In order to counter the climate change, there is a need for a radical revolution in the global energy supply. One of the most realistic scenarios is a substantial shift towards renewable energy sources, supported by a flexible baseload from carbon-free nuclear energy. The main requirements for this new generation of nuclear reactors are intrinsic safety, high fuel efficiency and a minimum amount of (long-lived) radioactive waste. A promising candidate for the near future is the lead (alloy) cooled fast fission reactor. An ideal long-term solution are nuclear fusion reactors, which produce only a small amount of short-lived radioactive waste. A problem that occurs in both these reactor types is the undesired production of the highly radiotoxic polonium-210 isotope. In this PhD thesis, the total production of this isotope in a nuclear fusion reactor is determined using neutron transport and inventory calculations. Next, quantum chemical calculations are performed to predict the molecular form in which Po-210 will occur in both discussed reactor types. Combining both aspects allows to estimate the risk associated with the presence of Po-210 in a nuclear reactor and can help to design efficient Po-210 extraction systems.