The easiest risks to manage are the known risks. When you have managed a project before, you know from experience what can go wrong. You know what to do to head the risks off early, and you know how to minimize damage if they do occur.
Known, unknown and unknowable ... project managers categorize risks as a way to devise the best strategy to plan for and mitigate when or if they materialize. ITER, as a first-of-a-kind project, is no exception.