Despite the restrictions on knowledge and materials of the Anglo-American nuclear monopoly in the early Cold War, Norway and the Netherlands managed to build and operate a joint nuclear reactor by July 1951. They were the first countries to do so after the Great Powers. Their success was largely due to the combination of the strategic materials of heavy water (Norway) and uranium (the Netherlands). Nonetheless, they had to overcome significant political and technical obstacles. In that process a number of specific nuclear secrets played a central role. This case is used to study how and why knowledge circulation was impeded by secrecy. Specifically, I will explore four different secrets that illustrate how the Netherlands and Norway, being outside the British and American secrecy regimes, chafed against those regimes. Knowledge circulation was enabled through relations within networks that were at the same time scientific, diplomatic, and personal. I will identify three main factors that affected the mobility of information: the availability of strategic nuclear materials, the scientists’ individual interactions, and national interests.