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What is to be done with nuclear waste that remains radioactive for thousands of years? Since we have not yet learned how to perform transmutation, two options remain: either planning for long-term storage which, barring a catastrophe, will have a substantial regular cost, or storing the waste ‘in a safe location’ 500 metres below ground (the Cigéo Project), which involves major investment. The decision requires a Socio-Economic Assessment (SEA) of the advantages and disadvantages of each of these solutions against a time horizon of several thousand years — particularly long by comparison with normal foresight work.Julie de Brux, Patrice Geoffron, Pierre-Benoit Joly, Reza Lahidji, Jacques Percebois and Émile Quinet, who contributed to this SEA, set out here how the assessment was made, taking account not only of very long-term economic perspectives but also the durability of public institutions and, more generally, of society. In an admittedly somewhat caricatural manner, two scenarios are sketched out here, called the OK and KO scenarios, though the costs and benefits of the two options have to be calculated in each case (the probability of each is roughly estimated), employing figures adjusted by a formula which the joint authors explain. Carried out in this way, the study shows that the plan to bury waste (the Cigéo Project) is, “when applying an entirely reasonable discount rate, the most advantageous from the general public’s standpoint, as soon as one assigns a probability of at least 15% to the KO scenario becoming reality”. Such an exercise is undoubtedly hazardous and the method is a daring one. Does anyone have another solution?