peer reviewed / It is well established in science and technology studies that participation and expert analysis should not be seen as contradictory. Key analytical questions include how both public and expert knowledge contribute to “closing down” and “opening up” appraisals and commitments, and how important these dynamics are in assessing the process and the conditions of democratizing technology. This article examines how the participatory turn has affected nuclear waste governance options in France and Canada. Through cross-case analysis, it describes how at each constitutive step of management programs, public and expert knowledge has followed a variety of pathways in (in)forming commitments, resulting in asymmetrical trade-offs. The term “closing up commitment” is introduced to refer to the way both national governments finally opted for closing the technological options at hand while introducing new conditions that might challenge future actions. We argue that paying attention to this mutation in nuclear governance allows for a more detailed analysis of power distributions in science and technology governance than a critical approach that rejects any closure because it can be (and often is) the result of an instrumental approach undertaken by the incumbent actors.