In Belgium, the long-term management of radioactive waste is under the exclusive competence of the Belgian Agency for Radioactive Waste and Enriched Fissile Materials (knew as ONDRAF/NIRAS). Unlike low-level waste, no institutional policy has yet been formally approved for the long-term management of high level and/or long-lived radioactive waste (knew as B&C waste). In this context, ONDRAF/NIRAS considers the public and stakeholders’ participation as an essential factor in the formulation of an effective and legitimate policy. This is why it has decided to integrate them in different ways during the elaboration of the Waste Plan (ONDRAF/NIRAS-document containing guidelines to make a principled policy decision about nuclear waste management). To do so, social scientists have been regularly mobilized either as external evaluators, follow-up committee members, or participatory observants. Hence, the Waste Plan is only the first step in a long decision-making process. For a PhD student under contract with ONDRAF/NIRAS, this mandate consists of thinking out a way to construct an inter-organizational innovative communication system that would be participative, transparent and embedded in a long- term perspective, thus integrating all the further legal steps to take throughout the decision-making process. In this regard, two paradoxical constraints must be taken into account: on the one hand, my own influence on the legal decision-making process should remain limited, because of a series of constraints, lock-ins and previous decisions which have to be respected; on the other hand, ONDRAF/NIRAS expects the research conclusions to be policy relevant and useful. In this paper, the purpose is twofold. Firstly, the issues raised by this policy mandate is an opportunity to question the performative dimensions of the social scientist in the decision- making process and, more specifically, to have a reflexive view on our position as PhD Student. Secondly, assuming the role of “embarked” social scientist, numerous of answers will discuss to face the different dilemmas of the researcher “in action”. Those reflections follow on, among others, those from previous papers discussed in Quimper in April 2013 [1] and in Leuven in June 2013 [2].