In the preface of the English version of the official report by the Fukushima Nuclear Accident Independent Investigation Commission of the National Diet of Japan, the commission chair Kurokawa Kiyoshi writes that the nuclear accident in Fukushima “was a disaster ‘Made in Japan’.” According to him, “Its fundamental causes are to be found in the ingrained conventions of Japanese culture” (NAIIC 2012). Curiously, such a culturalist explanation of the disaster only appears in the English version of the report, and the Japanese version gives significantly more fine-grained analysis of institutional cultures of the relevant organizations. Such “self-Orientalism” appears even in depiction of science, as was the case with Yukawa Hideki’s self-fashioning as an Oriental physicist (Yukawa 1973). Criticizing such historiography that essentializes Japanese national culture, this paper studies nuclear physics in post-WWII Japan up to the mid-1950s and discusses how Japan’s national identities in relation to nuclear power and nuclear states were invented and shaped. To achieve this, the paper will do two things. First, it will explore transnational aspects of nuclear physics in Japan. Recent historiography of nuclear power stresses transnational aspects, as exemplified by Abraham(2006) and Hecht(2012). In Japan, too, relations to other countries, in particular other nuclear States, the United States among others, played a crucial role in the introduction of nuclear power into Japan. By examining this early history, the paper will show that both nuclear physics and nuclear engineering in Japan of this period was hardly “made in Japan”; rather, it was a result of political and cultural negotiations across various boundaries. Second, it will show cultural diversity within nuclear physics in Japan of this period. Having suffered twice from the American nuclear weapons in the recent past, Japan in the 1950s had problematic relations to the United States and nuclear power. To attain “nuclearity”(Hecht), post-WWII Japan had to mobilize actors with diverse cultural backgrounds and invoke various cultural representations. Scientists, engineers, politicians, and industrialists were involved, each displaying cultural diversity among themselves. Through such considerations, this paper will conclude that it is wrong to assume essential cultural uniqueness of Japan's nuclear physics; Japan's nuclear physics shared many aspects with other countries, both strengths and problems.