2. Expression of need (feasibility phase: milestone "0" to milestone "1")

The aim of the feasibility phase is to answer the questions: what solutions can be proposed to satisfy the need, as known at the start of the phase at milestone "0", and what are the possible implementation schedules and corresponding cost estimates? Let's assume that the requirement is for a very simple purification circuit, for which the initial CDCF's expression of need can be summarized in the "sketch" shown in figure 1 .

The aim is to study a wide range of possible solutions, from existing solutions to be adapted or not, and new solutions to be developed. It also involves refining the functional requirement to ensure that it corresponds to...