soviets as well as all Nevada Test Site workers not directly involved in the countdown or detonation were either off work or confined to Camp Mercury, a base camp about 40 miles from ground zero.