A low-key Japanese diplomat could become head of the International Atomic Energy Agency when the organization charged with blocking the spread of nuclear arms meets this week to replace chief Mohamed ElBaradei. ElBaradei's successor will influence how the world meets the nuclear challenges posed by Iran, Syria or of extremists thought to be seeking the bomb. Nonproliferation is the IAEA's most high-profile task and the agency's director-general can determine the style and intensity of how the agency carries out its duties. The change in leadership comes at a potentially pivotal time in US-Iran relations: the new American administration has signaled it is ready for direct negotiations with Tehran over nuclear and other issues. Formally, it is still a two-way race between men who are a study in contrasts. Both Yukiya Amano of Japan and South Africa's Abdul Samad Minty are the chief IAEA delegates of their countries and wield other formidable credentials, including senior national nonproliferation and chairmanships of IAEA and other nuclear meetings. Both say they see the IAEA's role as essentially that of a technical organization that follows the direction of the 35-nation board.