Water and nuclear reactors are much closer intertwined than usually perceived. First, water is the source of the steam that drives the turbines of most nuclear power plants around the world. Next to generating electricity, water is the key to preventing accidents in nuclear plants. As uranium keeps on generating heat when the power plant is turned off, its core needs to be cooled continuously. This crucial connection between water and nuclear is focus of the paper. Nuclear safety will appear as relying heavily on earlier knowledge, institutions, and regulatory frameworks, which were related to water. The three parts of this article discuss technologies, actors and risks of nuclear power. Studying water as a resource in a much broader sense than being boiled for steam shows how determining water is to make nuclear power function. As this paper is part of a special issue, Water History in the time of COVID-19, it has undergone modified peer review.