peer reviewed ; We focus on the new governance practices in Belgian nuclear waste management from its ‘participatory turn’ in the late 1990s. Rather than praising (or rejecting) participation versus expert analysis, we make use of a theoretical and analytical framework in which the relevant dynamics for the analysis are ‘opening up’ and ‘closing down’ technological appraisals and commitments. Even though nuclear waste management agencies often plead for an integrative approach between expert analysis and stakeholder participation, in practice both exercises are often kept separate. We address this separation and its consequences and we find that societal concerns remain subsumed in the technical options that have long been favoured by the Belgian agency. This article encourages scholars, waste managers and decision-makers to scrutinise the moments and situations in which opening up would be desirable, and when, by contrast, it would be better to close down options in nuclear waste management.