https://en.wikipedia.org/wiki/…_produced_via_irradiation
The starting material captures a Neutron and then decays into the product. I don't know how this is done in Real-Life, however i think, that the starting material is in some sort of blanket. This would be perfect to balance, because you obviously need to extract the products. Maybe you could just place plates of the starting material near a fuel rod. In any case, the Reactor won't produce any power.