Any fusion reactor in this century will likely be putting out a Lot of fast neutrons. These fast neutrons can turn otherwise benign heavy elements into weapons

Any fusion reactor in this century will likely be putting out a Lot of fast neutrons. These fast neutrons can turn otherwise benign heavy elements into weapons-grade fissile material. There are detection techniques for this kind of breeding and I’d imagine any country capable of building a fusion reactor would also be okay with audits. If you just want weapons-grade fissile material, there are far more economical avenues than a fusion reactor. Also, if fusion reactors become more viable than fission reactors, then we could put a global ban on all fissile materials. They would serve no purpose except for weapons.