August 16, 1986

The Chernobyl nuclear disaster in the Soviet Union last April occurred when workers running a test shut off key emergency equipment, then ignored warnings of problems, according to a report prepared by Soviet officials. The 382-page report, to be publicly presented at an international symposium in Vienna on Aug. 25, said human error was the primary cause of the disaster, which killed at least 30 people and was the worst commercial nuclear accident in history. Safety violations by operators were noted. Copies of the report, in Russian, were presented to the International Atomic Energy Agency in Vienna on Thursday and distributed to the embassies of foreign nations there over the last two days. Translated details were obtained by The New York Times today from Asahi Shimbun, the Japanese daily, and discussed with American nuclear experts, who added their own explanations.

The Senate tonight ignored the objections of the Reagan Administration and overwhelmingly approved a set of tough economic sanctions designed to undermine the system of racial separation in South Africa. The vote was 84 to 14, with 37 Republicans and all 47 Democrats combining to form a broad, bipartisan majority for the measure. New American investment in South African companies would be barred by the bill, and a wide range of business relationships would be tightly restricted. 'Reshaping Foreign Policy' Senator Richard G. Lugar of Indiana, the chairman of the Foreign Relations Committee, assessed the measure this way: ''Very clearly we are involved in reshaping foreign policy. This is a very important shift.''

The United States issued a strong protest to Mexico today over what the White House called ''the unprovoked and totally unjustified detention and torture'' of a Drug Enforcement Adminstration agent by the Mexican police this week. The Mexican Government acknowledged today that the agent had been detained for questioning, but it denied he had been mistreated. But Attorney General Edwin Meese 3d said, ''There is no question that our agent was badly mistreated, and we're not going to stand for this kind of conduct.'' 'Vigilantism,' White House Says The White House, in a separate statement, described the detention of the agent, Victor Cortez Jr., by eight Mexican police officers in Guadalajara on Wednesday as ''vigilantism'' that ''causes serious harm to the relationship necessary for our two countries to be able to combat drug trafficking.''

President Reagan has ordered the space agency to yield its commercial launching business to private industry and to build a fourth space shuttle to replace the lost Challenger, the White House announced today. The decision foreshadowed a major change in the role of the National Aeronautics and Space Administration, which has used its shuttle fleet to carry a mixture of military, scientific and commercial satellites. Technological Role Envisioned ''NASA will no longer be in the business of launching private satellites,'' Mr. Reagan said in a statement. Instead, the space agency will concentrate on pioneering the development of new technology, and its shuttles will carry aloft satellites that are important to national security, foreign policy, and, ''even more,'' the exploration of space and development of new space capabilities. ''NASA will keep America on the leading edge of change,'' Mr. Reagan said. ''The private sector will take over from there.''

Congress struggled to clear the way for its three-week summer recess early today after the Democratic-controlled House approved a 1987 military programs bill Friday that rejects Reagan Administration policies on arms control, new chemical weapons and a space-based defense against missiles. After two weeks of wearying late-night sessions, a fight over an amendment that would provide a new automatic spending-cut mechanism for the budget-balancing law stalled approval of a short-term increase in the debt ceiling. The increase had to be approved to avoid a Government default on interest payments and a delay of other payments during the Congressional recess. The short-term extension will accommodate Government borrowing through Sept. 30.