This episode's headline: Disaster in Africa; Report on Chernobyl; Closed Frontier; Splintered Relations; Invented City?. The guests include In Washington: M. PETER McPHERSON Administrator, Agency for Int'l Development; In Vienna: RICHARD WILSON, Harvard University; HANS BLIX, International Atomic Energy Agency; In San Francisco: Dr. JACOB FABRIKANT, Radiation Scientist; In New York: ROBERT JOEDICKE, Airline Industry Analyst; REPORTS FROM NEWSHOUR CORRESPONDENTS: LESLIE OLSEN (WISH-TV), in Indiana; TOM BEARDEN, in Denver; VICTORIA FUNG (KCTS; SUSAN SHREVE, in Washington. Byline: In New York: CHARLAYNE HUNTER-GAULT, Correspondent; In Washington: JIM LEHRER, Associate Editor