During and immediately after the Second World War, physicists and engineers in several countries worked intensively and in competition to develop nuclear weapons and to control the chain reaction creating nuclear energy. An experience of urgency and a sense of revolutionary future promise permeated the activities and largely outweighed the risks as they were calculated at the time. As a result, small experimental reactors were built at research institutes or universities relatively close to city centres and densely populated areas—the key localization factor being the physicists’ own geography. This chapter focuses on three of the early pioneering urban reactors, located in Chicago, Moscow, and Stockholm, which were all symbols of national prowess as humanity was entering the nuclear age, and later became objects of heritage processes. We scrutinize the early operations as well as the making of atomic heritage, through the conceptual lens of the ‘security theatre’. The concept highlights the relationship between, on the one hand, calculable risk and security, and on the other hand, perceived risk and security. We argue that, overall, the security theatre displays reversed characteristics if comparing the establishment period with the processes of heritagization in the way that the calculable risks were initially high but downplayed, while subsequently being low but exaggerated. This tension between calculable risk and perceived risk, we suggest, forms the key to the attraction of contemporary atomic heritage. This chapter is based on historic and contemporary written and visual sources, together with interviews and on-site visits.