cross sections at high energy are much lower than at low ones (neutrons lose their energy as they interact with matter, until eventually getting captures, ie activating the material ie "become radioactive").
Front facing material will have "less probability", but much higher fluxes (low chance of winning, but more lottery tickets), the deeper you go, the less flux you get, but the "probability of turning radioactive" increase as the neutron energy decreases.

As you can guess, the vessel wall of a regular nuclear reactor is fairly radioactive, so a ballpark mark is everything "blue" and higher will be pretty radioactive come decommissioning. Plus from a pure mass point of view, ITER is much much bigger than an NPP reactor vessel. The cryostat might end up only slightly activated, but the vacuum vessel and all will end up at comparative radioactivity level as NPP reactor vessels (and a dozen time the volume/mass).

ping u/beerorist to correct me.