The "Superphénix" fast breeder nuclear reactor was a Franco-Italian-German project resulting from a shared ambition to develop new technology through European industry collaboration in the 1960s-1970s. Transnationality contributed to rendering the industrial-scale investment possible. Such projects had to be sited between partner countries, therefore it was located in Creys-Malville, between Lyons and the Italian border, yet even closer to the Swiss border near Geneva. This 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 shutdown in 1998. The conclusion addresses the shifting geographical scales and distribution of power over the course of the project.