The nuclear disaster in Fukushima which followed in the wake of the 3/11 Tohoku earthquake and tsunami has given rise to one of the most significant public health crises in modern world history, with profound implications for how nuclear energy is perceived. This paper analyzes the most dire phase of the Fukushima nuclear crisis, showing how the level of risk was assessed by nuclear experts and state-level actors who worked largely out of view of public scrutiny. In addition to examining how the accident progression in the reactors was addressed and conveyed to the general public, the paper addresses how the exclusionary zones were determined by Japanese and foreign governments in Japan. As the crisis unfolded and efforts to bring the reactors under control were initially proving ineffective, concerns increased that the dispersion of radioactive fallout was unmitigated, and with radiation monitoring by the U.S. military indicating levels significantly beyond TEPCO's conservative assessments, the United States broke with Japan, recommending an 80km exclusionary zone, and initiating military assisted departures for embassy staff and Department of Defense dependents from Japan. These actions deviated significantly from Japan's assessments (which had established a 30km evacuation zone), creating a dynamic where the U.S. provided technical consultation for the nuclear response while striving to maintain a delicate diplomatic balance as they attempted to impose a qualitatively different crisis management response. Because this crisis had significant implications for Japan's international relations, diplomatic considerations have helped to suppress the complex, often fractious relations between Japan and foreign governments - especially the United States - whose collective efforts eventually turned the tide from managing the nuclear meltdowns to ameliorating their long-term consequences. Based on interviews with political officials in both the Japanese government and foreign embassies in Japan, and nuclear experts and military officers who worked the crisis, the paper analyzes how technical assessments drove decision making and were translated into political policy.