I'm wondering if a fusion reactor would make its walls more radioactive thru neutron activation than a fission reactor, for a given amount of energy. It seems to me that (hydrogen) fusion produces most of its energy as neutrons that are unlikely to absorbed by the sparse near vacuum plasma. Fission produces most (90%?) of its energy to the fission products and neutrons are absorbed by the fuel to further fission and transmute uranium into plutonium. So I figure a lot more neutrons are hitting the wall in a fusion reactor and at higher energies than the moderated neutrons of a fission reactor. I guess a LOT depends on what the walls are made of. Fission plants are are of steel and I believe iron doesn't readily absorb neutrons or produce long enough lived radioisotopes to be a problem. Fusion reactor walls need to absorb the neutrons and turn them into heat. I believe lithium is proposed because it would allow breeding of Tritium, creating more fuel.