U.S. and Soviet scientists, ignoring a diplomatic blunder half a world away, are conducting rehearsals for this week's historic nuclear experiment that will mark a milestone in the atomic age.

Wednesday's test blast comes nearly four years after President Reagan first proposed such a project during a speech to the United Nations.The joint verification experiment is designed to measure each country's ability to monitor the size of the other's test. Verification could lead to ratification of treaties limiting the size of nuclear tests, which were signed in the 1974 and 1976 but never ratified.

Scientists from the United States and the Soviet Union will be measuring the explosive punch of a thermonuclear device encased in a steel canister buried 2,050 feet beneath a picturesque mesa, 130 miles northwest of Las Vegas.

The canister, painted red, white and blue, is 8 feet tall, 88 inches in diameter, and weighs 7 1/2 tons. Soviet scientists were allowed to look at the canister, but only after the nuclear device had been sealed inside.

While the test will measure the size of a blast, it is not directly related to a possible test ban. However, scientists and diplomats say Wednesday's experiment is a critical first step down that long road.

The tests mark the first time nuclear scientists from either country have been allowed on the other's highly sensitive sites. Up to 45 Soviet nuclear scientists will be at the Nevada Test Site.

A like number of U.S. scientists, who conduct nuclear testing programs for the United States and Great Britain, will be on hand for a test at the Soviet's site at Semipalatinsk sometime next month.

U.S. officials were embarrassed last week when it was revealed that three members of an American team had tried to take unauthorized materials from the Soviet nuclear test site. The White House said the three had engaged in "unauthorized souvenir collection."