TO PREVENT WAR: Archibald S. Alexander (center), assistant director of the U.S. Arms Control and Disarmament Agency, chats with Norman L. Thoburn (left) of the University's Institute of Science and Technology, and Joseph I. Coffee (right) of Bendix Systems Division, prior to delivering the keynote address at the opening of the Second International Symposium on Arms Control and Disarmament here last night. Thoburn and Coffee are co-chairmen of the symposium and represent the sponsoring organizations. The Arms Symposium will continue through Friday.