U.S. and Soviet scientists checked elaborate detection equipment atop a picturesque mesa as the countdown continued for a historic joint nuclear weapons test Aug. 17.

"I never thought I would see the day the Soviets were involved with us here," test director Joe Behne said as he and other officials stood at ground zero Thursday.Officials talked to reporters while standing on a platform atop a 2,200-foot shaft containing a nuclear device nearly 12 times as powerful as the bomb that destroyed Hiroshima.

U.S. Ambassador C. Paul Robinson, who helped forge the agreement with the Soviet Union, called the test unprecedented and a breakthrough in negotiations on the sticky issue of nuclear testing by the two superpowers.

Robinson called this test, and a similar shot in the Soviet Union next month, a major step in the new policy of "trust but verify."

The Joint Verification Experiments at the 1,350-square-mile Nevada Test Site this month and the Soviet site in Semipalatinsk next month are designed to measure each country's ability to monitor the size of the other's tests.

The verification could lead to ratification of treaties limiting the size of nuclear tests. The treaties were signed in 1974 and 1976 but never ratified.

The two countries had accused each other of cheating on the size of their tests. The United States has denied exceeding the 150-kiloton limit set by the treaties. "There's no question but what there was a problem, with both sides accusing the other of cheating," Robinson said.

The ambassador credited new technology which measures the size of a nuclear blast and a changing political climate between Washington and the Kremlin responsible for the breakthrough in negotiations.

President Reagan named Robinson as chief U.S. negotiator at the nuclear test talks in Geneva last February.

The Aug. 17 blast will have an explosive punch of up to 150 kilotons, nearly 12 times the force of the atomic bomb that destroyed Hiroshima 43 years ago Saturday.

Behne, of Lawrence Livermore Laboratories, said no one would be allowed within 15 miles of ground zero when the device is set off.

It would too dangerous to be closer, even though the device is buried four-tenths of a mile deep under tons of concrete, sand and gravel, said Energy Department spokesman Chris West.

U.S. and Soviet trailers packed with monitoring equipment sit hundreds of yards from ground zero, tied down to keep them from being thrown in the air and placed on special stands to absorb the shock.

About three dozen Soviet scientists will be on hand for the blast, code-named Kearsarge after a ghost town in the Sierra Nevada.

U.S. scientists now in the Soviet Union will monitor that test.

The device to be detonated later this month was delivered July 20 and placed 180 feet above the bottom of the 2,200-foot vertical shaft.

The Soviet scientists, who have been on the Nevada site since spring, were not allowed to see the device itself, Behne said.

But the Soviets were allowed to examine the canister holding the device once the container, painted red, white and blue, was sealed. The steel canister is 8 feet tall, 88 inches in diameter and weighs 71/2 tons.