This paper deals with the transnational history of the “Superphénix” fast breeder nuclear reactor, a Franco-Italian-German project resulting from a shared ambition to develop new technology through European industry collaboration in the 1960s-1970s. Transnational projects were usually sited near the border between partner countries, so the plant was located East of Lyons, 100 km from the Italian border, yet closest to the Swiss border near Geneva. This innovative, industrial-scale FBR prototype triggered a cross-border controversy fuelled by scientists in France and Switzerland, as well as mass protests on a European scale, followed by successive lawsuits. Focussing on the mutual influence the borders and the project exerted on each other, the paper examines institutional responses leading to the project’s reframing as a research facility in the 1990s, and subsequent developments facilitating its 1998 shutdown. The conclusion addresses the shifting geographical scales and distribution of power over the course of the project.