Very few papers exist in the field of social sciences that follow and study nuclear infrastructure design projects from the inside. Such a perspective would make it possible to understand the mechanisms of their successes or difficulties at their very origin. At a time when high hopes are placed on civil nuclear energy to solve the climate issue, but when simultaneously, nuclear industry actors are facing major difficulties in a great number of flagship projects, this situated understanding of any given project’s trajectory becomes crucially important. This paper proposes an analysis, from the inside, of a project that, about ten years ago, raised great expectations from both the French and global nuclear industry, but which, in 2019, was finally halted. This project is that of a so-called fourth generation sodium reactor: the Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID). ASTRID was a new sodium-cooled fast reactor. Begun in 2010, the project’s development was suspended at the end of 2019 by the French authorities. Through an inductive study of the project in the making, conducted from 2015 to 2019, the research team was able to follow the project trajectory and carry out nearly 30 interviews with actors directly involved in the project itself. By studying ASTRID as an infrastructure development project and building on the concepts of scale and infrastructure from the literature, the ASTRID project’s halt can be understood. The project’s suspension was the result of the increasing complexity and ambiguity faced by project members and stakeholders in aligning the local and global scales as the new infrastructure was developed, and more precisely, the ASTRID project infrastructure and the global nuclear infrastructure. Our analyses show that ASTRID’s trajectory gradually drifted as a result of three misalignments between the project’s infrastructure and the global nuclear infrastructure: a temporal, social, and physical misalignment. As a result, the project identity can be described as having been vague and ambiguous. This paper sets out how such a lack of clarity impacted design practices, the project organization as a whole, and the ASTRID trajectory. In consequence, it is crucially important that lessons are learned from the project’s cessation to understand both the difficulties related to the nuclear renaissance and in terms of the field of project management in general.