Right answer: They have been successful, but they are very expensive to operate.
A fusion reactor is a facility in which nuclear fusion reactions take place in a fuel formed by hydrogen isotopes (deuterium and tritium), releasing energy in the form of heat, and then transforming it into electrical energy.
At the moment there is no commercial fusion reactor that allows to obtain electrical energy, although there are experimental research facilities where fusion reactions are studied, as well as the technology that will be used in the future.
In other words:
All commercial nuclear reactors currently in use work with nucler fission (separation of the components of the atom nucleous), not nuclear fusion (joining two light nucleous to form a heavier nucleous). And this is because, although experimental reactors have already shown us that nuclear fusion works, it is such a complex process that has not been yet found a way to get the fusion reaction to behave in a stable way for a long time and it is expensive as well.