Nuclear power plants require immense amounts of water to cool the reactors. Therefore, NPPs are almost exclusively built near rivers, lakes, and the sea. In the 1960s and 1970s, Switzerland and Germany had grandiose plans to nuclearise the Rhine and its tributaries. These plans came at a time when the environmental status of the Rhine was deteriorating, which did not go unnoticed. This forced the riparians to negotiate the limited cooling resources and to create new knowledge on the thermal resilience of the Rhine as well as the Aare. Earlier literature and coverage of the case was tending towards a potential escalation of the negotiations. There was talk in newspapers of a “cooling water war” between Germany and Switzerland. So far, research on NPPs on borders mostly focused on anti-nuclear protest and political conflicts relating to nuclear projects. In most cases, cooperation was limited to technical cooperation in the construction and operation of NPPs and in environmental monitoring. By contrast, this article demonstrates the close cooperation between two countries in finding a solution for the fair allocation of the scarce resource of cooling water which worked for both. The focus of this article is on the informative discussions and the subsequent negotiations on the fair distribution of heat load capacity and cooling water between the individual countries within the framework of water diplomacy and water regimes.