Neutron bombardment of your fusion containment vessel creates radioactive material. I am not sure where you get this idea that fusion is 'clean', but everything that involves pushing around fundamental particles creates radioactive waste. There are theoretical designs for fusion reactors that limit the radioactive side-effects somewhat but nothing in this area is clean.
Scaled down fission reactors like the Toshiba 4S can be sized to power an apartment building or city block. There are numerous micro reactor designs available and a variety of companies working to try to create a market for them.
When it comes to terrestrial reactors then fusion reactors will be enormous (need to generate a lot of power to compensate for the huge risk and the massive costs that it will take to build them) and if you are talking about putting reactors in space then it is foolish to consider anything but fission for at least our lifetime.
Scaled down fission reactors like the Toshiba 4S can be sized to power an apartment building or city block. There are numerous micro reactor designs available and a variety of companies working to try to create a market for them.
When it comes to terrestrial reactors then fusion reactors will be enormous (need to generate a lot of power to compensate for the huge risk and the massive costs that it will take to build them) and if you are talking about putting reactors in space then it is foolish to consider anything but fission for at least our lifetime.