You don't get to precisely choose which isotopes are produced by the reaction. Even among the light elements, there are radioactive isotopes, such as tritium. And it doesn't take much of it, to render the entire mixture hazardous unless refined in some fashion. One hope is that the refinement process can return the radioactive stuff to the cooking pot, so that the ultimate waste products are safe.

Considering the DT reaction D + T = a + n.

Neutrons produced in this reaction, being chargeless, cannot be contained within the electromagnetic field, thus leaving plasma and reacting with the surrounding material (e.g. tokamak chamber walls) and producing radioactive elements. This leads to activation and radioactive degradation of the reactor itself.