The cost of fusion, relative to other electricity sources, will be the primary decider for commercial power. Estimates from literature are presented, together with new estimates, based on simple arguments. The conclusion is that the cost of fusion is likely to be 10 times more than fission.
The neutrons from fusion reactions make certain materials radioactive via neutron activation. This will produce large volumes of radioactive waste which must be managed for hundreds or even thousands of years.
The materials in a reactor must survive damage from the enormous flux of neutrons emitted from the fusion reactions. This problem is as yet unsolved and with current materials reactor components will have to be replaced on a regular basis.
Safety is often evoked to suggest that fusion is preferable to fission, but many risks exist for fusion reactors which are similar to those of fission reactors. A major risk for both fission and fusion is loss of coolant followed by hydrogen explosions.
The quantity of tritium naturally occurring is small and essentially all tritium supplies come from CANDU fission reactors. In a few decades most of these special reactors will stop operations and the global supply of tritium will be close to zero before commercial fusion reactors can be brought into operation.
Because of the system complexity, the time a fusion power plant is fully operational to produce electricity could be low. The fusion reactor availability will be reduced by necessary scheduled maintenance as well as component breakdowns.
The enormous physical size of the ITER tokamak and the complexity of the associated systems required for operations, raise serious doubts that commercial reactors of even bigger scale could ever be viable.