During the period of Soviet occupation, Lithuania, Latvia, and Estonia played integral roles in the Soviet nuclear program. Following the USSR’s collapse, numerous military and civilian nuclear facilities were left in these states, serving as remnants of the Cold War. These facilities include nuclear power plants, military training centers, uranium mines, and scientific institutions housing research reactors. They serve as tangible evidence of the Soviet Union’s technological, scientific, and military expansion. However, documenting this expansion has been difficult due to the destruction or relocation of archives to Russia after the Soviet Union collapsed. This project focuses on the Lithuanian case, explicitly linking the Ignalina Nuclear Power Plant, constructed in Lithuania in the early 1980s, with the Soviet nuclear infrastructure in the Baltic Sea region. This project aims to investigate Soviet nuclear expansion in the Baltic Sea region not only through top-down relations but also by examining horizontal cooperation in terms of infrastructure of knowledge – circulation of scientific knowledge, nuclear safety expertise, personnel and materials among the Baltic States. This approach helps understand how various techno-scientific and political actors in the Baltic States perceived technological expansion during the Soviet era, and how research, military and civilian nuclear infrastructures in the Baltic Sea region were interconnected.