In Belgium, the long-term management of radioactive waste falls under the exclusive competence of the Belgian federal Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF). Contrary to the existing situation for low-level waste, no institutional policy has yet been formally approved for the long-term management of long-lived and high level waste. In this context, ONDRAF considers the public and stakeholders’ participation and engagement as essential in the formulation of an effective and legitimate policy. This is why, over the past few years, it has decided to engage them in different ways (public dialogues, interdisciplinary conference, consensus conference, legal public consultation) to elaborate the “Waste Plan”, an ONDRAF-document containing guidelines to make a principled policy decision about nuclear waste management. To achieve “successful” public participation exercises, social scientists have been regularly mobilized either as external evaluators, follow-up committee members, or participative observers. Our previous research on such public participation processes showed that little had been said as to how stakeholders and the “public” have been mobilized by the Belgian Agency during the elaboration of the Waste Plan. How has the wider public opinion been integrated in the Waste Plan so far? To what extent could public participation and engagement redefine the internal debate within ONDRAF on the relevance and usefulness of such exercises for the long-term management of long-lived and high level waste? Based on official document analysis, semi-structured interviews and participatory observation, this paper explores how ONDRAF actually assesses the quality of public/stakeholders participation and how it makes use of it in its communication and management strategies.